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Group Title: Bulletin of the Florida State Museum
Title: Plio-Pleistocene megalonychid sloths of North America
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00001529/00001
 Material Information
Title: Plio-Pleistocene megalonychid sloths of North America
Series Title: Bulletin of the Florida State Museum
Physical Description: 213-296 p. : illus. ; 23 cm.
Language: English
Creator: Hirschfeld, Sue Ellen
Webb, S. David ( Sawney David ), 1936- ( joint author )
Publisher: University of Florida
Place of Publication: Gainesville
Publication Date: 1968
 Subjects
Subject: Sloths, Fossil   ( lcsh )
Paleontology -- Pleistocene   ( lcsh )
Paleontology -- Pliocene   ( lcsh )
Paleontology -- North America   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 294-296.
General Note: Cover title
Statement of Responsibility: by Sue E. Hirschfeld and S. David Webb.
 Record Information
Bibliographic ID: UF00001529
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA0839
notis - ABS4653
alephbibnum - 000298271
oclc - 00115374
lccn - 74626635

Table of Contents
    Front Cover
        Front page 1
        Front page 2
    Table of Contents
        Front page 3
    Title Page
        Front page 5
    Copyright
        Front page 6
    Main
        Page 213
        Page 214
        Page 215
        Page 216
        Page 217
        Page 218
        Page 219
        Page 220
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        Page 223
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    Bibliography
        Page 294
        Page 295
        Page 296
    Back Cover
        Page 297
Full Text


BULLET


N


OF THE


FLORIDA


STATE MUSEUM


BIOLOGICAL SCIENCES



VOLUME 12


UNIVERSITY


OF FLORIDA


Gainesville


1968









Numbers of the BULLETIN OF THE FLORIDA STATE MUSEUM are pub-
lished at irregular intervals. Volumes contain about 300 pages and are not
necessarily completed in any one calendar year.












OLIVER L. AUSTIN, JR., Editor
WALTER AUFFENBERG, Managing Editor












Communications concerning purchase or exchange of the publications and all
manuscripts should be addressed to the Managing Editor of the Bulletin, Florida
State Museum, Seagle Building, Gainesville, Florida 32601.


Publication date
No. 1, 19 April 1968 ......................................
No. 2, 10 May 1968 ..................................
No. 3, 17 June 1968 .................- .........-.......
No. 4, 28 June 1968 ........................... .....
No. 5, 16 December 1968 ...............................-


Price
$1.20
.55
.85
.60
1.20

















TABLE OF CONTENTS


No. 1. Variation and Evolution of Gulf Coast Populations
of Beach Mice, Peromyscus polionotus.
W. WEDGWOOD BOWEN __--------------------------- 1


No. 2. A Review of the American Lizards of the
Genus Xenosaurus Peters.
WAYNE KING AND FRED G. THOMPSON ______________ __ 93


No. 3. Some Mexican Land Snails of the Family Urocoptidae.
FRED G. THOMPSON _____-__ ____-- -----___-__ 125


No. 4. Geographic Variation in Rhineura floridana (Reptilia:
Amphisbaenidae).
GEORGE R. ZUG ____ ___- ---------------185


No. 5. Plio-Pleistocene Megalonychid Sloths of North America.
SUE E. HIRSCHFELD AND S. DAVID WEBB ______ ------- 213






BULLET


N


OF THE

FLORIDA STATE MUSEUM


BIOLOGICAL
Volume 12


SCIENCES
Number 5


PLIO-PLEISTOCENE MEGALONYCHID
SLOTHS OF NORTH AMERICA

Sue E. Hirschfeld and S. David Webb


UNIVERSITY


OF FLORIDA


Gainesville
1968




Numbers of the BULLETIN
lished at irregular intervals.
necessarily completed in any


OF THE FLORIDA STATE MUSEUM are pub-
Volumes contain about 300 pages and are not
one calendar year.


WALTER AUFFENBERG, Managing Editor
OLIVER L. AUSTIN, JR., Editor


Consultants for this issue:
BRYAN PATTERSON
DONALD E. SAVAGE




















Communications concerning purchase or exchange of the publication and all
manuscripts should be addressed to the Managing Editor of the Bulletin, Florida
State Museum, Seagle Building, Gainesville, Florida. 32601.


Published December 16, 1968


Price for this issue $1.20





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 213

PLIO-PLEISTOCENE MEGALONYCHID SLOTHS

OF NORTH AMERICA




SUE E. HIRSCHFELD AND S. DAVID WEBB




SYNOPSIS: Although ground sloths appear in South America in Mustersan (late
Eocene) time, they are not found in North America until the Hemphillian (mid-
dle Pliocene) when three genera appear, two of them megalonychids. It now
appears that the genus Megalonyx was widely distributed through North Amer-
ica by that time. The most primitive is M. mathisi, new species, from the upper
Mehrtens formation, Merced Co., California. Additional material of M. curvi-
dens Matthew from the Upper Snake Creek channels of Sioux Co., Nebraska,
differs in its shorter cheek teeth and narrower caniniforms from later species.
Likewise a fine collection of late Blancan (early Pleistocene) M. leptostomus
Cope from Cita Canyon, Randall Co., Texas, exhibits small cranial, dental and
podial differences from typical late Pleistocene species of Megalonyx. M. lep-
tonyx (Marsh) is a nomen dubium. The general features of Megalonyx are
reviewed and the great variability of its dentition is analyzed.

The earliest megalonychid from North America is a new genus and species,
Pliometanastes protistus, from early Hemphillian deposits at McGehee Farm, Ala-
chua Co., Florida. Relatively complete material of this new genus is described.
Another new species from New Mexico, ?P. galushai, and other records from
western North America are presented. The genus bears no special relationship
to Megalonyx nor to any of the West Indian megalonychids, but stems from some
Mio-Pliocene stock that gave rise to all of these.


1 Sue E. Hirschfeld is Teaching Assistant in the Department of Paleontology at
the University of California, Berkeley. She completed most of the work reported
in this paper as a Master's candidate at the University of Florida.
SS. David Webb is Assistant Curator of fossil vertebrates at the Florida State
Museum and Assistant Professor of Zoology at the University of Florida, Gaines-
ville. His principal research is on Pliocene mammals of Florida.


Hirschfeld, Sue E., and S. David Webb. 1968. Plio-Pleistocene Megalonychid
Sloths of North America. Bull. Florida State Mus., vol. 12, no. 5, pp. 213-296.





BULLETIN FLORIDA STATE MUSEUM


TABLE OF CONTENTS
INTRODUCTION .......-....... ---.......-..-----------......-----...-.. ----------------- 214
ACKNOWLEDGEMENTS -......-- .........---- ........ ....----------.... ----------------- 215
Megalonyx Harlan ..............--..---.......---- .....--------.. .........------------- 216
M. leptostomus Cope ...-------... --.............- ----.------------------------ --- 219
M. leptonyx (Marsh) .....-... -.....---..- ..----.... --....... ....-------- 231
Other Blancan Megalonyx ........------.....-..---- ----. .. ----... ....---- ...------ .. 231
M. curvidens Matthew .........- ...---- -----.... ..-------....-------....---------- 235
M. mathisi new species ........---.................-------..-- --------------------- ...... 239
Other Hemphillian Megalonyx ---.........------..-..------.--------. ....-----------. 244
Pliometanastes new genus ............-..-...------- ...----------.. ..~. .---....--- ---- 246
P. protistus new species .--.-- .........-----....--- .......------------------....----- .. 246
?P. galushai new species ... --.....---... -- ..... ..... ------------... .... .........------- 283
Other Hemphillian Pliometanastes ................- ..........------ ...-----........-- 284
Other Hemphillian Megalonychidae ..---.............. ....------.. .......---- ..-..- 286
RELATIONSHIPS .......----------.. ------.........--....---.....-------- ....--------------- 287
LITERATURE CITED .....................................-...... ................ ..-.. ....... 294


INTRODUCTION
Ground sloths appear in South America in the MliiI,'s.,ii (late Eo-
cene), but the group is unrecorded in North America until the Ilem-
phillian (middle Pliocene). Fossil records of these earlier North Amer-
ican forms have hitherto been few and fragmentary. For this reason,
the discovery of relatively abundant remains of two genera of Hem-
phillian ground sloths in central Florida adds significantly to our
knowledge of the phyletic and zoogeographic relationships between
North and South American forms. The genera are Thunobadistes Iay,
a mylodontid from Mixson's Bone Bed, and a new megalonychid
genus from McGehee Farm. This contribution concerns the new
megalonychid and a review of other early North American mega-
lonychid sloths. A nearly complete skull from the late Hemphillian
of California representing a new structurally primitive species of
Megalonyx, new material of Al. curvidens from latest Hemiphillian
deposits, as well as an excellent collection of Al. leptostomus from
the late Blancan of Texas are described and problems of defining
species of MA',ialoinqf on the basis of isolated teeth are analyzed.
When the family Mh gl ,luclhiil.i (as constituted by Simpson,
1945) first appears in the Deseadan (early Oligocene) of Patagonia, it
is represented by fragmentary material. By the Santacrucian (middle
Miocene) stage in Patagonia seven mjic,.iloi ychid genera are distin-


Vol. 12





215


1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


guished, most of them represented by abundant material. Within the
most completely known genus, IIalal, '/, 22 species have been pro-
posed.
Skipping to the Pleistocene, Metgahlny the best known Pleisto-
cene megalonychid, is well represented by skulls, teeth, and two com-
plete skeletons. Although it is not found in South America, it ranges
widely through North America, reaching north to Alaska (Stock,
1942). In the West Indies nine genera of megalonychid sloths have
been recognized (Paulo Couto, 1967), all presumably Pleistocene in
age.
This substantial kInos\ ldge of megalonychid sloths in the Miocene
of South America and in the Pleistocene of Nearctica and the West
Indies contrasts strikingly with our ignorance of the group in the Plio-
cene of the entire hemisphere. The new genus from Florida is now the
most completely known Pliocene megalonychid. Its description, to-
gether with the review of other early North American megalonychids,
is intended to fill the Pliocene gap and thereby to provide new insight
into the taxonomic and geographic relationships of megalonychid sloths.

ACKNOWLEDGMENTS
The present contribution grew from the analysis of several unpublished col-
lections of early North American megalonychids. We began with the new Hem-
phillian genus from Florida in the collections of the Florida State Museum,
University of Florida (abbreviated UF). The first specimens of this form were
discovered by Florida State Museum parties under Clayton E. Ray during pre-
liminary excavation of the McGehee Farm Site in the summer of 1963. Inten-
sive work began in 1964 with support from the Frick Corporation and continued
from 1965 through 1967 aided by NSF grant GB 3862. Then Donald E. Sav-
age suggested that we study the megalonychid material from the extensive
Cita Canyon fauna that he and others had collected for the Panhandle Plains
Museum at West Texas State, Canyon, Texas (hereafter WT). Thirdly, (we
investigated the Tertiary megalonychids in the incomparable Frick collection
of Cenozoic vertebrates now in the American Museum of Natural History in
New York (FAM), and the Blancan material from Ilagerman, Idaho in the
U.S. National Museum collections (USNM). We studied Pliocene megalony-
clids in the Museum of P l, ,.,t.l. ,\t of the University of California at Berke-
ley (UCMP) and those in the collections of the University of Oregon (UO).
And finally we compared these earlier materials with the fine collection of late
Pleistocene Megalonyx in the Academy of Natural Science of Philadelphia
(ANSP).
\We wish to thank the many colleagues who received us graciously and loaned
us material from their institutions. We are particularly grateful to Malcolm C.
McKenna, Bryan Patterson, Donald E. Savage, R. J. G. Savage, Morris Skinner,
Ted Galushla, S. J. Olsen, J. A. Shotwell, Horace Richards, II. K. Brooks, and
R. A. Edwards for their advice and assistance.






BULLETIN FLORIDA STATE MUSEUM


All measurements in this paper are in millimeters unless otherwise indicated.
Statistical methods and abbreviations follow Simpson et al. (1960). Additional
abbreviations of collections are AM (American Museum of Natural History),
OMP (Oklahoma Museum of Paleontology), and PU (Princeton University). Our
dental nomenclature distinguishes the caniniforms from the molariforms, and
therefore the abbreviation M1 indicates the first lower molariform tooth, which
some systems label M,.

Megalonyx Harlan
Megalonyx Harlan, R. 1825. Fauna Americana, Anthony Finley, Philadelphia:
201. (Type species M. jeffersoni Wistar, West Virginia).
Morotherium Marsh, '. C. 1874. Notice of new Tertiary mammals III. Amer.
Jour. Sci., 1874: 531. (Type species M. gigas Marsh, Central California).
DIAGNOSIS. (Modified from Stock, 1925.) Skull short, broad,
and deep, with anterior end truncated and strong sagittal crest. Man-
dible deep, without spoutlike predental region. Dentition 5. First
tooth separated by diastema from cheek tooth series; cross-section
meniscoid with inner bulge. Last superior tooth subtriangular in
cross-section. Appendicular skeleton more robust than in Nothro-
therium. Winglike process of calcaneum broad; astragalus retain-
ing essentially the structure seen in Hapalops and distinctly less
modified than in Nothrotherium. Astragalus possessing V-shaped
notch for reception of the tibia. Shaft of fibula curved dorso-medial
to distal end in region of peroneus brevis attachment. Metatarsal
IV short; metatarsal V with relatively slender lateral process.

DENTAL VARIABILITY
It has long been recognized that the dimensions of sloths are among
the most variable of any mammals. This is exemplified among fossils
by the 22 "species" of Ilapalops described from the Santa Cruz de-
posits of Patagonia. Similarly in Mcdal,,nyx. nearly every good speci-
men has been described as a different species. Whether this variability
is inter- or intraspecific is a crucial, but rarely answered, question.
Simpson (1959) shows that while considerable individual variation
is manifest within a single collection of Megalocnus rodens, this varia-
tion is continuous and appears to represent a single population. We
present a similar analysis of all the complete adult megalonychid teeth
from the Port Kennedy Bone Cave collection in the ANSP (Tables 1
and 2). Measurements were taken just below the occlusal surface of
each tooth. Cope (1871) described four species of Megalonyx from
this collection and, for the sake of objectivity, we have followed his
assignments for all teeth used in this analysis.


Vol. 12









HIRSCIIFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


co 0 cQi co r -I I In






M rc CD0 o01 0C ( CO- CO
CL4 O u c ^ M cl t- I-- t-- 0





_r CO t- => t- 01 t--







e co co r co cq co t o
-1 O cO CD i 01 O CO







1- 1 -- 10 C CO 10 O0
- --q C-i C- C0 c0 C10 -






P O 10 co -t 00 CO 1 c
a MC oi 10 IM 1 10 CO -i
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'- CO C cO 0 o CO C)
10C 1 { 1-4


1968


217


:10
6u5


10
10COd


00 1-
00 1
O- i-


ci. o
co co




BULLETIN FLORIDA STATE MUSEUM


Two sets of calculations were made on these teeth: The first uses
just those teeth Cope designed as M. wheatleyi; the second includes
all adult sloth teeth from the deposit, among them one lower and
two upper caniniforms of M. tortulus and one upper caniniform of
M. loxodon (the specimen of M. sphenodon was omitted because it
is a juvenile). The second set of calculations yields coefficients of
variation that are not unusually high and graph in a homogeneous
cluster of points (Fig. 1). This strongly suggests that the Port Ken-
nedy Bone Cave specimens belong to a single population.


0 5 10

Width


15 20 25


FIGURE 1. Size distribution of upper caniniform teeth of adult Megalonyx
wheatleyi, M. loxodon, and M. tortuous from Port Kennedy Bone Cave, Pa.
Measurements taken just below occlusal surface.


Vol. 12


218




1968 HIRSCHFELD/WEBB: PLIO-FLEISTOCENE SLOTIIS 219

In study of megalonychid teeth too little attention has been paid
to ontogenetic variation. Juvenile specimens of hlcalonyx and of
Recent tree sloths have consistently failed to yield deciduous teeth.
Owen (1840-45) describes a newly erupted (permanent) tooth of
Bradypus as conical and completely covered by cement. As the tooth
erupts, the cement wears away and exposes the dentinal layers. We
have observed in a series of X-ray plates of a young Choloepus hoff-
wani that in younger stages the teeth taper sharply from root to
crown, whereas in more mature stages the sides of the teeth become
nearly parallel. The same variations occur in Megalonyx teeth.
The length of the inner dentinal layer may also serve as an age
indicator. For example, in a tapered lower caniniform of a young
Megalonyx from Devil's Den, Florida, the inner dentine layer occu-
pies only the lower 10 per cent of the tooth length, but in a parallel-
sided mature specimen from Santa Fe I, Florida, this layer occupies
72 per cent of the total tooth length.
We have observed considerable change in the shape of the occlusal
surface accompanying these other dimensional changes in living
Choloepus. Such changes evidently occurred in Megalonyx as well.
A comprehensive investigation of Megalonyx probably will show that
several of the proposed species are actually ontogenetic stages of a
single natural species.

Megalonyx leptostomus Cope
t,- Ll. 'I.n leptostomus Cope, 1893. A preliminary report on the Vertebrate Pa-
]contology of the Llano Estacado. 4th Ann. Rept. Geol. Surv. Texas: 49.
TYPE. The type of M. leptostomus consists of some skull frag-
ments and an upper molariform tooth from the Blanco Beds near the
eastern edge of the Llano Estacado in Crosby Co., Texas. Meade
(1945) referred several additional teeth and jaw fragments from the
same area to this species. The new materials described below were
collected in the lowest stratigraphic unit in the Blancan section
in Cita Canyon, 31/, miles south and 13 east of Canyon, Randall Co.,
Texas (Johnston and Savage, 1955). Thus all of the material known is
derived from Blancan age deposits in the Panhandle of Texas. These
new materials are the most complete known.
DIAGNosIS. Skull smaller than Megalonyx brachycephalus (Sto-
vall and McAnulty, 1950), narrow at postorbital protuberances;
height and breadth of occiput considerably less than in other spe-
cies, jugals flaring anteriorly, mastoid process rounded and not





BULLETIN FLORIDA STATE MUSEUM


Figure 2. Skull of Megalonyx leptostomus, WT 1956, dorsal, lateral, and
ventral views.


220


Vol. 12




HIRSCIIFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


,, i;like' foramen ovale surrounded on the posterior side by a round
protuberance, wider posterior divarication of palatines, adult cani-
i1forms with prominent median bulge, and M2 and M3 with rela-
te -cly greater mediolateral diameter than in other species, phalanges
S:nd 2 and digit III of pes not co-ossified.
SKULL. The skull, WT 1956 (Fig. 2) is well preserved but lacks
1 pterygoids, palatines, nasal region, and most of the zygomatic
;:ch. All teeth are missing. Sutures in the occipital region are not well
S,ed, although those between the frontals and parietals are almost
l, The sagittal crest is well developed as in all species of Megalonyx
d about 112 mm long, as compared with 78.9 in M. brachycephalus
A ,d 127 in M. jeffersoni.

The jugal bones flare anteriorly and laterally, whereas in M. jeffer-
I ;i they project more ventrally and posteriorly. The postorbital pro-
1erance is poorly defined and extremely narrow. The distance
Sross the postorbital protuberance is 92 mm, compared to 120 in
ac mc pl'cepidui, (Stovall and McAnulty, 1950) and 142 in M.
I. gani (Stovall, 1940).

The Cita Canyon skull presents a flat dorsal profile similar to that
it Owen's specimen of M. jeffersoni (Leidy, 1885) and in M. miller
; yon, 1938), unlike the profile in Dickeson's specimen of M. jeffer-

Table 3. MEASUREMENTS OF Megalonyx leptostomus SKULL,
(WT 1956) IN MM

I gth of skull from posterior end of occipital
ondyles to posterior border of caniniform alveolus 270.0
\ th of maxillary at posterior internal angle
f caniniform alveolus 65.0
v ith of palate anterior to first cheek teeth 42.5
I st width of palate 22.5
v' ith across anterior end of glenoid fossa 166.0
! iimum orbitotemporal width 72.8
Simrnum postzygomatic width 117.0
T nrsverse diameter of occipital condyles 70.2
\'th of basisphenoid anterior to basilar tubercles 48.5
Al liolateral width of process lateral to foramen ovale 16.8
Median length of palate from point of bifurcation
Sposterior margin 106.3










Table 4. COMPARATIVE MEASUREMENTS OF KNOWN SKULLS OF Megalonyx SPECIESt
to

Measurement M. brachy- M. i, fft r- M. jeffersoni M. lidyi1 M. miller1 M. hogani1 M. lepto- M. mathisi
cephalus' soni (Owen)l (Dickeson)l stomus

Length of skull from
occipital condyles to
anterior margin of
first cheek teeth 256 356 334 343 291 245 230 223
Breadth of face at post-
orbital protuberance 120 127 138 123 129 142 92
Breadth of cranium at
narrowest part of
temporal region 86 90 102 91 75 98 73
Length of sagittal crest 78 127 127 144 95 95 112
Height of occiput from
dorsal margin of fora-
men magnum 70 76 70 46 51
Height of occiput from
basioccipital 110 110 107 72 75
Breadth of occiput at
mastoid process 141 159 165 165 153 175 126 123
Length of diastema 41 50 49 40 50
Maximum width first
cheek tooth alveolus 24 21
Maximum width second
cheek tooth alveolus 24 27 28 28 25

'From Stovall and McAnulty (1950).





IIIRSCIIFELD/WXEBB: PLIO-PLEISTOCENE SLOTHS


.)li Leidy, 1855) or in M. hogani, and M. bruchycecphal is, in which
the anterior portion of the frontal bones arch above the nasal region.
These differences in the dorsal profile may reflect age differences.
Dickeson's specimen of M. icffecr,,ni appears to be the oldest, as most
of the sutures are ollitrralt-dl and the sagittal crest is closed and
fu,.cd. In Owen's specimen the sagittal crest is an open fissure and
los,,t of the sutures are fused but still distinguishable. In the Big
r!:e Cave specimen sutures are fused and the sagittal crest is just
, !ni iu, thus indicating a juvenile.
(i :iparison of the Big Bone Cave juvenile and Dickeson's adult
,,__i, that, as the individual becomes older, the vaulting of the
fnrIi. I region becomes more pronounced and the frontal ridges run-
1nin from the sagittal crest to the postorbital protuberances diverge
nr-. Thus the flat dorsal profile of the City Canyon skull probably
cat;, h1rin a relatively young specimen.
"i: -(rtical distance from the dorsal border of the foramen mag-
I "!; :.; a1 supraoccipital crest is extremely short, measuring 45.5 mm,
(c.;'i'riid to 70.0 for M. brachycephalus. Similarly the height from the
r.:-a!- border of the foramen magnum measures 71.7 mm compared
,ii 1077.0 in M. leidyi and 110.0 in M. jeffersoni. The Cita Canyon
-l ,imen is only slightly higher in this area than in the new genus
fri McGehee where it is 67.8 mm. The breadth of the occiput meas-
r, .! across the mastoid processes is considerably less than in the other
sip -..s of Megalonyx, 126 mm in this specimen, 141 in M. brachy-
S'''i:s.', and 175 in M. hogani.
i median occipital ridge does not extend to the foramen mag-
inu : in Owen's specimen of M. jeffersoni but is lost in a depression
aboit halfway down the occiput. As a whole the occiput is consider-
i;: eIss rugose and lacks the large tuberosities on either side of the
: !i i, ridge in Owen's M. jeffersoni. The juvenile specimen from Big
m:ne Cave also lacks these prominent tuberosities; the base of its
';tistoid process is not depressed below the supraoccipital surface nor
I it concave posterolaterally as in M. jeffersoni. Also, the process is
lnailer and more rounded than in that species. The occipital condyles
': th Cita CrnI n skull have shorter mediolateral diameters than
tll' condyle Cope (1893) figured for M. leptostomus from the Blanco
I '] l] lit.
In the p.tlh.i,,r portion of the skull the excavation between the
lilsilar tubercles of the basioccipital is not so deep as in M. jeffersoni.
Ple projection of bone medial to the glenoid fossa and surrounding


1 ,-




BULLETIN FLORIDA STATE MUSEUM


the posterior border of the foramen ovale is large and rounded,
whereas in M. Jeffersoni it is short and narrow. The basilar tubercles
in the Cita Canyon skull are situated more ventrally in relation to the
condyles than in M. jeffersoni. The median lacerate foramen is lo-
cated farther posteriorly in relation to the basilar tubercles than in
that species, in which it opens adjacent to the anterior border of the
tubercles, and the foramen ovale and foramen rotundum are farther
apart. The pterygoids and palatines project at a more obtuse angle
from the basisphenoid and the parasphenoid does not project as far
ventrally as in M. jeffersoni.
The epitympanic ring is complete on the left side of the skull and
forms a circular loop of bone which is firmly fused with the mastoid
process of the squamosal posteriorly, the entotympanic medially, and
the squamosal anteriorly. The external auditory meatus thus formed
is directed laterally. The entotympanic is fused with the stylohyal
process anteriorly and the epitympanic laterally. The custachian tube
apparently occupied a groove running obliquely anteroventrally from
the anterolateral margin of the entotympanic toward the midline of
the skull. The stylohyal process appears to be abnormal, either it is
exostosed or contains a fused fragment of the stylohyal. At the base of
the stylohyal process, posterior to the epitympanic, is the opening of
the stylomastoid foramen. The foramen immediately posterior to the
stylomastoid (second stylomastoid, Leidy, 1855: 10) opens into a ca-
nal that leads into the mastoid process of the squamosal. Posterior to
this foramen a groove runs along the posterior side of the mastoid
process, fading out just ventral to the supraoccipital crest. In other
specimens this groove is roofed over and opens in the mastoid process
as the mastoid foramen. Apparently the latter two foramina are for
venous drainage from the cranium and exhibit considerable individual
variation.
On the right side of the skull the stylohyal process, entotympanic,
and ectotympanic are missing, expsi ng the petrosal quite clearly. The
opening of the fenestra ovalis in the dorsolateral portion of the petro-
sal is directed anterolaterally. The facial foramen is located anterior
to the fenestra ovalis. Posteroventral to the fenestra ovalis is the fen-
estra rotunda, the opening of which is directed posteroventrally. Lat-
eral to the petrosal, the epitympanic recess opens dorsally into the
epitympanic sinus, which extends anteriorly within the squamosal to
the posterior margin of the glenoid fossa.
In its anterior region the palate resembles that of M. jeffersoni, but


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IIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


nore posteriorly the angle of bifurcation between the palatines be-
,,, s greater than in M. jeffersoni, thus resembling the type specimen
oif I. leptostomus (Cope, 1893). The posterior alveolar border for the
cinlliniform does not display any of the distinct grooves or concavities
in ihe M. jeffersoni teeth.

DENTITION. Isolated megalonychid teeth available from Cita Can-
,a.r1 include four upper caniniforms and one lower caniniform, four
\,: or M3's, one upper or lower M1, one M2, and two M3's. In ad-
lit;:! a left mandible, WT 580, with a fragmentary caniniform, M1,
,li '. and a right mandible with a caniniform, M2, and Ms. A
fr-;:: tary palate includes the left M1 M4, right M3 M4 and a
p I; alveoli of M1 M2 (Table 5).
His collection is of interest because it comprises a series of speci-
; ranging from very young to presumably fully adult. The varia-
,,: i the teeth are presumed to represent stages in ontogenetic age
,:A; i irn specific differences. The left mandible, WT 580, is from an

}TJble 5. MEASUREMENTS OF TEETH OF Megalonyx leplostomus IN AMM


Catalog
ot! Number
C, WT 579
c! WT 2548
\ WT 580
N WT 580
WT 2321
WT 579
WT 579
WT 2376
WT 2376
: WT 1746
WT 1746
WT 1746
tg WT 2376
\N WT 580
I WT 580
SWT 580
\ r WT 580
1\1 or M3 WT 1746
'\ or M3 WT 1746
- orM3 WT 2548
Sor' N WT 1789
M _or M1 WT 2548
1 AP anteroposterior
2 ML = mediolateral
3Estimated


Estimated
Age

juvenile
juvenile
very young
very young
adult
juvenile
juvenile
juvenile
juvenile
adult
adult
juvenile
juvenile
juvenile
juvenile
juvenile
juvenile
adult
adult
juvenile
juvenile
juvenile


Measurement
AP1 ML2


26.9
22.8
7.9
8.0
15.5
12.6
13.73
12.9
13.5
30.1
30.0
25.8
26.9
10.33
11.9
11.7
7.93
15.5
15.0
14.2
13.0
13.4


13.4
12.0
10.6
10.0
24.0
18.5
13.4
18.0
18.0
19.6
19.4
15.0
16.4
13.53
16.23
15.8
12.9
24.6
25.3
19.6
19.9
17.0





BULLETIN FLORIDA STATE MUSEUM


extremely young individual, for it is very small, short, and rather
rounded anteroposteriorly in comparison with the longer, slimmer
adult jaw. The bone of the mandible is rather porous in texture. The
distance from the anterior border of M1 to the posterior border of M2
is 17 mm. the teeth are embedded in the jaw in such a way that their
taper cannot be measured, nor can the amount of internal dentine be
determined. Observed differences between the presumed juvenile and
adult teeth are larger size and greater relative width in the latter. Only
the lateral wall of the caniniform is preserved. It is long-ovate in shape
and measures 16.6 mm in length.
The caniniform associated with the adult mandible WT 579 differs
from all the other caniniforms from Cita Canyon, as well as from the
caniniforms of later species of Megalonyx, in lacking the prominent
median bulge on the linbual side and in being more convex on the
lateral side. In this respect it closely resembles the canine of M. cur-
videns. M2 and M3 show no significant differences from the other
specimens and Ms closely resembles one from the Snake Creek beds
that Matthew (1924) referred to M. curvidens.
Two isolated upper caniniforms, a right and a left, WT 1746, repre-
sent adult individuals. The extreme development of the median bulge
on the lingual side makes both teeth almost triangular in cross section.
This bulge is better developed in these specimens than in the other
Cita Canyon caniniforms, as well as in any other species of M'ega-
lonyx. The anteroposterior dimension is shorter than in most specimens
of M. wheatleyi and the mediolateral dimension is greater. The oc-
clusal surface is concave, and the median bulge projects above the
rest of the occlusal surface.
A third upper caniniform, also numbered WT 1746, is smaller than
the two just described and represents a younger individual, as the
sides are not quite parallel and the internal dentinal layer is not so
deep as in the preceding specimen. Otherwise this tooth has the same
characters as the adult specimens.
Another upper caniniform, WT 2376, appears to be an adult but
does not have so large a median bulge as the WT 1746 specimens.
The occlusal surface is worn evenly and thus lacks the central excava-
tion and projecting lingual bulge characteristic of later Ac.L'(aIu!'r
caniniform teeth. The lower caniniform, WT 2548, is similar to WT
2376 in lacking a prominent median bulge and having an almost flat
occlusal surface. These two teeth do not differ in age from those with


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227


IIlRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


the prominent median bulge; the differences may represent extreme
individial variation or sexual dimorphism.
Tw o molariform teeth, a left and a right M2 or M3, both numbered
\T -17 1 pir, ii'.il ly represent fully adult individuals, while two
other right upper M2 or M3, WT 2548 and WT 1789 represent younger
indiviulidLls. The former are considerably longer mediolaterally than
in (,it. r WT 2548 or WT 1789. The mediolateral widths are com-
paraltil to those of M. jeffersoni, but the anteroposterior dimensions
are conijiderably less than for that species. The mediolateral widths
fall within the upper limits of the observed range of M3 and near

2!:' | 6. MEASUREMENTS OF Megalonyx leptostomus POSTCRANIAL
MATERIAL IN MM

Tibia 1716)
Length nteroexternal surface 251
Max. bnrt of head 1321
Max. )br-ca". i of tarsal end 112
Max. ante: ,posterior diameter of tarsal end 63
Calcanc,,c (WT 2599)
Widttl neck 25.3
Lentli t. neck 44.9
Dist,!i from inner border of external astragalar facet
', eroexternal prominence of articulating end 72.0
D>i1 from dorsal border of external astragalar facet
i '(itral border of cuboid facet 73.0
MnAr,,,l: II (WT Uncatalogued)
Max. ,gth 54
Breadt: oIf shaft 22
1)Dptlh 'I proximal end 381
Deptih distal articulation (obliquely) 371
Aletat, al III (WT 1836)
Antir,, ',sterior diameter through middle 39.9
D,,p ,f median vertical convexity at distal end 55.0'
DI( i ,fP proximal articular face 385
l ..ith 51.5
S I of Digit III of Pes (WT 1714)
\I, nteroposterior length 345
S, t 46.8
/ f', ,P proximal end 55.6
ji Of distal condyles 41.7
II of Digit III of Pes (WT 1714)
,; ,p1' ,t" riolr length 53.0
\i,, proximal end 39.5
I !t; ,t proximal end51.4
,'t:,,f distal contyles at middle 27.4
St disltal condyles 36.6
i: -timactcd





BULLETIN FLORIDA STATE MUSEUM


the upper end of the range for M2 from the Blanco beds. The tooth
of M. leptostonus that Cope (1893) thought was a last superior
molar is probably an M2 or M3. Its dimensions correspond to two
M2 or M3 (WT 1746) from Cita Canyon. Cope's tooth measures 14
mm anteroposteriorly and 25 mediolaterally, whereas the Cita speci-
mens measure 15.0 anteroposteriorly and 25.3 mediolaterally for one,
and 15.5 anteroposteriorly and 24.6 mediolaterally for the other.
The shapes of these teeth agree with those Cope (1893) de-
scribed for the Blanco specimens of M. leptostomus. In cross section
they are subangular trapezoids with one side of the occlusal surface
bevelled outward. The posterior side is slightly concave, the anterior
side slightly convex.

TIIA. The left tibia (Fig. 4G, H), WT 1716, is smaller than that
of M. jeffersoni described by Leidy (1855) by about one-fifth, but
is larger than that described by Stirton (1939) from the Mulholland
Fauna in California. The element is essentially complete but lacks
the external femoral facet and posterior border of the internal fe-
moral facet. In overall outline, the Cita Canyon tibia does not
differ significantly from that of jeffer.soni figured by Leidy (1855).


FIGURE 3. Left calcaneum of Megalonyx leptostomus, WT 2599. A.-inner
view, B.-outer view. X1/1.


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228




HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


it differs from the megalonychid from Mulholland in the greater
veiiral projection of the internal malleolus and the smaller and less
c,,( pitIu'Is astragalar spine. The facet for the astragalus is not as
wi.e as in that specimen; in this respect it resembles M. jeffersoni.
(:ALCANEUM. The left calcaneum (Fig. 3), WT 2599, from Cita
C:l,,on lacks only the extreme posterior border of the tuber calcis
a i! the medial edge of the internal astragalar facet. It is slightly
so ..ller and less robust than the calcaneum from Hagerman, Idaho,
u \M 23209. The tuber calcis of M. leptostomus is similar in shape
t h that of the new Hemphillian genus (below) in its greater pos-
t<: or ( \p.l,'ion. The proximal articular surfaces lack the distinct
(g o'es between the facet seen in M. iclffci.ii californicus Stock
( 25), and the cuboid facet lacks the inner lip flattened at right
a .:les to the rest of the surface observed in that species. The dis-
toi cce between the internal astragalar facet and the cuboid facet is
'i :tter in M. leptostomus than in the specimen from Idaho. The
a; :i for attachment of the peroneus muscle is not so prominent
ai.dI does not project laterally as a narrow ridge of bone as in that
specimen.
METATARSALS. A poorly preserved left metatarsal II is avail-
aie from the Cita Canyon collections. It is smaller than the same
c; icent in the Hagerman, Idaho sample, but in overall appearance
t:C two specimens agree.
A third metatarsal in the Cita Canyon collections lacks only the
1 tral extremity. It has about half the bulk of metatarsal III of
jeffersoni, and differs from that of Hapalops and M. jeffersoni
i having the medial and lateral distal articular facets for phalanx
I raised above the surrounding surface and very well defined. How-
Sr, the carina is less robust than in M. jcffcrwii and the facet for
,, iculation with metatarsal IV projects farther ventrally. The facet
t the ectocuneiform is similar in shape to that of M. i'ffcr.woii,
<'opt that it is slightly convex at the center of the facet rather than
uI formly concave.
PHALANGES. The Cita Canyon material includes phalanges I
aid II of the third pedal digit (Fig. 4). The remarkable feature of
t'"s specimens is that they are unfused even though they appear
t, presentt a mature individual or individuals. The same is ob-
" I\ed in specimens from the Santa Fe I site in Florida and from
l kerman, Idaho, both of Blancan age. This is in sharp contrast
to all observed later Pleistocene Mlc'aloihnii specimens, including


229






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230


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1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 231

those of M. jeffersoni d( scrili-d by Leidy (1855) and Stock (1925),
in which phalanges I and II are completely co-ossified.
Phalanges I and II from Cita Canyon are less robust than those
from Idaho, but do not differ significantly in shape. The anterior
facets on phalanx I are much more flattened in these early species
of Megalonyx, than in later Pleistocene specimens.
The posterior surface of the phalanx II is similar to that of
Hapalops. A facet is located on the dorsal and another on the ven-
tral end of the median ridge between the articular surfaces for
phalanx I. These facets contact the dorsal and ventral margins of
phalanx I and prevent much movement between the two phalanges.
Considerably less movement is possible between these phalanges
than in HIapalops.

Megalonyx leptonyx (Marsh)
Morotherium leptonyx Marsh 1874, Notice of new Tertiary mammals III,
Amer. Jour. Sci., Vol. 7, p. 532.
TYPE. Marsh (1874) described a small ungual phalanx of the
third digit of the manus from the Snake River Basin, Idaho, as the
type of IMorotfhrium leptonyx. Evidently it represents a small meg-
alonychid, probably Mthcalonyx. but more refined determination is
impossible. The only locality data Marsh provided in the original
description is "Pliocene beds of Idaho." According to Hay (1927)
this claw is the same one that Leidy (1871) mentioned as having
come from Castle Creek, Owyhee County. Gazin (1935) tentatively
referred two mandibles, some teeth, and a few footbones from the
Hagerman beds (early Blancan) to Megalonyx leptonyx? (Marsh)
on the basis of their small size and the lack of determinable ma-
terial of other ground sloths from the area. The type of lorothIcrium
leptonyx has been misplaced. As the description of type is inade-
quate for specific diagnosis and the age and locality are uncertain,
we consider 31cgalonryv 'leptfonU. a nomen dubium.

Other Blancan Megalonyx
HAGERMAN, IDAHO. We doubt Gazin's (1935) tentative ref-
erence of the Megalonyx sample from Hagerman, Idaho to M.

FIGURE 4. Postcranial elements of Megalonyx leptostomus. A. Phalanx I of
digit III of pes, WT 1714, lateral view, x. B and C, proximal and distal
views of same, X1. D. Phalanx II of digit III of pes, lateral view, X 2. E
and F, proximal and distal views of same, X1/. G. Left tibia, WT 1714, an-
terior view, x0.38. H. Inner view of same, X0.38.





BULLETIN FLORIDA STATE MUSEUM


leptonyx (Marsh). It seems more probably referable to M. lep-
tostomus. In any case we defer specific assignment until the dis-
crimination of species within Icgaloiiyx is placed on a more secure
and consistent basis.
Gazin (1935) described the following material from the Hager-
man beds: imperfect mandible with complete lower dentition, as-
sociated with upper caniniform, upper cheek tooth, claw and patella
USNM 13477; mandible edentulous except for left caniniform of
immature individual, USNM 12669, both from Plesippus quarry; last
upper molariform tooth, USNM 13568, co-ossified trapezium and
first metacarpal, USNM 13570; and two fourth metacarpals, USNM
12675; a claw and several toe bones, from "various localities south
of the horse quarry." Two additional lots of associated specimens
Elmer Cook and George Pearce collected about 5 miles south of

Table 7. MEASUREMENTS OF ASSOCIATED RIGHT HIND LIMB ELEMENTS OF
Megalonyx FROM HAGERMAN, IDAHO, (USNM 23209) IN MM

Fibula
Estimated length 280.0
Anteroposterior diameter at middle of shaft 33.5
Transverse diameter at middle of shaft 26.5
Anteroposterior diameter of distal end 63.3
Max. transverse diameter of distal end 63.8
Calcaneum
Max. length 178.0
Max. diameter of tuber calcis 171.0
Max. thickness posterior border 30.0
Width of neck 28.3
Thickness of neck 48.0
Distance from inner border of external astragalar facet
to inferoexternal prominence of articulating end 71.3
Distance from dorsal border of external astragalar facet
to ventral border of cuboid facet 85.4
Astragalus
Max. anteroposterior diameter parallel to fibular facet 92.0
Max. transverse diameter measured at right angles to fibular facet 66.2
Depth of fibular facet at distal end 44.1
Max. transverse diameter of head 49.9
Anteroposterior length of external calcaneal facet 63.2
Cuboid
Max. diameter from astragalar to cuneiform surface 43.9
Max. diameter from calcaneal to dorsal surface 32.4
Max. diameter from medial to lateral surface 43.5
Navicular
Max. diameter across cuneiform facets (width) 60.1
Diameter taken at right angles to greatest (dorsoplantar) diameter 53.0
Max. thickness through convexity articulating with astragalus 27.6


232


Vol. 12





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 233

Cunciform
Max. dorsoplantar diameter 49.9
Max. mediolateral diameter 31.8
Max. thickness through navicular to mtt III surface 18.2
M\, I/,/,I/. ali II
Max. length 35.0
Breadth of shaft 37.8
Depth of proximal end 39.9
Depth of distal articulation 29.7
Metatarsal III
Anteroposterior diameter through middle 51.3
Depth of median vertical convexity at distal end 66.6
Depth of proximal articular face 58.8
Max. width 47.6
Metatarsal IV
Anteroposterior diameter through middle 57.9
Depth of median vertical convexity at distal end 50.1
Depth of proximal articular face 51.9
Max. width 41.0
Metatarsal V
Anteroposterior diameter 51.1
Depth of major convexity at distal end 24.1
Depth of proximal end 25.3
Max. width 21.1
Phalanx I of Digit III of Pes
Max. anteroposterior length 583
Max. width 49.3
Depth of proximal end 54.6
Height of distal condyles 45.8
Phalanx II of Digit III of Pes
Max. anteroposterior length 45.9
Width of proximal end 45.5
Depth of proximal end 36.7
Width of distal condyles at middle 31.5
Depth of distal condyles 38.3
Phalanx I of Digit IV of Pes
Max. anteroposterior length 28.9
Max. width 38.3
Depth of proximal end 36.7
Height of distal condyles 30.2
Phalanx II of Digit IV of Pes
Max. anteroposterior length 62.3
Width of proximal end 34.0
Depth of proximal end 37.1
Width of distal condyles at middle 23.2
Depth of distal condyles 29.1
Phalanges I and II of Digit II of Pes
Max. anteroposterior length 37.5
Width of proximal end 21.0
Depth of proximal end 28.9
Width of distal condyles 17.9
Depth of distal condyles 17.0





BULLETIN FLORIDA STATE MUSEUM


Plesippus Quarry, near Iagerman, Idaho, contain: 10 podial frag-
ments, USNM 23208 and a right fibula, calcaneum, astragalus, cub-
oid, navicular, cuneiform and four metatarsals, three sesamoids and
many of the phalanges of the three lateral digits, USNM 23209.
The unfuscd condition of phalanges I and II in the third pedal
digit has been noted above. Phalanges I and II of the second pedal
digit are fused. The measurements are presented in Table 7 and
the foot is illustrated in figure 5.

PROCTER PITS, TEXAS. A Megalonyx palate with the right first
molariform, left caniniform, and alveoli for the remaining upper
teeth, FAM 77811, was collected in Proctor Pit C in the Texas Pan-
handle. The widest dimension of the palate is 86.6 mm across the


40MM


FIGURE 5. Left pes of Megalmoyx
dorsal view.


from Ilagcrman, Idaho, USNM 23209,


234


Vol. 12


i
'.~ cs,


~





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


caniniforms. The palate is constricted posterior to the caniniforms
to a width of 37.2 mm. The width across the posterior molariforms
is 58.0 mm; between them it is 16.8. The posterior narial opening
lies opposite the last molariforms. The caniniform measures 29.8
mm long by 16.6 wide and has the lingual convexity characteristic
of advanced Mcgaloiy.x. M1 measures 17.2 by 14.0 mm. The cheek
tooth row is 65.5 mm long.
In anterior view the rostrum presents an elliptical narial open-
ing with a maximum width of about 38 just dorsal to the palate
and narrowing to about 20 between the lateral edges of the nasal
bones. The tips of the nasals stand 8 behind the anterior edges of
the canines. The anterior edge of the jugal lies immediately pos-
terior to the caniniform. The jugal plate slopes ventrad, posteriad,
and slightly laterad. The posterior edge lies posterolateral to M2.
SANTE FE I, FLORIDA. A medial phalanx of the third pedal
digit, UF 10441, was collected by D. Webb, R. Allen, and J. Rob-
ertson in June, 1965 from locality I, Sante Fe River, Gilchrist
County, Florida. Its length is 64.9 mm, its width 32.1, and its depth
at the proximal end 46.5. The significance of this specimen lies in
the fact that it is unfused to the proximal phalanx as in other Blan-
can M'egaliyonx.
JACKASS BUTTE, IDAHO. An isolated intermediate lower molari-
form tooth, UO 2404, from locality 6, Grandview Fauna, Jackass
Butte, Idaho measures 17.9 by 12.4 mm at the occlusal surface.

AIcgalonyx curvidens Matthew
Megalonyx curvidens Matthew, W. D. 1924. Third Contribution to the Snake
Creek Fauna. American Mus. Nat. Hist. Bull., 50; 149.
TYPE.- Tooth, probably RM3, AM 17601, from upper level,
Quarry 1. Upper Snake Creek Beds, Sioux Co., Nebraska, collected
by Albert Thomson in 1918.
REFERRED MATERIAL. Left ramus and symphysis with canini-
forms and left cheek teeth, FAM 77800, collected by M. F. Skinner
in East Pliohippus Draw about 10 feet above the site from which
the type of Plioliippus leidyanus came along with rhinoceros, mas-
todon, horse, camel, and peccary material (Skinner field notes, vol.
2, p. 44); two isolated upper cheek teeth, AM 20493 and 21460;
humerus, FAM 77801, from East Pliohippus Draw; navicular, PU
12079 (Sinclair, 1915); two proximal phalanges of the third manual
digit, AM 81034 and 85970, the latter collected by the late Harold


235




236 BULLETIN FLORIDA STATE MUSEUM Vol. 12

Cook 6 September 1949 while walking with Glen Evans and Gray-
son Meade, in high exposures about 100 yards east of the Pliolip-
pus leidyanus site (Cook's notes); second phalanx of third manual
digit FAM 77802; and an ungual phalanx, AM 14051.
DIAGNOSIs. Caniniforms long ovate with concave lingual sides;
symphysis protruding relatively farther and more rounded at an-
terior tip than in M. leptostomus. Alveolar ridge posterior to the
caniniform teeth same width as adjacent cheek teeth. Molariform
teeth slightly more curved than in later species.
DESCRIPTION. The mandible, FAM 77800, (Fig. 6) belongs to
a young animal as indicated by the tapering teeth. This is most
evident in the caniniforms, the occlusal dimensions being 14.8 by
8.0 in contrast to basal dimensions of 21.5 by 11.1.




20mm

























FIGURE 6. Mandible of Megalonyx curvidens, FAM 77800, occlusal and lat
eral views.





1968 IIIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


The caniniform teeth present a long subcircular occlusal surface
that wears nearly flat, much as in typical Megalonyx. The canini-
forms differ in having faintly concave lingual sides, whereas canini-
forms of M. jeffersoni show a pronounced lingual bulge. The absence
of this bulge also has been noted above in some M. leptostomus
specimens.
The symphysis protrudes somewhat more than in late Pleistocene
MeA g,(lonyx. specimens, reaching 14.3 anterior to a line between the
anterior ends of the caniniforms. But as the canines erupt and
reach mature proportions the protrusion presumably decreases ac-
cordingly. The keel on the anterior slope of the symphysis is weak
and confined to the upper third of the slope. The anterior border
of the symphysis rises in a nearly straight profile, in contrast to the
sigmoid curve, concave at the base, convex at the top, typical of
later species of Megalonyx. The mental foramen appears 10 mm
posterior to the "chin" and 9 mm below the alveolar border, about
as in typical Megalonyx. The symphysis attains a depth of about
42 mm below the caniniform teeth. The posterior end of the sym-
physis lies between the roots of the caniniforms as in other Mega-
lonyx.
The alveolar ridge posterior to the caniniform retains the same
width as the adjacent cheek tooth. In this respect M. curcidens
differs from typical late Pleistocene Megalonyx specimens and
agrees with Meade's (1945) description of a mandible of M. leptos-
tomus from Mt. Blanco, Texas.
The cheek teeth closely approximate those of later Megalonyx
species in shape and the development of wear facets. The principal
wear is on the posterior edge of each tooth.
The last molar in the mandible agrees closely with Matthew's
type tooth, except that the type is slightly larger and comes from
the right side. Measurements of the base of left M3 are 16.9 by
13.4 mm as compared with 17.8 and 15.0 in the type. These teeth
are slightly narrower on the labial than on the lingual side. A
concavity passes down the posterior side and a faint one down the
labial side. The highest corners are the posterolabial and antero-
lingual. Each tooth curves gently along its length producing a
slight arch posteriorly and a lesser one lingually. This feature of
the type tooth inspired the species name and distinguishes it from
late Pleistocene Megalonyx.
The upper half of the ascending ramus is missing. A large fora-


237





238 BULLETIN FLORIDA STATE MUSEUM Vol. 12

men through which the mandibular nerve and blood vessels passed
occupies the base of the coronoid process, but somewhat higher
than in most Megalonyx specimens.
An upper molariform tooth, AM 21460, measures 14.0 mm an-
teroposteriorly and 19.8 transversely. It is slightly concave on the
posterior and convex on the anterior side. Another such tooth, AM
20493, measures 13.7 by 19.0 mm.
The left humerus from the Snake Creek channels is an incom-
plete distal half of the shaft. The deltoid and pectoral crests are
well developed, being especially well defined at their junction near
the midpoint of the shaft. Between this point and the distal condv-
lar expansion the narrowest width of the shaft is 44.2.
Sinclair (1915) described a navicular from the Upper Snake
Creek channels, P.U. 12079. Measurements taken from the illus-
tration of the Snake Creek specimen show its natural size to be
about 52 mm across the cuneiform facets and 46 mm in the dorso-
palmar diameter. The Upper Snake Creek navicular is similar to
that of Pliometanastes in that the mediolateral diameter is greater
than the dorsopalmar diameter and unlike Megalonyx jeffersoni, M.
sierrensis, and the Idaho specimen where the opposite occurs.
The Snake Creek navicular differs from Pliometanastes but is sim-
ilar to the navicular of egahlonyx in having the mesocuneiform
facet elevated above the ectocuneiform facet.

Table 8. MEASUREMENTS OF PHALANGES OF Megalonyx curvidens IN MM

Proximal Phalanges (AM 81034) (85970)
Prox. Length 53.3 53.1
Prox. Width 42.9 46.8
Max. Depth 31.8 34.2
Dist. Length 43.1 43.5
Dist. Width 32.1 30.2
Second Phalanx (FAM 77802)
Max. Length 62.2
Median Length 55.3
Prox. Width ca. 39
Dist. Width 32.4


The phalanges from the Upper Snake Creek channels closely
correspond to specimens of later Mcgalon v., except for their gen-
erally smaller size. Their principal dimensions are presented in'
Table 8.




239


1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


Megalonyx mathisi new species

TYPE. Ventral half of skull, UCMP 80416 (Figs. 7 and 8).
TYPE HORIZON AND LOCALITY. Black Rascal Creek, V-67223,
Upper Mehrten Formation, east of Merced, Merced County, Cali-
fornia. Hemphillian, probably late Hemphillian, tentatively corre-
lated with the Pinole Tuff site, Contra Costa County, California,
which has a potassium-argon date of approximately 5.2 million
years.
ETYMOLOGY. Patronymic for Glen E. Mathis whose generous
assistance made collecting the type specimen possible.
DIAGNOSIS. A relatively small animal, slightly smaller than Mega-


FIGURE 7. Skull of Megalonyx mathisi, UC 80416, lateral and palatal views.





BULLETIN FLORIDA STATE MUSEUM


lonyx leptostomus but larger than Pliometanastes protistus. Caniniform
smaller than first molariform, oval in cross section with great-
est diameter obliquely anteroexternal to posterointernal, unlike all
other species of Megalonyx in which the greatest diameter is ob-
liquely anterointernal to posterointernal. Caniniform without me-
dian bulge, lingual side of tooth relatively flatter than labial side.
Maxillary plate between caniniform teeth wide and very thin.
Palate distinctly arched, concave posterior to caniniform teeth, be-


S S


C


C
cf


FIGURE 8. Basicranial region of skull of Megalonyx mathisi, UC 80416. Ab-
breviations: cf, condyloid foramen; ent, entotympanic bone; er, depression
for cpitympanic ring; est, groove for eustachian tube; f, facial foramen; fo,
foramen ovale; fr, foramen rotundum; mf, mastoid foramen; mlf, median
lacerate foramen; mps, mastoid process of squamosal; o, fenestra ovalis; p,
pctrosal; plf, posterior lacerate foramen; o, fenestra ovalis; p, petrosal; plf,
posterior lacerate foramen; r fenestra rotunda; saf, canal for inferior ramus of
stapedial artery; smf, stylomastoid foramen; stp, stylohyal pit; th, tympanohyal;
x, unnamed foramen posterior to stylomastoid foramen.


240


Vol. 12





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 241

coming convex at level of M2. Palate with long narrow canals for
nutritive foramina. Nasal region not as highly vaulted as in other
species. Sagittal crest present and supraoccipital crest prominent.
Rectus capitus ventralis excavations posteromedial to basilar tuber-
cles. Occipital condyles relatively closer together and do not pro-
ject so far posteriorly as in M. leptostomus. Dorsal margin of the
condyles nearly confluent with the occiput.
SKULL. The skull, UCMP 80416, lacks the nasals, dorsal part
of the frontals and parietals, expanded portion of the jugal, ventral
part of the pterygoids, ectotympanic, and all teeth. All sutures are
firmly fused indicating a fully adult individual. The skull is slightly
smaller than hIcgalonyx leptostomus but larger than Pliometanastes
protistus.
The most striking differences between M. mathisi and the other

Table 9. MEASUREMENTS OF Megalonyx Mathisi SKULL, (UCMP 80416) IN MM

Max. length 286.0
Length posterior end occipital condyles to posterior
border caniniform alveoli 263.0
Width maxillary at level of caniniforms 92.7
Width palate anterior to first molariforms 45.8
Min. width of palate between molariforms 21.3
Width across zygomatic arches anterior end glenoid fossa 152.0
Max. width across mastoid processes 122.8
Min. postzygomatic width across squamosal 104.0
Transverse diameter inside foramen magnum 23.6
Dorsoventral diameter foramen magnum 23.2
Max. width across occipital condyles 66.0
Max. width across one condyle 22.8
Max. depth of one condyle 28.0
Height occiput from dorsal margin foramen magnum 51.01
Height maxillary, anterior alveolar border to nasal suture 50.0
Width between premaxillary sutures 56.5
alveoli2 left right
Max. length caniniform alveolus 18.5 18.5
Max. width caniniform alveolus 12.0 11.3
Max. anteroposterior alveolar diameter M1 16.5 16.0
Max. mediolateral alveolar diameter M' 21.0 21.5
Max. anteroposterior alveolar diameter M2 17.0 17.5
Max. mediolateral alveolar diameter M' 24.0 24.5
Max. anteroposterior alveolar diameter M' 17.0 17.5
Max. mediolateral alveolar diameter M3 24.0 23.0
Max. transverse alveolar diameter M4 16.5 17.0
1 Estimated
2 Measurements of molariforms taken just below level of palate where alveolar walls are straight.
These measurements do not take into account distortion due to preservation and reconstruction and
thus may be as much as 1 mm rrore than true alveolar diameter.





BULLETIN FLORIDA STATE MUSEUM


species of Megalonyx are found in the anterior region of the skull.
The nasal bones are lacking, but from the curvature of the maxil-
laries apparently the nasal region was not so highly vaulted as in the
other species. The maxillary between the caniniform teeth is a thin
broad plate in M. mathisi, whereas in M. leptostomus it is consid-
erably thicker, attaining its greatest thickness in M. jeffersoni. This
difference in thickness of the maxillary is greater than would be ex-
pected if it were due solely to allometric increase in skull size.
Medial to the caninform alveoli, on the antero-internal side, is a pair
of depressions for the premaxillary. The distances between these
articular surfaces is relatively farther apart in M. mathisi than in
other species.
The size, shape, and orientation of the caniniform teeth distin-
guishes M. mathisi from all later species of the genus. The canini-
form alveoli are smaller than the alveoli of the M1 and are distinctly
oval, the internal margin being straighter and less convex than the
external to posterointernal, in marked contrast to other species
where the greatest diameter is anterointernal to posteroexternal.
The posterior margin of the tooth projects medially in M. mathisi,
whereas in other species it projects laterally. The size and cross sec-
tion of the caniniform alveolus are structurally intermediate between
the small triangular tooth of Pliometanastes and the typical long
ovate tooth of lMcgalonyx leptostomus.
The molariform alveoli are subtriangular and do not differ in
shape from the later species of the genus.
The palate is distinctly arched, concave immediately posterior to
the caniniform teeth, becoming convex at the level of the M2. M.
mathisi resembles M. jeffersoni in this character and differs from M.
leptostomus in which the palate is straight along its length. The
palate contains long narrow canals for nutritive foramina. These run
the length of the palate as in Pliometanastes and they are especially
prominent in the anterior part of the palate, just posterior to the
caniniform teeth.
There is some distortion and displacement in the dorsal part of
the occiput, but not enough to obscure the major characters. The
supraoccipital crest is strongly developed, as is the vertical occipital
crest. The occiput rises nearly perpendicular to the basi-cranial
axis. The occipital condyles do not project so far from the occiput
as in other species and are nearly confluent with the occiput along


Vol. 12






1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 243

their dorsal margin. The occiput crest projects outward posteriorly
above the condyles.
Compared with Megalonyx leptostomus: The mastoid process is
not quite so prominent and is somewhat pointed; the area of attach-
ment of the digastric muscles dorsal to the mastoid process is more
rugose and more deeply excavated; the occipital condyles are closer
together and the foramen magnum more rounded; the bone of the
dorsal margin of the foramen magnum is considerably thicker and
has a prominent pair of processes projecting posteriorly on either
side of the midline and articulating with the atlas; the occipital con-
dyles are more elongate medio-laterally; and the triangular notch so
prominent in M. leptostomus is barely evident in M. mathisi.
The basicranial region is remarkably well preserved; only the
ectotympanic ring is lacking. In ventral view the occipital condyles
are positioned closer to the skull and are relatively closer together
in M. mathisi than in M. leptostomus. The condyloid foramen opens
anterior to the condyles and postermedially to the posterior lacerate
foramen, the largest of the basicranial foramina. The stylohal process
borders the lateral edge of the posterior lacerate foramen and ap-
pears to be formed by the paroccipital process, tympanohyal, and a
process of the mastoid region, which together form an articulating
surface for the stylohyal. Anterolateral to the tympanohyal and post-
erlateral to the epitympanic recess is an oval depression for the epi-
tympanic ring. The entotympanic forms the anterior margin of the
posterior lacerate foramen, sutured to the paroccipital process at the
anterior margin of the stylohyal process on the lateral side and the
basioccipital on the medial side, lying against the lateral margin of
the basilar tubercles. It is widest posteriorly, narrowing anteriorly
where it meets the pterygoid and lateral margin of the median lacer-
ate foramen. Dorsal to the entotympanic is the petrosal which lies in
the epitympanic recess. The fenestra ovalis opens from the dorsal
portion of the petrosal, with the opening directed laterally. Postero-
ventral to the fenestra ovalis is the fenestra rotunda which opens
posterolaterally.
Some confusion exists in the literature concerning the designa-
tion of the stylomastoid-mastoid foramina. The skull of Megalonyx
mathisi has two foramina at the lateral base of the stylohyal process
and a dorsal foramen in the mastoid process on the posterior side.
The anterior foramen lies immediately posterior to the oval depres-
sion for the epitympanic ring, lateral to the tympanohyal, and at the





BULLETIN FLORIDA STATE MUSEUM


base of the stylohyal process. This foramen, the stylomastoid fora-
men, communicates via the sulcus facialis with the facial foramen,
which is located antero-dorsal to the fenestra ovalis. The canal of the
foramen immediately posterior to the stylomastoid bends posterodor-
sally into the mastoid region. This latter foramen and the mastoid
foramen on the occiput probably function as openings for venous
drainage leaving the skull. The latter foramina are highly variable,
as is demonstrated by the skull of M. mathisi in which the mastoid
foramen is lacking on the right side of the skull. This variability is
further indicative of venous circulation.
In the anterior part of the ear region, the eustachian tube lies in
a very shallow and poorly defined groove that begins at anterolateral
margin of the entotympanic and continues forward obliquely across
the pterygoid towards the midline of the skull. Lateral to this groove
are two structures of uncertain function. Anteroventral to the epi-
tympanic recess is a dorsoventrally aligned furrow that may have
served to house the anterior portion of the epitympanic ring. Medial
to this furrow a canal runs within the squamosal but close to the
pterygoid from the anterodorsal base of the petrosal to a foramen
opening posterior to the foramen ovale. This prominent foramen may
represent an enclosure of the inferior ramus of the stapedial artery.
The foramen ovale, foramen rotundum, and orbital fissure are in the
same position as in other species.
The basilar tubercles are excavated internally along the posterior
and medial sides. Anterior to the basilar tubercles is a pair of deep
sinuses in the basisphenoid, ventral to the anterior portion of the
brain cavity.
The zygomatic process of the squamosal narrows anterior to the
glenoid fossa and is mediolaterally compressed. The glenoid fossa is
more concave and its anterior side lies more dorsal than in M.
leptostomus. No sutural connection is apparent between the zygo-
matic process of the squamosal and the jugal as in M. leidyi. The
articular surface for the mandibular condyle in M. mathisi appears
to be restricted to the lateral margin of the glenoid fossa where the
zygomatic process flares laterally, whereas in M. leptostomus the
articular surface appears to have extended medially almost to the
lateral margin of the pterygoid.
Other Hemphillian Megalonyx
In addition to the above described material, a number of speci-
mens have been discovered that do not warrant specific taxonomic


244


Vol. 12






1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


assignments. They are arranged below according to geographic
origin.
BEAR TOOTH SLIDE, NEBRASKA A lower caniniform, FAM 77803,
M. F. Skinner collected in the same channel at Bear Tooth Slide,
middle fork of Deep Creek, in the middle of the Ash Hollow Forma-
tion, Brown Co., Nebraska, has a long oval cross section with a flat,
slightly convex, lingual border. This border is not concave as in M.
curvidens, but it is far less convex than typical late Pleistocene
A3,l galiiyx. This tooth measures 22.0 x 13.8 mm.
COGSWELL QUARRY, KANSAS FAM 77805 represents an imma-
ture stage of development in an upper caniniform of Ihcgalonyx.
The upper surface is worn flat and measures only 14.1 by 9.3 mm.
It expands rapidly toward the root, but is broken off 25 mm above
the crown. At that point its dimensions are 15.9 by 10.4 mm. In cross
section its shape approaches the long ovate form characteristic of
Mylon-hmyr. A strong lingual convexity exactly resembles that ob-
served in caniniforms of typical Pleistocene Mlcgalonyx.
OPTIMA, OKLAHOMA Savage (1941) described an upper and
a lower caniniform tooth from the Optima Fauna. The lower, OMP
41-26-S1, has an occlusal length of 24.8 mm. This specimen is from a
mature individual as the tooth walls are parallel. A faint bulge on
the lingual side suggests the more massive-bulge of later species of
AMlilnyr.. The upper caniniform, OMP 41-BE-S2 is 19 mm long
and also from a mature sloth. Its strong lingual bulge is postero-
lingual rather than mediolingual as in most later Megalonyx speci-
mens.
PINOLE, CALIFORNIA Stirton (1939) described an upper or
lower first molariform tooth, UCMP 22110. Its transverse diameter
is 17.0 mm and its length 12.3. It agrees closely in size and wear
pattern with a tooth of M. leptostomus from Cita Canyon, Texas
(WT 2548) described above.
WESTEND BLOWOUT, OREGON Two astragali from Hemphillian
sites in Oregon (Shotwell, 1958) pertain to Alegal'( onx. UO 6332 is
an incomplete specimen but retains enough of the tibial and navicu-
lar surfaces to reveal features characteristic of Alegaloiyv, most
conspicuously the deep tibial notch.
KREBS RANCH, OREGON UO 8080, the second Oregon astraga-
lus, is complete. It differs from astragali of Pliometanastes and re-
sembles those of Megalonyx in its more nearly equal medial and


245





BULLETIN FLORIDA STATE MUSEUM


lateral sides, less rectangular and more convex internal calcaneal
facet, and deeper navicular facet. It also differs from P. protistus
in its greater size (anteroposterior diameter 75.7 mm, transverse
diameter 64.6, transverse diameter of head 44.1, length of external
calcaneal facet 61.6).

Pliometanastes new genus
GENOTYPE: Pliometanastes protistus. As only a single species
is certainly attributed to this genus, the characterization of the genus
is identical to that of the species below.
ETYMOLOGY: Greek; Plio = pliocene, meta = change, nastes
= occupants; thus referring to a Pliocene wanderer.

Pliometanastes protistus new species
TYPE: UF 9479, a partial skull, including the posterior two-
thirds of the cranium (from the postorbital constriction posteriorly)
as well as the left lacrimal, jugal, lateral portion of the maxillary
containing the alveoli of M1 M3, the right maxillary containing the
alveoli of M1 M2, the right portion of the palate containing the
alveoli of M2 M3.
ETYMOLOGY: Greek protistos = the first.
REFERRED MATERIAL: Anterior skull fragments, UF 10337,
10338, and UF 9561; posterior skull fragment, UF 13211; anterior
portion of a right mandible, UF 9450; posterior portion of a left
mandible, UF 9480; left superior second of third molariform, UF
9449; left superior fourth molariform UF 9613, and UF 10341; tho-
racic vertebrae, UF 9464 and UF 9468; lumbar vertebrae, UF 9465,
UF 9466 and UF 9467; caudal vertebrae, UF 10342; fragmentary
left humerus, UF 9445; proximal half of ulna, UF 9447; right meta-
carpal II, UF 9453; left metacarpal II,UF 9454; associated right
metacarpal II, UF 11524; right metacarpal III, UF 11525; right
metacarpal IV, UF 11526, and right metacarpal V, UF 11527; patella,
UF 9774; right fibula, UF 9446; right navicular, UF 9474; right cal-
caneum, UF 9437; left astragalus, UF 9440; phalanx I of digit III
of the pes, UF 9459; ungual phalanges, UF 9640, UF 9461, UF 9462,
and UF 10340.
TYPE HORIZON AND LOCALITY: McGehee Farm, about 3 miles
north of Newberry, Alachua County, Florida. Alachua Formation,
Hemphillian (Middle Pliocene).


Vol. 12





HIRSCHFELD/WEBB: PLIO-PLE1STOCENE SLOTHS


For many years diverse nonmarine sediments occurring as sink-
hole, Qca\e, and fissure fillings, and as isolated accumulations in vari-
ous depwssions in the Ocala Limestone, have been assigned to the
Alachual Formation (Simpson, 1930; Cooke, 1945; Pirkle, 1956; and
Webbi i:f64). The lithologies of the sediments assigned to the Ala-
chua Formation are quite heterogeneous, ranging from sinkhole
fillings of reddish sandy clay, blue-gray in unweathered condition,
to gra', phosphatic sands in larger basins with commercially signifi-
cant /z nes of hard-rock phosphate.
Th type locality of the Alachua fauna and Alachua Formation is
Mixoi, Bone Bed northeast of Williston, Levy County. This locality
has \i'. lted a vertebrate fauna of Hemphillian age. Vertebrate fossils
four in other Alachua sediments range in age from early Miocene
thru.. b Recent. Typically these fossil collections are not mixed, but
occur .s discrete faunas in historically distinct sinkhole fillings and
solution deposits.
T': vertebrate-bearing deposits at McGehee lie unconformablv
upon ;in irregular surface of Ocala Limestone of Late Eocene age.
Two main strata can be distinguished: a lower clayey sand lying
diree,'!, upon the limestone, and above this a coarse, poorly sorted
and v cakly stratified gravelly sand. The greater part of the sloth ma-
terial lias come from the lower clayey sand layer, as has most of the
asso i.;"Ied vertebrate fauna. The upper gravelly layer contains shark
teeth ray crusher plates, gar and drum scales, turtle shells, large
amoi,,ns of pebble phosphate, chunks of Ocala Limestone and black-
ened .1 aterworn bones and teeth. Much postdepositional solution of
the !i,:estone has occurred, and in many places the deposits have
drop. ,1 10 feet or more into large solution pipes.
Mc1 McGehee Farm deposits accumulated in an estuary in a
coast:;i solution valley when the sea stood approximately 90-100 feet
above, its present level (Webb, 1964). Vertebrate hard parts accumu-
lated ; solution pockets in the estuary.
Of the 14 mammals from McGehee Farm (Table 10 and Fig. 9)
that lo ,,e been identified to genus, 12 (Mylagaulus, Osteoborus, Ser-
ridentiul"s, Tapiravus, Teleoceras, Aphelops, Calipp)l., Nannippus,
Neopi ,pparion, Pliohippus or Dinui (ippjus, Prosthennops, and Mega-
tylopuo are known to occur in both the Clarendonian and Hemp-
hillia. Tanupolanma is known from Hemphillian faunas but is more
typical ,e the Blancan and later. The species of Ncohipparion from
Mc`_.- (near N. eurystyle) and the species of Natunippu's (near





BULLETIN FLORIDA STATE MUSEUM


Table 10. MCGEHEE FARM FAUNAL LIST
CHONDR1CHTHYES1
Odontaspis cuspidata (Agassiz)
0. macrota (Agassiz)
Isurus hastalis (Agassiz)
Carcharodon megalodon Agassiz
Hemipristis serra Agassiz
Carcharhinus cf. leucas (Muller and Henlc)
C. limbatus group
Galeocerdo cuvieri (Peron and Le Sueur)
G. aduncus Agassiz
Rhizoprionodon sp.
Negaprion brevirostris (Poey)
Pristidae
Dasyatidae
Myliobatidae
OSTEICHTHYES
Lepisosteus
Elopidael
Ariidae1
Sciaenidael
AMPHIBIA
Siren sp.
REPTILIA
Macroclemmys sp.
Trionyx aff. T. ferox (Schneider)
Chrysemys williamsi Weaver and Rose1
C. carri Weaver and Rose'
Geochelone alleni Auffenberg
Geochelone aff. G. hayi (Sellards)
Gopherus sp.
Gavialosuchus americanus
Alligator cf. A. mississippiensis
Ophisaurus sp.
Farancia sp.
Crotalidae
AVES
Phalacrocorax wetmorei Brodkorb
Nycticorax fidens Brodkorb
Ereunetes rayi Brodkorb
MAMMALIA
Talpidae
Mylagaulus kinseyi Webb'
Lagomorpha
Pliometanastes protistus n. gen. et sp.'
Osteoborus spp.
Mustelidac'
Felidae'
Serridentinus floridanus (Leidy)


248


Vol. 12





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 249

Tapiravus sp.
Teleoceras proterus (Leidy)
Aphelops longipes (Leidy)
Calippus sp.
Dinohippus or Pliohippus
Nannippus nr. N. ingenuus (Leidy)1
Neohipparion nr. N. eurystyle (Cope)l
Hipparion pli, urilr Leidy1
Prosthennops sp.
Protoceratidae
Tanupolama sp.
-I 1 0_'i11/, l i p sp.
_\ i, ll ,tI' .p ri' l, e
SAdditions to preliminary list in Webb (1964).
Megalonychidae Tanupolama
BLANCAN






HEMPHILLIAN





Osteoborus
Osoborus Prosthennops
CLARENDONIAN Serridentinus Nn Aph
Neohipparion Aphelops
Protoceratidae
Pliohippus Topirovus
Teleoceras
Mylogoulus Calippus Nannippus Megatylopus

FIGURE 9. Stratigraphic ranges of mammalian genera from McGehee Farm.

N. ilngclnut.s) are both characteristically Hemphillian species. No im-
migrants from Asia such as Indarctos and Plesiogulo, valuable cri-
teria for defining many Hemphillian faunas, have been found at
McGehee. No sloths, here represented by Pliometanastes, have yet
been reported from pre-Hemphillian localities in North America.
Protoceratidae previously were not known in post-Clarendonian
deposits. Taking all of these facts into consideration, the McGehee
. in,'1- ld>g.- is ;iignled an early Hemphillian age.

DIAGNOSIS: Dental formula 5/4. A relatively small animal, the
next larger North American megalonychid being the Hemphillian






BULLETIN FLORIDA STATE MUSEUM


Megalonyx mathisi (new species). Mandible with long, deep, and
relatively wide predental spout; posterior end of symphysis below
caniniform tooth; caniniform nearly vertical in orientation, nearly
circular in cross section, but flattened lingually. Dorsal ridges not
united to form a sagittal crest and diverging just anterior to the ex-
ternal occipital protuberance. A pair of depressions present on either
side of the median ridge on the occiput at the external occipital pro-
tuberance. M. rectus capitus ventralis excavations lateral rather than
posteromedial to the basilar tubercles. Dorsal border of occipital
condyles nearly confluent with the occiput. Mastoid foramen at dor-
sal margin of mastoid region. Palate with long, narrow canals for
nutritive foramina. Shaft of ulna straight on anterior side. Meta-
carpal II with prominent dorsoexternal process of proximal end.
Patella proportionally longer than in Ilapalops or c'iLah,'iy. Fibu-
lar shaft not curved dorsomedially to the distal end in the region of
attachment of the peroneus brevis muscle as in Achgalonei\. Tuber
calcis and its neck intermediate in development between M3l.glonyx
and Hapalops. Astragalus lacking V-shaped tibial notch. Metatarsals
slender with restricted muscle scars.
SKULL. The skull of Pliometanastes is known from a number of
fragments. The most complete specimen, UF 9479, consists of the
posterior two-thirds of a cranium (from the postorbital constriction
back), as well as the left lacrimal, jugal, and lateral portion of the
maxillary containing the alveoli of M1 M3, the right maxillary con-
taining the M1 M2, the right portion of the palate containing the
alveoli of M2 M3, and a fragment that may represent the right dor-
sal projection of the jugal. No teeth are associated with this skull.
Isolated skull fragments include four specimens UF 9561, 10337,
10338, and 13211, two of which appear to belong to the same indi-
vidual. These specimens contain the lacrimal, the jugal, and the an-
terior portion of the maxillary with the alveoli of M1 M2, and the
base of the caniniform alveolus. The other skulls, UF 9479 and 13211
appear to represent adults as all but the squamosal-supraoccipital-
exoccipital sutures are obliterated.
The skull of Pliometanastes is about half the size of the skull of
MctialtoUi\" jeffersoni and is considerably smaller than that of M.
leptostomus (Figs. 10 through 13).
A prominent ridge running from the dorsal margin of the fora-
men magnum to the external occipital protuberance has a depression
on each side just ventral to the supraoccipital crest similar to the


250


Vol. 12






1968 HIIRSCIIFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


20mm
20mm


FImUiE 10. Skull of Pliometanastes protistus, UF 9479, dorsal view.

condition in Eucholoeops front (Scott, 1903-1904, pl. 55, fig. lb).
Aeg.alonyx lacks these depressions. In Pliometanastes the dorsal oc-
cipital depressions are bordered laterally by a pair of semicircular
tuberosities that merge dorsally with the supraoccipital crest. Lateral
to these tuberosities and medial to the suture of the exoccipital
l)one is a smaller and more pointed pair, each bearing a ventrally
projecting ridge of bone. A deep groove lies on the suture between
the supraoccipital and squamosal bones.








BULLETIN FLORIDA STATE MUSEUM


it


Vol. 12


20 r_
20 rrrn


FIcURE 11. Skull of Pliometanastes protistus, UF 9479,


252


-ventral view.







1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


FIGURE 12. Basicranial region of skull of Pliometanastes protistus, UF 9479.
Abbreviations as in figure 8, and alc, anterior lacerate canal; alf, anterior
lacerate foramen; s, sinus of basisphenoid bone.

Considerable variation in this region of the occiput has been ob-
served in three specimens of M. jeffersoni: A cast of Owen's speci-
men preserved in the Florida collections, Dickeson's specimen and
a juvenile from Big Bone Cave, Tennessee, both in the collections
at the ANSP. The Owen skull has a depression in the region of the
squamosal-supraoccipital suture, the squamosal bone being depressed
below the level of the supraoccipital bone; just medial to the suture





254 BULLETIN FLORIDA STATE MUSEUM Vol. 12

e-

I c















-'< 20 mm

FIGURE 13. Skull of Pliometanastes protistus, UF 9479, occipital view.

is a relatively flat space, concave at its dorsal extremity, about 15
wide; medial to this a large tuberosity about 13 wide projects ven-
trally from the transverse area of muscle attachment on the occiput.
An entirely different situation exists in the Dickeson skull: Medial
to the squamosal-supraoccipital suture, which is deeply indented, a
projection of bone about 18 wide projects laterally over the suture
at its dorsal end. Medial to this projection is an excavated area with
the rest of the occiput rather smooth. The occiput of the juvenile
from Big Bone Cave is quite similar to that of M. leptostomus from
Cita Canyon, Texas. In the region of the suture the bony protuber-
ances, prominent in the Owen and Dickeson specimens, are lacking.
In overall shape the occipital region in Plionetanastes most
closely.resembles that in M. leptostomus. The mastoid process is not
hooklike as in M. jeffersoni, but is rounded as in M. leptostomus.
The occipital condyles are nearly confluent with the back of the
skull as in Eucholeops fronto, whereas in M. jeffersoni and M. lep-
tostomus a depression separates the condyles from the general pos-
terior surface of the occiput. The condyles are set farther dorsally
from the basilar tubercles than in M. jeffersoni.
In Pliometanastes the excavation of the basioccipital, posterior
to the basilar tubercles, is not so deep as in Megalonyx. The most





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 255

Table 11. MEASUREMENTS OF Pliometanastes Protistus SKULL,
(UF 9479) IN MM
Height of occiput from dorsal margin of foramen magnum 36.0
Height of occiput from basioccipital 67.8
Max. width of occiput across mastoid process 108.0
Max. width across occipital condyles 60.51
Max. width of individual condyle 16.0
Max. depth of individual condyle 27.0
Transverse diameter inside foramen magnum 26.8
Dorsoventral diameter of foramen magnum 23.7
Width across the zygomatic arches at the anterior end
of the glenoid fossa 132.0
Min. orbitotemporal width 71.01
Min. postzygomatic width across squamosal 94.0
Width of basisphenoid anterior to basilar tubercles 45.0
1 Estimated.

prominent basicranial muscle scar, for the rectus capitis muscle, is on
the lateral side, whereas in Megalonyx it is on the medial side of the
basilar tubercle. The area between the tubercles is nearly flat in
Pliometanastes as in E. front, whereas it is slightly concave in
M. leptostomus, and deeply concave in M. jeffersoni.
The stylomastoid foramen lies at the base of the stylohyal proc-
ess on its anterolateral side, as in all the other megalonychids ob-
served. The position of the mastoid foramen varies considerably
between individual specimens of M. jefersoni, as well as between
that species and the unique skulls of M. leptostomus and Pliometa-
nastes. In the most complete skull from McGehee, the mastoid
canal enters the squamosal bone starting at the base of the stylo-
hyal process on its posterolateral side, ascending through the mas-
toid process, passing near the posterior surface for about 16, then
turning anteriorly and entering the squamosal bone at its dorsal
extremity. The location of the mastoid foramen at the dorsal border
of the squamosal is similar to that in E. front, but the canal lead-
ing to the foramen is not enclosed in bone as in Pliometanastes.
In Pliometanastes the stylohyal pit is widest at the posterior
margin, whereas in M. jeffersoni it is widest anteriorly. The stylo-
hyal pit in Pliometanastes is not so deep as in M. effersoni. Plio-
metanastes lacks the bony protuberances surrounding the pos-
terolateral side of the foramen ovale; these are very prominent and
rounded in the skull of M. leptostomus from Cita Canyon and are
long, narrow, and less prominent in M. jeffersoni.
The median lacerate foramen lies farther anteriorly to the basi-





BULLETIN FLORIDA STATE MUSEUM


lar tubercles in Pliometanastes than in M. leptostomus and partic-
ularly M. jeffersoni, but not so far forward as in E. front. The
foramen ovale opens in the same position in relation to the glenoid
fossa in Plimnclaianstfcr as it does in both Mcgaloniyx and Eucho-
loeops. The McGehee skull is broken in the area of the opening of
the foramen rotundum, but the canal leading to it is preserved.
Although it is not possible to say positively where the foramen is
located, it appears from the position of the canal that it lay just
anterior to the glenoid fossa, ahead of its position in Megalonyx
and more nearly in the anterior position observed in Hapalops
ruetimeyeri.
Pliometanastes lacks the prominent sagittal crest seen in Mega-
lonyx, Eucholoeops, and Megalonychotherium. Instead a pair of
low ridges extend anteriorly from the supraoccipital crest a dis-
tance of 24 and then diverge. It appears that in some juvenile
specimens of Hapalops, as illustrated by Scott (1903-1904), a pair
of ridges run along the midline of the skull, but these do not
diverge in the same manner as in this adult skull of Pliometanastes.
In Pliometanastes the supraoccipital crest is low and relatively flat;
in Megalonyx it rises well above the parietals and forms a V-shaped
ridge. In the McGehee crania a deep triangular depression below
the external occipital protuberance marks the insertion of the nu-
chal ligament. This depression is not observed in M(c'alouin!l\. Hap-
alops, or Eucholoeops.
Large sinuses beneath the frontal and parietal bones extend
farther posteriorly beneath the parietal in Pliometanastes than in a
partial skull of M. cf. jeffersoni from Hornsby Springs, Florida
(UF 4045). The distance from the posterior margin of the sinus
to the supraoccipital crest is about 26 mm in the McGehee cranium,
in UF 4045 it is about 63. Furthermore the parietal sinuses in the
McGehee cranium have a lateral pair of vertical partitions in ad-
dition to a median partition, whereas Megalonyx (UF 4045) ex-
hibits only the median one. lMegloinv and Pliometanastes differ
from Nothrotherium, which lacks a parietal sinus, and from Cho-
loepus hoffmani in which the parietal sinus extends all the way
posteriorly to the occiput.
The posterior portion of the zygomatic arch arises as a ridge
just above the squamosal-supraoccipital suture. The shape of the
zygomatic arch dorsal and anterior to the glenoid fossa is similar
in the McGehee cranium to that in the M. leptostomus, but it does


Vol. 12




257


1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


not project so far laterally as in the Cita Canyon specimen. As this
part of the zygomatic arch is not preserved in the specimens of
M. jeffersoni observed, no comparison with this species can be
made. The dorsal border of the zygomatic arch curves laterally as
it projects from the skull in Pliometanastes but curves more medi-
ally in M. leptostomus and M. jeffersoni.
The sutures of the maxillary, jugal, and lacrimal bones in Plio-
metanastes lie in the same positions as in Hapalops ruetimcyeri,
the suture between the lacrimal and jugal running just posterior
to the lacrimal foramen, that between the lacrimal, jugal, and max-
illary running between the anterior opening of the infraorbital
canal and the lacrimal foramen and meeting the squamosal bone
at the dorsal side of the posterior opening of the infraorbital canal.
The jugal in Pliometanastes is directed outward and posteriorly
as in Hapalops, quite unlike that of M. leptostomus in which it
projects outward and slightly anteriorly. In M. jeffersoni the jugal
is directed downward and posteriorly very close to the skull. In
UF 9479 the posterior part of the jugal is flattened mediolaterally
opposite the third molariform tooth, at which point it twists and
widens laterally resembling the jugal in Hapalops longiceps (Scott
1903-1904, pl. 31).
A fragment believed to represent the dorsal projection of the
right jugal, associated with UF 9479, is more slender anteropos-
teriorly than in either Mlcgalon~yx or Hapalops. It is flat on the
lateral side, rounded on the medial side, and flattened at the dorsal
extremity. The overall shape of this fragment resembles the paral-
lel-sided jugal of Megalonyx and Nothrotherium and not that of
Hapalops, in which the anterior side of the projection is concave
and the posterior side convex.
The base of the alveolus for the upper caniniform tooth is sub-
circular in each of five specimens. Estimated internal measure-
ments of the two specimens in which the anterior alveolar wall is
preserved are: UF 10337, 10.7 mm anteroposteriorly, 7.0 medio-
laterally; UF 10338, 10.4 mm anteroposteriorly and 6.7 mediolat-
erally. The base of the caniniform alveolus is no larger than the
base of the alveolus for the first upper molariform tooth. This is in
sharp contrast to Mc'Ic'alon.yx in which the caniniform is consider-
ably larger than the first molariform tooth.
A small fragment of the right posterior side of the palate pre-
served in UF 9479 differs from Megalonyx, Hapalops, and Eucho-




BULLETIN FLORIDA STATE MUSEUM


loeops in the manner in which the nutritive foramina open into the
ventral surface. In the McGehee palate long narrow canals run
anteroposteriorly on the ventral surface of the palate, whereas in
the other genera they run dorsoventrally so that only small round
foramina appear on the palatal surface.
MANDIBLE. The mandible is represented by two specimens,
UF 9450 and 9480, from McGehee Farm. A third specimen from
the Withlacoochee River, UF 11941, a symphysis, is referred to the
same species. UF 9450, the right anterior portion, includes the
ventral part of the symphysis, caniniform alveolus, and anterior
margin of the alveolus of M1, but lacks most of the predental
"spout." UF 9480 includes the posterior two-thirds of the left
ramus (Figs. 14 and 16).

Table 12. MEASUREMENT OF Pliometanastes protistus MANDIBLES IN MM
(UF9450) (UF9480) (UF11941)
Measurement right left

Depth of horizontal ramus at middle
of diastema 42.0
Length of diastema 20.0 16.5 16.5
Length of symphysis from caniniform
alveolus to tip 37.4 37.9
Anteroposterior diameter of
caniniform alveolus 10.0 13.2 12.9
Max mediolateral diameter of
caniniform alveolus 8.6 10.9 11.1
Height of condyle above ventral
border of angular process 53.0


The two symphyseal specimens of Pliometanastes reveal a num-
ber of distinctive features. The symphyseal spout is long, extend-
ing 34.0 mm anterior of the caniniform alveoli in UF 11941 and
evidently about the same in UF 9450. The predental spout is
rounded at the tip as in Paulocnus and not pointed as in Acratoc-
nus. The sides of the spout are parallel, not converging anteriorly
as in Paulocnus. In side view the spout is flat along the dorsal
surface as in Paulocnus, not decurved as in Mesocnus. The sym-
physis is deep with a slightly concave outline ventrally. It does not
bear a keel as in Pleistocene Megalonyx. The posterior edge of
the symphysis lies between the roots of the lower caniniform teeth.
In this respect Pliometanastes agrees with Megalonyx and differs


Vol. 12


























































FIGURE 14. Mandibular symphysis of Pliometanastes protistus, UF 11941,
occlusal and lateral views.





BULLETIN FLORIDA STATE MUSEUM


from Paulocnus and other West Indian megalonychids in which
the symphysis extends posteriorly at least as far as the first cheek
tooth.
The caniniform alveolus presents a rounded to triangular cross
section. The lingual wall is slightly convex, nearly flat. The apex
of the triangle faces labially. Although no caniniform has been dis-
covered, we are confident that the tooth wore on a near vertical
anterior surface producing a sharp point as in similar teeth of
Choloepus, Eucholoeops, Pliomorphus, and Paulocnus.
The alveolar ridge between the caniniform and first cheek teeth
is considerably thicker in Pliometanastes than in typical M3cgna1hiyl.
(for example, UF 10348 a small Rancholabrean Mcgaloniy.r jaw
from Sumter Co., Florida), but as noted above, the same broad-
ened ridge is observed in Blancan specimens of Megalonyx. The
postcaniniform diastema attains a length of 20.1 mm in UF 9450
and a greater but indefinite amount in UF 11941, thus exceeding
the diastema in Paulocnus and approximating that in Megalonyx
of similar size.
UF 9450 includes the condyle but lacks the coronoid process
and the posterior portion of the angle. The horizontal ramus is
preserved as far forward as the alveolus of M1 and contains the
lateral walls of the alveoli of M2 M3. The poorly developed mus-
cle scars and porous texture of the bone suggest that this specimen
is from a young individual. The angular process does not project
ventrally as in Hapalops, but is more horizontal as in Megalonyx.
The angle does not attain the dorsoventral height seen in M. jefer-
soni (see Leidy, 1855, pl. 1). The condylar process does not curve
dorsally and is set in a more horizontal position than in Megalonyx.
The dental canal passes through the alveolus of Ms, whereas in
31cgalonyx. it runs outside the alveolus, being enclosed in the side
of the wall of the jaw. The foramen lateral to M3 is situated in the
same position in the McGehee mandible as in Megalonyx and Eu-
choloeops, not on the ascending ramus as in Hapalops.
DENTITION. Only three isolated teeth have been found at the
McGehee Farm: two left M4's, UF 10341 and UF 9613, and a left
upper M2 or M3, UF 9449. The two M4's differ in size, the antero-
posterior diameter of UF 10341 being 9.0 mm, while that of UF
9613 is 7.1 mm and the mediolateral diameter of UF 10341 being
12.8 mm, that of UF 9613, 10.7 mm. The fourth upper molariform,
UF 10341, probably represents an older individual as the sides are


Vol. 12







1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 261

parallel. In UF 9613 the tooth tapers slightly, the crown measuring
10.7 mm in length and the root 11.2 mm. Both teeth resemble the
triangular tooth of M. wheatleyi figured by Cope (1871), though
within the whole sample of M. wheatleyi this tooth varies consider-
ably in shape. As Cope (1899) states, "The last superior molar,
however, differs considerably in form. In some individuals it is tri-
angular in section; in others the section is a transverse oval. All
intermediate forms occur in Megalonyx wheatleyi." The pattern of
wear of the fourth upper molariform teeth in Pliometanastes is
similar to that in Megalonyx.
The left M2 or M3 of Pliometanastes resembles that of M. lep-
tostomus in shape and wear of the crown; both have a slight con-
cavity on the posterior side and a slight convexity on the anterior
side. Both Pliometanastes and Megalonyx have irregular triangular-
shaped second and third molariform teeth, whereas in Hapalops
these teeth are squared. In these genera M4 most nearly resembles
that of Hapalops in shape, but here too the tooth of Hapalops is
more squared.
The posterior mandibular fragment, UF 9480 described above,
indicates that the lower molariform teeth were of the broad trape-
zoidal type observed in Megalonyx, Megalocnus, and Paulocnus,
with the labial side longer than the lingual.
THORACIC VERTEBRAE. In the McGehee collection are two
thoracic vertebrae, UF 9464 and UF 9468. UF 9464 is complete,
while UF 9468 lacks the epiphyses and neural spine. The exact
position of these vertebrae in the column cannot be determined.
In UF 9464 the neural spine slopes posteriorly, the articular
surface for the tuberculum of the rib is convex, and the dorso-
external angle of the posterior face of the centrum is not truncated
by an articular facet. The articular surface for the capitulum is
ovate and concave. The ventral part of the centrum is pierced by
two pairs of nutritive foramina and numerous smaller ones. These
larger foramina are considerably smaller than those observed in
the lumbar vertebra. A single median foramen opens on the dorsal
side of the centrum below the neural canal.
UF 9468 differs from UF 9464 in that the ventral part of the
centrum is less rounded and has a slight keel, the concavity be-
tween the prezygapophyses is deeper, the articular surface for the
tuberculum of the rib is larger, and the neural canal has a slightly
greater diameter.
These thoracic vertebrae differ from the vertebrae of M. jeffer-








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263


1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


soni californicus figured by Stock (1925, pl. 18) in having relatively
shorter neural spines, the dorsal end of the spine longer, less
rounded, and more horizontal, the centrum relatively thicker antero-
posteriorly, the prezygapophyses not sloping ventrally but more
horizontal, and the articular surface for the capitulum of the rib
more oval and slanting more anteriorly.
LUMBAR VERTEBRAE. Three lumbar vertebrae, UF 9465, UF
9466, and UF 9467, are available. UF 9465 lacks the transverse
processes, UF 9466 lacks the transverse processes and neural spine,
and UF 9467 lacks the centrum. Each of the specimens differs con-
siderably from the others, suggesting that they belong to different
parts of the series. In UF 9465, the posterior face of the centrum
is deeper and narrower and the fragment of the transverse process
is inclined more dorsally than in UF 9466. A small foramen pierces
the ventrolateral side of the transverse process in UF 9465, but is
lacking in UF 9466. In UF 9467 a large foramen pierces the trans-
verse process posterior and slightly lateral to the prezygapophyses.
The medioventral edges of the medial postzygapophyses are closest
together in UF 9467, and are farther apart in UF 9465 and 9466,
the distance between them being 20.6, 24.2 and 26.4 mm, respec-
tively. The neural spine preserved in two of the vertebrae is broad
anteroposteriorly and flattened. At the dorsal end a terminal en-
largement bears two prominent posterior projections. The winglike
transverse processes in UF 9467 are broad, flattened, and intricate
in construction. In dorsal view a concavity is observed on the pos-
terolateral margin of each process, medial to it a prominence that
projects posteriorly, and, medioventral to it, the postzygapophysis.
The vertebrarterial canal opens anterior and slightly ventral to the
postzygapophysis.
CAUDAL VERTEBRAE. Two caudal vertebrae are available, UF
10342 and UF 10343. In both the neural arch is missing and the
transverse processes are incomplete. Both vertebrae are probably
from the middle of the series.
The centrum of UF 10342 is oval shape, the anterior face meas-
uring 40.2 mm in width and 31.3 in height, the posterior face meas-
uring 37.4 and 29.3 mm respectively. A single large foramen pierces

FIGURE 15. Vertebrae of Pliometanastes protistus. A.-Anterior thoracic ver-
tebra, UF 9464, lateral view. B.-Anterior view of same. C.-Caudal vertebra,
UF 10342, ventral view. D.-Lumbar vertebra, UF 9465, anterior view. E.-
Dorsal view of same. Scale x%.






-/ t


A




<-r |- ^ r^-


B
I.







C









E





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 265

the median ventral surface of the centrum. Two pairs of promi-
nences, at the anterior and posterior extremities of the ventral surface
of the centrum, are present for articulation with the haemapophyses
(Fig. 15 and Table 13).
CLAVICLE. A right clavicle, UF 10344, is represented in figure
16. The compressed shaft forms a single arc between the extremities,
the curvature being more arcuate than in M. jeffersoni but lacking
the sigmoid curvature in Nothlrulhi'rinum. It is much more slender,
especially medial to the articular facet for the scapula, than in M.
jeffersoni. The most rugose area for muscle attachment extends
about halfway up the shaft from a point just lateral to the sternal
facet. The shaft attains its greatest depth near the middle at the
end of this most prominent muscle scar. It also broadens medially
to form the articular facet for the sternum. This surface is not so
wide as in Nothrotherium, but is relatively wider than in M. jeffer-
soni. The head is convex, almost circular in side view, compressed,
and slightly wider and deeper than the shaft.
HUMERUS. Three incomplete humeri, a left (UF 9445) and two
rights (UF 9444 and 9443), are represented. All lack the proximal
end, and only in UF 9445 is the lateral portion of the distal end pre-
served. (Fig. 16).
The humerus appears to be a relatively conservative element
compared to the Santa Cruz megalonychids. The major changes ap-
pear to be in the size and development of the pectoral, medial, and
deltoid crests. In Hapalops the junction of the pectoral, medial, and
deltoid crests stands prominently above the surrounding shaft. The
pectoral crest rises posteriorly and then curves anteriorly, forming
a hooklike projection on the medial side. This projection is less
prominent in Pliometanastes. Along the anterolateral border of the
shaft just posterior to the deltoid crest is a prominent ridge in Plio-
metanastes and 1-allont.!l absent in Hapalops. In both Plioinitatlus-
tes and M'hialoniyv the median ridge formed between the pectoral
and deltoid crests is more prominent than in Hapalops.
In lMcgdlonuim dorsal and slightly lateral to the entepicondylar
foramen is a convex protuberance that is absent in Pliometanastes
and Hapalops.

FIGURE 16. Limb elements and mandible of Pliometanastes protistus. A.-
Clavicle, UF 10344. B.-Left mandible, UF 9480, lingual view. C.-Labial
view of same. D.-Right mandible, UF 9450, labial view. E.-Occlusal view
of same. F.-Patella, UF 9774, femoral view. G.-Left humerus, UF 9445,
anterior view. Scale x1/2.





BULLETIN FLORIDA STATE MUSEUM


Table 13. MEASUREMENTS OF Pliometanastes protistus
POSTCRANIAL MATERIAL IN MMi
Thoracic Vertebrae (UF 9464) (UF 9468)


Length of centrum
Width of centrum measured over anterior face
Depth of centrum measured over anterior face
Max. width across transverse process
Length of neural arch along middle at base
of neural spine
Height measured from middle of ventral border of
posterior face of centrum to end of neural spine
Dorsoventral diameter of neural canal measured at
anterior end
Transverse thickness of neural spine at middle
Transverse thickness at end of neural spine
Transverse width across anterior zygapophyses
Lumbar Vertebrae (UF 9465)
Length of centrum 45.6
Width of centrum measured across
anterior face 60.4
Depth of centrum measured across
anterior face 40.9
Width across supports for
anterior zygapophyses 59.01


Width across transverse process
Max. width across posterior zygapophyses
Length of neural arch along middle
and at base of neural spine
Height measured from middle of ventral
border of posterior face of centrum
to end of neural spine
Dorsoventral diameter of neural
canal measured at anterior end
Transverse thickness of neural
spine at middle
Transverse thickness at end of
neural spine
Clavicle
Max. length
Min. depth of shaft (medial to scapular facet)
Max. depth of shaft (about halfway between
extremities)
Long diameter of scapular facet
Humerus (1
Max. width of shaft at middle
Min. width of shaft
Thickness of shaft at end of
deltoid crest
Thickness of bridge over
entepicondylar foramen


42.6

53.5


131.0

31.3


38.4
44.4
34.4
84.2

51.0

102.0


24.9
34.0
18.0
47.2
(UF 9466)
42.5


61.0

39.3



44.0

48.0


42.51
32.8'
84.0

51.7



27.8


49.0
(UF 9467)


154.0
40.3

56.0


27.2


52.0


25.8






UF 9443)
49.2
31.0

43.8


Ulna
Max. mediolateral diameter of olecranon process


27.0
(UF 10344)
127.0
15.7

25.4
24.4
(UF9444) (UF9445)
40.7 44.7


40.5


43.4


22.4
(UF 9447)
37.5


Vol. 12






1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


Width of shaft at concavity of sigmoid cavity
Dorsoventral diameter of radial notch
Metacarpal III
Max. length
Max. depth of proximal end
Max. width of proximal end
Max. depth of distal end
Max. width of distal end
Min. depth of shaft
Min. width of shaft
Metacarpal IV
Max. length
Max. depth of proximal end
Max. width of proximal end
Min. depth of shaft
Min. width of shaft
Metacarpal V
Max. length
Max. depth of proximal end
Max. width of proximal end
Max. depth of distal end
Max. width of distal end
Min. depth of shaft
Min. width of shaft
Metacarpal II (UF 11524)
Max. length 56.6
Max. depth of proximal end 30.7
Max. width of proximal end 27.5
Max. depth of distal end 32.4
Max. width of distal end 22.2
Min. depth of shaft 14.0
Min. width of shaft 17.3
Patella
Dorsoventral diameter of femoral facet
Transverse diameter of femoral facet
Thickness through femoral facet
Total length
Fibula
Length through shaft
Max. anteroposterior diameter of proximal end
Transverse diameter of proximal end
(perpendicular to tibial facet)
Anteroposterior diameter at middle of shaft
Transverse diameter at middle of shaft
Anteroposterior diameter of distal end
Max. transverse diameter of distal end
Calcaneum
Max. anterioposterior diameter
Max. diameter of tuber calcis
Max. thickness at posterior border
Width of neck
Thickness of neck


40.8
30.0
(UF 11525)
66.5
32.9
32.0
34.9
24.0
15.3
21.4
(UF 11526)
73.5
31.0
19.5
14.8
12.1
(UF 11527)
64.3
23.7
25.0
22.4
17.6
11.0
14.1
(UF9453) (UF9454)
54.9 61.1
30.5 34.4
23.4 29.2
31.7 35.7
22.0 23.4
13.7 15.6
20.1 20.4
(UF 10347) (UF9774)
33.5 32.0
41.1 43.0
24.0 20.5
80.0
(UF9448) (UF9446)
224.0
31.0 33.8


42.2




(UF 9437)
131.0
110.4
24.4
24.0
34.4


37.6
19.0
16.4
44.5
47.0
(UF 9442)
128.0

24.0
20.7
34.4






BULLETIN FLORIDA STATE MUSEUM


Distance from inner border of external
astragalar facet to inferoexternal
prominence of articulating end
Distance from dorsal border of external
astragalar facet to ventral border of
cuboid facet
Astragalus
Max. anteroposterior diameter
parallel to fibular facet
Max. transverse diameter measured
at right angles to fibular facet
Depth of fibular facet at distal end
Max. transverse diameter of head
Anteroposterior length of
external calcancal facet
Navicular
Max. diameter across cuneiform facets
(width)
Diameter taken at right angles to greatest
(dorsoplantar) diameter
Max. thickness through convexity
articulating with astragalus


Phalanx II of Digit III
Length through middle
Width of proximal end
Depth of proximal end
Width of distal end
Depth of distal condyles
Ungual Phalanges
Max. length from
posterior end of over-
hanging process to tip
of claw process
Max. depth from dorsal
surface of hood to
discoid area on ventral
surface
Length of subungual base
Width of ventral surface
of claw process just
anterior to hood


53.0 50.8


67.2 63.5
(UF9440) (UF9441) (UF 10339)


67.0

57.2
31.2
35.2

43.5


(UF9460) (UF9554)


55.0
20.0


23.0


50.0



22.4


65.4


58.0
33.9
39.0

46.0


57.5
34.0
36.5

43.3
(UF 9474)


42.8

41.5

19.4
(UF 9459)
27.5
37.5
38.3
35.6
34.4
(UF9462) (UF 10340)


31.0


11.0 15.3


49.8



21.4



10.3


'Estimated

ULNA. The proximal half of a right ulna, UF 9447, is illustrated


in Fig. 17. The coronoid process and the area
brachialis muscle are lacking.


for attachment of the


The McGehee ulna is intermediate between Hlapalops and Mega-
loniiy in several characters. In IIapalops the anterior border of the
shaft from the olecranon process to the styloid process forms a single
convex are, whereas in MIcgalonr this area is concave. In Pliome-


Vol. 12





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 269

tanastes the anterior border of the shaft is straight dorsoventrally.
The lateral projection of the olecranon is considerably greater in
Megalonyx than in either Pliometanastes or Hapalops. In Hapalops
the posterior border of the shaft is slightly convex dorsal to the sig-
moid cavity, in Pliometanastes it is nearly straight, and in Mega-
lonyx it is concave, and slopes less steeply posteriorly than in either
Hapalops or Pliometanastes.
The olecranon process in Plioimctaonatc', is relatively wider across
the anterior border than in Hapalops. The vertical ridge along the
posterior border of the olecranon process is sharper and less rounded
than in Hapalops. The dorsoventral height of the radial notch is
much greater in Pliometanastes than in Hapalops or even in Mega-
lonyx. In Pliometanastes and Hapalops the radial notch is not dis-
tinctly separated from the sigmoid cavity by a groove as in M. jef-
fersoni californicus (Stock, 1925). The radial notch faces almost
directly anteriorly in MIcgalJnyr, considerably to the side in Hapa-
lops; in Pliometanastes its orientation is intermediate.
MANUS. Four right metacarpals, numbers II, III, IV, and V,
(UF 11524-UF 11527), were collected together at McGehee and
appear to belong to one individual (Fig. 18). Metacarpal IV is
d.atlg'd and lacks the anterolateral posteromedial portion of the
distal end above the carina. There are also two isolated metacarpal
II, a right (UF 9453) (Fig. 19), and a left (UF 9454). The corre-
sponding metacarpal elements of Pliometanastes, Hapalops and Meg-
alonyx are all closely similar, the major differences being functions
of the dilerences in size.
The fourth metacarpal of Pliometanastes is the longest, as in
Mcalo,.!/xv. The fifth metacarpal is slightly shorter than the third,
whereas in lMchalh,,o ry the fifth is second in length.
The available specimens of metacarpal II of M. jeffersoni, ANSP
12507 and ANSP 12475, differ notably in the width of the shaft,
length (ANSP 12507 is shorter and more robust than ANSP 12475),
and degree of concavity of the facet for metacarpal III. The trape-
zoid facet of M. ijffcroni is almost uniformly concave, whereas in
Pliometanastes the surface becomes concave near the dorsal extremity
of the facet, and convex ventrally as it does in Hapalops. The dorso-
lateral process of the proximal end is extremely prominent, thus ac-
centuating the sigmoid shape of the facet. This process is not quite
so prominent in Hapalops and is almost lacking in M. jeffersoni.
The palmar extremity of the trapezoid facet curves anteriorly in
Pliometanastes, whereas in M. jeffersoni it is flattened in the dorso-





















A B


IC





C


U'














D


:t.


!r





HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


palmer plane. The dorsal end of the facet proximal to the facet for
metacarpel III is convex in Pliometanastes, concave in jeffersoni. The
dorsolateral extremity of the trapezoid facet proximal to the facet
for me I does not extend so far dorsally as in jeffersoni.
UF 9453 has a ridge from the dorsomedial end of the facet for
metacarpal I to the medial projection for the articulation with
phalanx I, and on either side of this ridge the shaft is concave. In
UF 9454 and 11524 no ridge is apparent and the shaft is concave
dorsally and convex ventrally. In both specimens of jeffersoni the
shafts are completely round. The shape of the lateral side of the
shaft in UF 9453 closely resembled that of Megalonyx, ANSP 12475,
but in UF 9454 the ventrolateral side of the shaft is more deeply
excavated. The shape of the distal end of metacarpal II is similar
in both Pliometanastes and Megalonyx. On the dorsomedial side
of the distal end proximal to the projection for the articulation with
phalanx I, a distinct ridge of bone runs from the dorsal end of the
carina ventrally, with a depression below it. This ridge and depres-
sion are much more pronounced in Megalonyx and Hapalops than
in the McGehee metacarpal II UF 9453 and 11524, and they are
even less distinct in UF 9454.
Metacarpal III of Pliometanastes, UF 11525, closely resembles
that of Megalonyx and Hapalops. The differences lie in size and in
the shape of the facets of the proximal end. In Pliometanastes, as
in Hapalops, the facet for metacarpal IV consists of two parts that
are continuous with one another. The dorsal end of the facet is oval
with the long axis extending down the shaft. The palmar part is also
oval but the long axis runs transversely; the facet is thus shaped like
an English saddle in side view. In Megalonyx the facet for meta-
carpal IV consists entirely of the dorsal part with little or no palmar
extension.
The facet for metacarpal II is similar in shape in Pliometanastes,
Megalonyx, and Hapalops. In Pliometanastes the palmar half of the
facet turns medially so that it almost faces ventrally; in Megalonyx
and Hapalops the facet curves more gently and does not turn so far
ventrally. The shaft in Pliometanastes is more slender and rounded
than in Megalonyx. This appears to be the result of increase in size,

FIGURE 17. Ulna and fibula of Pliometanastes protistus. A, B.-Proximal half
of right ulna, UF 9447, outer and radial views. C, D.-Right fibula, UF 9446,
tibial and outer views. E, F, G.-Left metatarsal IV, UF 11572, left, plantar,
and outer views. Scale, A-D X%/, E-F, natural size.


1968















B


1.,







D


I
I-:
r)


O
r i


&-
''


E


SI-
A,

34
4d"


F


A




273


HIIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


as metacarpal III of Pliometanastes most closely resembles the small-
er, more slender element of Megalonyx from the Hagerman beds in
Idaho.
Metacarpal IV of Pliometanastes, UF 11526, is a long, slender
bone with a dorsopalmar expansion of the proximal and distal ends.
It resembles that of Hapalops, and differs from both in the extreme
mediolateral compression of the proximal end. The shaft is more
slender and rounded than in Megalonyx, especially at the distal end.,
At the proximal end the facet for metacarpal V is similar in
shape to that of Mcgalonlyv. The facet for metacarpal III is also
similar, but it is flattened against the shaft, whereas in Megalonyx
and Hapalops it curves outward. The compression of the proximal
end where the facet is flattened for metacarpal III is striking; the
palmar portion of the proximal end in Pliometanastes is a narrow,
pointed projection, whereas in the other two genera this part is
wider and somewhat squared. The flattened facet for metacarpal
III brings the two metacarpals closer together, especially at the dis-
tal end. The facet for articulation with the metacarpal III extends
farther ventrally than in Megalonyx. The facet for the uniform is
similarly shaped in both Pliometanastes and Megalonyx, except for
the extreme narrowing of the palmar end in Pliometanastes. The
distal end of this element closely resembles that of Megalonyx.
Metacarpal V of Pliometanastes, UF 11527, is slightly shorter
than metacarpal III, UF 11525. This is in sharp contrast to the rel-
ative lengths of these elements in Megalonyx, where me V is con-
siderably greater in length than metacarpal III. The shorter distance
between the proximal and distal articular processes of metacarpal V
of Pliometanastes make it more robust in appearance than that of
Megalonyx. The proximal end is irregularly triangular in shape as
in Megalonyx. The distal end is considerably shorter in the dorso-
palmer direction than in Megalonyx.
On the proximal end the surface for articulation with the lateral
extension of the cuneiform is almost square in outline and presents
a relatively flat surface. Situated on the dorsomedial side of the
cuneiform facet is the uniform facet which is confluent medially

FIGURE 18. Associated metacarpus of Pliometanastes protistus. A.-Right met-
acarpal II, UF 11524, inner view. B.-Dorsal view of same. C.-Right metacar-
pal III, UF 11525, dorsal view. D.-Outer view of same. E.-Right metacarpal
IV, UF 11526, outer view. F. and G.-Dorsal and inner views of same. H.--
Right metarcarpal V, UF 11527, inner view. I. and J.-Outer and dorsal
views of same. All natural size.


1968




274


BULLETIN FLORIDA STATE MUSEUM


B

A


Vol. 12




1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


with the facet for the fourth metacarpal. The uniform facet is semi-
circular in outline, and the surface forms a smooth sigmoid curve
with the concave portion on the dorsal and the convex portion of
the palmar part. A similar sigmoid shape appears in the facet for
metacarpal IV but in reverse, the convex portion dorsal and the
concave palmar. On the far lateral side of the proximal end is an oval
area for ligamentary attachment; the shaft is depressed on either side
of it. The median part of the shaft is oval and compressed in the dor-
sopalmar direction. The facet for the first phalanx at the distal end is
broad and only slightly convex, unlike the other metacarpals of the
manus which have a large rounded carina. This indicates considerable
restriction of movement in this last digit.
Patella. The articular facet of UF 9774 is kidney-shaped, the
medial and lateral sides being almost symmetrical (Fig. 16). In UF
10347 one side of the facet extends farther ventrally than the other.
The patella has proportionally longer distal projection below the facet
than in either Hapalops or Mc.t'alony. : the ventral projection does not
narrow below the facet as in those genera, but remains relatively wide
to the distal extremity.
Fibula. One complete right fibula, UF 9446 (Fig. 17), a proximal
end of a right fibula, UF 9448, and an immature left fibula, UF 9555,
lacking the epiphyses, are represented.
In the McGehee fibula the shaft is essentially straight; whereas in
Mlcalo1,ntl the shaft is curved posteriorly distal to the region for at-
tachment of the peroneus brevis muscle. The shaft in this region is
more enlarged and rugose in IM\c-'alon.yx than in Pliometanastes. The
scar is considerably less distinct in Pliometanastes than in 3'cgalonyx.v
In Megalonyx fibulae a ridge running from the tibial facet to the dor-
sal margin of the muscle scar gives the shaft a triangular slope,
whereas in the McGehee fibula this ridge is lacking and the shaft is
round.
In Pliometanastes the calcaneal facet is distinctly concave with a
slight papilla at the junction of the astragalar and calcaneal facets. The
astragalar facet is likewise concave, whereas it is flat in Megalonyx
fibulae, which lack the raised area at the junction of the two facets.

FIGURE 19. Podial elements of Pliometanastes protistus. A.-Right metacar-
pal II, UF 9453, inner view. B.-Right navicular, UF 9474, astragalar surface.
C.-Cuneiform surface of same. D.-Left astragalus, UF 9440, tibial view.
E. and F.-Fibular and calcaneal views of same. G.-Right calcaneum, UF
9437, outer view. H.-Inner view of same. Scale X12.


275




276 BULLETIN FLORIDA STATE MUSEUM Vol. 12

In Pliomntcanaisates and 3cgalony iv the proximal end of the fibula is
shorter and broader than in Hapalops. A deep depression appears on
the anteromedial side of the proximal end in these genera but not in
Hapalops; a depression in Hapalops posteromedial to the tibial facet
of the proximal end is absent in the other genera.
Calcaneum. Four calcanea (Fig. 19) are represented, three
right, UF 9437, 9438, and 9442, and one left, UF 9439. Only UF 9437
is complete; UF 9439 from an immature individual lacks the epi-
physis. The calcaneum of Pliometanastes retains several primitive fea-
tures associated with Hapalops and its allies; the major differences re-
sult from an increase in size.
The length of the tuber calcis is intermediate between those of
Hapalops and Mcgalniyx v. An adult calcaneum of a small AIlcgalunyv
from Sumter County, Florida, is no larger than the McGehee calcan-
eum yet has considerably longer tuber calcis. The McGehee calcan-
eum also differs from the Sumter County Megalonyx specimen in lack-
ing an anterodorsal projection of the tuber calcis Pliometanastes differs
from all species of 3legalonyx observed including the Sumter County
specimen, in having the dorsal region of the tuber calcis proportion-
ally thicker.
Apparently the degree of proximity of the astragalar facets and the
depth of the grooves separating them vary considerably within the
genus Megalonyx. In Pliometanastes, as in the Blancan M. lepto-
stomus, the grooves do not appear to be so deep as those described
by Stock (1925) and Leidy (1855) for M. icff'rni.
In Pliometanastes the two astragalar facets are almost confluent,
whereas in Hapalops, M. leptostomus and the Sumter County MI ra-
lonyx the large external astragalar facet is set ventrally below the level
of the smaller internal astragalar feet.
In the McGehee calcaneum the cuboid and internal astragalar facets
are closer than in Hapalops and almost touch. The distance between
them varies in the different species of Megalonyx and appears to be
greatest in M. lcpto.tosmus; the distance is greater in all Megalonyx
than in Pliometanastes.
The shape of the external astragalar facet in Pliometanastes is
identical to that of Hapalops, both sides being mirror images of each
other. In Megalciny the side of the facet dorsal to the internal astrag-
alar facet is longer than the side dorsal to the cuboid facet.
The shape of the cuboid facet is similarly almost circular and
slightly concave in Hapalops, Pliometanastes, and Megalonyx, but
Pliometanastes does not have the inner lip flattened almost at right





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 277

angles to the rest of the surface as Stock (1925) describes for M. jef-
fersoni californicus. This inner lip is also absent in M. leptostomus and
the Sumter County Aflc'galoiny.r.
The internal astragalar facet in Pliometanastes is more ovate than
that of Hapalops, which is almost round. It differs from that of Mega-
lonyx mainly in the dorsal margin, which is convex and curves toward
the external astragalar facet as in Hapalops. In Megalonyx the dorsal
margin is concave and forms a distinct separation between the two
facets.
The lateral area for attachment of the peroneus muscle posterior
to the cuboid facet varies in shape and thickness in Pliometanastes,
Hapalops, and 3cgalonli.\. The McGehee calcaneum has a distinct
and relatively wide groove immediately lateral to the external astrag-
alar facet. This groove is less well defined in M. leptonyx, and is only
a slight depression in M. lcjptoltinhit and Hapalops. In Pliometa-
nastes, as in Hapalops, the projection for attachment of the peroneus
is large, relatively long, and wide; in jeffersoni, in M. leptostomus, and
the Mc-glonyx from Iagerman, Idaho, it is c,)iisid,-r..bly smaller,
shorter, and narrower. The area for attachment of the peroneus thus
decreases with increase of size in Megalonyx, but in Pliometanastes
more closely resembles Haplilopst than the small species of 3Mc'ial' J! r.
Astragalus. Three left astragali, UF 9440, 9441, and 10339, in the
collection from McGehee (Fig. 19) all are complete, well preserved,
and appear to represent adult individuals.
The most conspicuous difference between a McGehee astragalus
and those of M. jeffersoni and M. tcheatleyi is its more rounded shape.
The medial side is shorter than the lateral side in Pliometanastes
whereas in M. jeffersoni and wcheatleyi the medial and lateral sides
are more nearly equal. The McGehee astragalus more closely resem-
bles Hapalops in this condition. This increase in the width of the
medial side of the stragalus appears to be related to increase in size,
as a specimen of 3lcgaloityr from the Santa Fe River in Florida, which
has an astragalus smaller than the McGehee astragalus, is relatively
narrow on the medial end as in Pliometanastes.
All specimens of 3lcgalrJiyr examined have a deep, V-shaped
notch between the tibial and navicular facets to receive a process of
the tibia. UF 10339 and 9440 have only a vague depression, UF 9441
a very shallow notch. Although no tibia is represented in the McGehee
collection, the lack of a deep notch in the McGehee astragali implies
the lack of a very well developed tibial projection. This is of consid-
erable interest, because the tibia Stirton (1939) described from the






BULLETIN FLORIDA STATE MUSEUM


Mulholland Fauna in the San Francisco Bay Region has a well devel-
oped and quite prominent astragalar spine on the distal end of the
tibia. In Hapalops, PU 15913, a relatively deep notch is present.
The internal calcaneal facet is not separated from the external
calcaneal facet by a wide, distinct furrow as in all specimens of Mehga-
lonyx observed. The internal calcaneal facet in UF 9440 is connected
to the external calcaneal facet by a low ridge. In the other two speci-
mens there is only shallow depression between the two facets. In
Hapalops, PU 15913, as in Megalonyx, a distinct separation exists.
The internal calcaneal facet varies considerably in shape in the
specimens of Megalonyx available, but in all cases it is either rela-
tively flat or slightly convex. In Pliometanastes it is slightly concave
and somewhat diamond-shaped, with apex curving up toward the ex-
ternal calcaneal facet. The shapes of the external calcaneal facet and
fibular facet are similar in Pliometanastes, Hapalops and l[r'galoiyix.
The navicular facet in the McGehee astragalus is ovate, having almost
a tear-drop shape, whereas in 3Mrgalonyx it is considerably deeper in
a dorso-plantar direction.
NAVICULAR. A right navicular, UF 9474, is shown in Figure 17.
The McGehee navicular exhibits the condition seen in Hapalops
where the mesocuneiform facet is raised above the ectocuneiform
facet, whereas the opposite condition exists in M. jeffersoni and the
Hagerman megalonyx where the mesocuneiform is lower than the
ectocuneiform facet.
The two cuneiform facets do not meet each other as a distinct
ridge in Pliometanastes as they do in Hapalops, M. jeffersoni, M. sier-
rensis (Sinclair, 1905), the megalonychid from the Snake Creek Beds
of Nebraska described by Sinclair (1915), and the Mlhgalonyi from
Hagerman, Idaho. Instead in the McGehee navicular the mesocunei-
form facet slopes gently to merge with the ectocuneiform facet.
In the navicular of Pliometanastes and Hapalops, the mediolateral
diameter (width across the cuneiform facets) is greater than the
dorsoplantar diameter. This condition is also found in the navicular,
PU 12970, from the Snake Creek Beds (Sinclair, 1915). In M. jeffer-
soni, M. sierrensis and the specimen from Hagerman, Idaho the
mediolateral diameter across the cuneiform facets is less than the
dorsoplantar diameter.
The mammillate surface in the McGehee navicular is less rounded
than in M. jeffersoni.
The navicular in both Pliometanastes and Hapalops lacks the deep
notch on the median side of the ectocuneiform facet, between the


Vol. 12




1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


ectocuneiform and mesocuneiform facets, which is prominent in the
later species of Megalonyx and in M. curvidens from the Snake Creek
beds. Instead, in the McGehee navicular, as in Hapalops, the medial
border of the ectocuneiform facet slopes gently upward to meet the
median ridge surrounding the medial border of the mesocuneiform
facet.
METATARSAL IV. A nearly complete left metatarsal IV, UF 11572,
is represented in the McGehee collections. The element lacks the dor-
sal projection of the proximal end, broken slightly ventral to the level
of the dorsal part of the facet for articulation with metatarsal III, as
well as lacking the exteriodorsal projection of the distal end above the
carina.
Metatarsal IV of Pliometanastes is slightly shorter than in Mega-
lonyx from Hagerman, Idaho and about the same length as the mega-
lonychid from the Mulholland site II, California, described by Stirton
(1939). The greatest length of the McGehee metatarsal measures 82.5
mm while the Hagerman Mlgaloyx. element measures 87.3, and the
Mulholland metatarsal measures 82.7.
Metatarsal IV of Pliometanastes resembles Hapalops in retention
of a long and slender shaft and thus differs from the Hagerman Mega-
lonyx in which the element is short and robust with relatively less
distance between the more massive proximal and distal extremities.
The portion of the cuboid facet that is preserved is quadrilateral
in shape and extends to the posteroventral margin of the element as
in Hapalops and the Hagerman Megalonyx but differs in the latter
aspect from the Mulholland specimen in which the cuboid facet
slopes sharply toward the facet for the metatarsal III.
The facet of metatarsal IV for articulation with metatarsal III is
elliptical in outline and slightly convex in Pliometanastes. In this
character it is similar to the Mulholland metatarsal IV and differs
from the Hagerman Metalonyx and Hapalops in which the facet is
concave. This character, however, appears to vary considerably within
the genus Mci1galoiy\.
The facet for articulation with the metatarsal V is parallel to the
shaft and is subtriangular in shape as in the Hagerman IMcgalb'iiyx
and the Mulholland specimen. The facet of Pliometanastes differs
from the Mulholland specimen in that the lower portion of the facet of
Pliometanastes does not flare outward but is flat and very slightly de-
pressed into the shaft.
The region of the proximal end immediately posteroventral to
the facet for the metatarsal V is depressed in Pliometanastes, unlike


279




280 BULLETIN FLORIDA STATE MUSEUM Vol. 12

the situation in the Hagerman Megalonyx and the Mulholland meta-
tarsal in which this region is level with the facet or in the case of
3Icgalonyx, slightly bulging.
Pliometanastes differs from the Hagerman Mcgalhoyl and the Mul-
holland specimen in the posteroventral view of the proximal end. In
Pliometanastes the dorsal area of muscle attachment is relatively
small, somewhat square in shape, and projects outward from the shaft,
but not nearly so prominently as a projection in IIapalops. Win-.- the
shaft curves upward to meet this region of attachment lies a concavity
that is more distinctly demarcated in Pliometanastes than in the Ia-
german Me-galonyx or the metatarsal from Mulholland. In the Hager-
man specimen, the region for muscle attachment covers a much
greater area of the proximal end and shaft than in Pliometalnastes or
the Mulholland metatarsal. The Mulholland metatarsal differs from
those of Pliometanastes and the Hagerman Megalotiyx in that the
cuboid facet does not extend all the way to the ventral margin of the
proximal surface,but terminates just ventral to the level of the ventral
margin of the facet for metatarsal III. A specimen of M. cf. heatleyi
from Florida, discussed by Simpson (1928, Figure 8) exhibits a similar
condition with respect to the foreshortened cuboid facet and depres-
sion at the extreme ventral side of the proximal end.
The metatarsals of Pliometanastes are notable for their slender
proportions and for their terminally restricted muscle scars. In the
small Blancan 3legalonyx from Hagerman, Idaho for example, the
least depth of the shaft of metatarsal IV is 27.7 mm and the least
width is 23.5, whereas in P. protistus the corresponding dimensions
are 16.4 and 18.5 mm. Although the areas of muscular and tendinous
attachment at the extremities of this element are closely comparable
in these two taxa, they are smaller and occupy proportionally less of
the shaft in Pliometanastes. Although the genus .- galonliyx encom-
passes considerable variation in these same features-compare the
stocky specimen of M. ijffcr-,n(i californicus Stock (1925, fig. 49)
with the slender one of M jeffersoni in Leidy (1855, pl. XIII)-Plio-
metanastes falls well beyond the scope of variation observed in Mega-
lonyx.
PHALANX I OF DIGIT II. A single well preserved right phalanx I
of digit III of the pes, UF 9459, is represented. Its distal end differs
considerably from the pes of Iapalops and rcgalony.x in the shape of
the condyles, the depth of the median groove between the condyles,
and the proximity of the condyles to each other.
The distal condyles of the McGehee phalanx I are considerably





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 281

more rounded than in Hapalops or Megalonyx, where they are quite
flattened. In these two genera the lateral wall of each condyle is
flattened to form a ridge, whereas in Pliometanastes the lateral wall
is smoothly rounded with no ridge.
The groove between the condyles is relatively deeper and nar-
rower, the condyles being closer together in the McGehee phalanx I
than in either 3Mclialon.\ or Hapalops. A ridge around the dorsal mar-
gin encloses a pit at the dorsal end of the groove between the con-
dyles in Pilomctaiadtc,. This ridge is poorly developed in Hapalops
and is absent in Megalonyx.
Both Pliometanastes and Hapalops lack the dorsal lip above the
proximal articular surface for metatarsal III, which is well developed
in 3lcgaloi~y\. The proximal articular facets for the metatarsal III in
Pliometanastes are similar in size to those of Hapalops and lcegalotnyx.
In all three genera the medial facet is larger than the lateral and the
proximoplantar facet on the medial side is about twice as large as on
the lateral side.
The proximoplantar facets in the McGehee phalanx I are most
similar to those in the same element of IMgalonyx. In both genera the
medial facet is trapezoidal; the lateral facet is triangular in 31cglonUyx
and nearly so in Pliometanastes. In Hapalops these facets project dis-
tally, but not so in these two genera.
In both Fliomticail tcia- and Megalonyx the proximoplantar border
is nearly flat, whereas in Hapalops it is distinctly concave. The proxi-
mal facets for metatarsal III agree with those in Hapalops and differ
from those in Megalonyx in having the broad articular facets meet the
facet for the carina abruptly at the right angles, whereas in Mega-
lonyx the broad facets turn anteriorly and grade into the carinate
facet.
In Pliometanastes phalanx I lacks the hook-like projection above
the medial facet for mt III in 3Ic'al1orii/ and Hapalops. The distal
articular surface of the first phalanx is relatively well rounded in Plio-
metanastes, indeed even the phalanges of Hapalops are more flattened
on this surface. This contrasts sharply with the flattened articulation
in Megalonyx which restricts movement between the first and second
phalanges and leads to fusion of these elements in late Pleistocene
time.
UNGUAL PHALANGES.-Five ungual phalanges are preserved in the
McGehee collection. They are readily recognized as megalonychid
claw cores by their strong lateral compression, with a triangular
cross-section as in Mc,'aloti!. and the Santa Cruz megalonychids.





BULLETIN FLORIDA STATE MUSEUM


The largest claw in the collection, UF 9460 is believed to repre-
sent phalanx 3 of digit III of the pes. The bony sheath is partially pre-
served on one side, and the very anterior tip is lacking. The base is
relatively more robust than in any of the other McGehee claws. On
the anterior half of the base is a well-marked discoid area for tendon
attachment. Posterior to this area two nutritive foramina, equal in
size, open to the distal part of the claw. The dorsal surface of the claw
is broadly convex at the proximal end and narrows about halfway to
the tip of the phalanx. From this point to the tip it has a narrow dor-
sal edge and is triangular in cross section. The two articular concavi-
ties are symmetrical, of equal width and depth. At the dorsal and
plantar extremities of the median ridge between them the ridge is
squared instead of pointed as in the other claws. The posterior dorsal
surface above the median ridge is deeply indented.
The smallest claws, UF 10340 and UF 9554, are complete and lack
only the bony sheath. The area for tendon attachment on the base is
a small, raised area anterior to the two nutritive foramina, but it is
not a well-marked discoid area as in phalanx 3 of digit II of the pes.
The dorsal surface is not so broadly convex at the proximal end as in
the large claw of the pes. The dorsal surface does not narrow as in
that element to form a sharp ridge, but remains more nearly convex
to the anterior tip. The two articular concavities are not divided by a
distinct median ridge as in the phalanx 3 of digit III. A slight rise be-
tween the two concavities fades out completely at the dorsal half of
the inner surface. In UF 9554 the dorsal and plantar extremities of
the proximal end are more pointed than in UF 10340, and the dorsal
border of the proximal end is not indented as in phalanx 3 of digit III
of the pes.
The claw UF 9462 is intermediate in size between the claws dis-
cussed above and lacks the bony sheath and anterior tip. It differs
from the others in that the articular concavities are not so symmetri-
cal, and one concavity extends farther proximally than the other. The
median ridge between the two concavities is well defined. This median
ridge is pointed, at both dorsal and plantar proximal extremities, un-
like the squared condition in the phalanx 3 digit III. On the dorsal
border, above the articular surface for phalanx II, is an indentation,
but it is not so deep and well defined as in phalanx 3.
A claw from the small Sumter County, Florida Mcualotuni. is
slightly larger than the McGehee claw UF 9462. The base of the
Sumter County claw is more robust and the dorsal projection at the
proximal end is longer with a greater dorsoplantar height than in


282


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1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 283

Pliometanastes. Along the entire dorsal length of the phalanx the
claw core of the Sumter County specimen is less compressed than in
Pliometanastes. However, these may not represent the same digital
elements.
?Pliometanastes galushai, new species
TYPE.-FAM 77811, right half of mandibular symphysis with ca-
niniform root and part of right ramus containing two anterior molari-
form teeth.
ETYMOLOGY.-Patronymic for Ted Galusha of the American Mu-
seum of Natural History, in recognition of his extensive contributions
to vertebrate paleontology in general, and to North American gravi-
grade collections in particular.
DIAGNOSIS.-A small megalonychid with long, relatively deep sym-
physeal spout, posterior end of symphysis below caniniforms, triangu-
lar caniniforms, and transversely broadened, subrectangular molari-
forms.
AGE AND LOCALITY.-Hemphillian, from horizon stratigraphically
between Round Mountain and San Juan Quarry. Collected by Ted
Galusha in 1947 below lower Tuffaceous Zone, San Juan locality, north
of Espanola, Lyden Quadrangle, New Mexico.
DESCRIPTION.-The type and only specimen is from a subadult, as
indicated by the slight taper along the teeth and by the porous nature
of the bone. (Fig. 20).







20mm
FIGURE 20. Mandible of Pliometanastes galushai, FAM 77811, lateral and oc-
clusal views.

The symphyseal region produces a long spout that reaches at least
14 mm anterior to the caniniform. Whether the spout was pointed or
parabolic at the tip is not determinable. The anteroventral border
presents a convex to flat profile. As in Pliometanastes the symphysis
terminates posteriorly between the caniniform roots, not farther pos-
terior as in Acratocnus and most West Indian megalonychids. The
symphyseal depth at this point is 24.5 mm.






BULLETIN FLORIDA STATE MUSEUM


As in Pliometanastes and several West Indian megalonychids, the
caniniform tooth presents a subtriangular cross section. The lingual
wall is slightly convex; the apex of the triangle is labial.
The post caniniform diastema is 13.1 mm long; it is longer than
in Paulocnus and approximates that of Pliometanastes protistus.
The two molariforms are wider than long (10.0 x 5.3 mm). Unlike
cheek teeth of Pliometanastes and Megalonyx, they are subrectangular
rather than subtrapezoidal in cross section. The first and second
cheek teeth closely resemble one another. In their shape and small
size, they present a strong resemblance to those of Acratocnus or
Hapalops.
The unique combination of features in the type clearly separates
this species from other North American and West Indian megalony-
chids, but the limited material makes it difficult to decide to which
described genus it has closest affinities. If comparable juvenile speci-
mens of P. protistus were known, the two species might be seen to be
closely related. The structure of the symphysis and caniniform both
agree more closely with those in P. protistus than with those in other
known megalonychids, but the geometry of the cheek teeth suggests
fundamentally different and more primitive relationships. The post-
caniniform constriction may be a less important, possibly ontogenetic
difference. Further interpretations must await better material of these
taxa.
Other Hemphillian Pliometanastes
DUNNELLON PLANT 6, FLORIDA. A well preserved black second
left metacarpal (V-2877, Florida Geological Survey), collected from
Plant 6, Dunnellon Phosphate Company near Dunnellon, Marion Co.,
Florida, is almost identical in size and features to that described from
McGehee Farm. On that basis and because of its geographical prox-
imity it is assigned to Pliometanastes protistus.
Box-T, HIGGINS, OKLAHOMA. M. F. Skinner collected three iso-
lated megalonychid teeth in 1939 from Pit 1 of the Box T quarries.
A pair of lower caniniforms, FAM 77809, and a lower right first
molariform, FAM 77808, probably belong to a single individual. The
caniniforms clearly would fit the alveoli in P. protistus. They are
subtriangular in cross section, with the presumed lingual side slightly
convex, the apex of the triangle labial. At the occlusal surface the
tooth measures 11.7 mm anteroposteriorly and 10.4 transversely. The
shearworn surface slopes anteriorly, but the angle is only about 20
degrees below the horizontal, as compared with an angle of over 50


284


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1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 285

degrees in Choloepus. Pliometanastes caniniforms do not change
shape in any important way with age, as the cross sectional shape
remains unchanged through a length of over 48 mm in these speci-
mens. A faint ovate wear facet occupies the posterior vertical wall
of the caniniform. The first molariform is slightly longer on the labial
than on the lingual side. It measures 12.6 long by 15.0 wide. Faint
grooves occupy the lingual and anterior faces.
NORTH SANTA CLARA CANYON, NEW MEXICO. The posterior por-
tion of a braincase, FAM 77810, was collected in 1946 by Ted Galusha
from the third promontory in Santa Clara Canyon north of Pueblo
San Juan. Its stratigraphic position lies between those of the San
Juan and Leyden quarries. The specimen agrees in size and detailed
morphology with the cranial material of Pliometanastes from McGehee
Farm. In particular this specimen exhibits the low occiput, absence
of sagittal crest, protrusive occipital condyles and nuchal depres-
sions characteristic of Pliometanastes.
KLIPSTEIN RANCH, CALIFORNIA.- From the Klipstein Ranch 3,
Upper Caliente formation of the Caliente Range of California, UCMP
V-5684, are several fragments of an upper dentition plus M M3 of
the right side and M1 M2 of the left side (Table 14). The teeth of
the right side are all broken below the occlusal surface, except the
caniniform which has a sharp-pointed wear surface.

Table 14. MEASUREMENTS OF UPPER TEETH OF Pliometanastes FROM
KLIPSTEIN RANCH, CALIFORNIA IN MM
Right Left
Anteroposterior Mediolateral Anteroposterior Mediolateral
C1 13.4 9.0 13.5 9.3
M1 12.2 16.4 12.6' 17.81
M2 13.2 19.5 14.4 19.81
M" 11.3 19.5
1Measured at the occlusal surfaces; all others measured at uppermost margin.

The caniniform is subtriangular approaching trihedral as in Plio-
metanastes. It measures 13.4 mm anteroposteriorly and 9.0 mm
mediolaterally and is smaller than any of the molariform teeth,
unlike the situation in Megalonyx where the caniniform is larger
than the molariform teeth. The narrowest part of the caniniform is
anterior, opposite that of the caniniform alveolus of the mandible
from McGehee Farm. The medial side is slightly convex forming
an acute angle at the posteromedial side. On the posterolateral
side the tooth bulges outward completing the triangle. The shape




BULLETIN FLORIDA STATE MUSEUM


of the caniniform is the only feature that distinguishes this specimen
from Megalonyx. The molariform teeth are smaller than those of
Megalonyx leptostomus but do not differ significantly in shape.
Other Hemphillian Megalonychidae
JIM SWAYZE QUARRY, KANSAS. A left intermediate upper molar-
iform tooth, FAM 77804, was collected by M. F. Skinner from the
Jim Swayze Quarry of Hemphillian age. It has a heavily worn
anterior slope, is narrower labially than lingually, and has a shallow
concavity on the labial side. Its dimensions are 19.7 by 12.5 mm.
It cannot be determined with certainty whether this tooth pertains
to Megalonyx or to a large individual of Pliometanastes.
GUYMON, OKLAHOMA.- A mature left intermediate upper molar,
FAM 77806, is from the Guymon Quarry. Its dimensions are 14.0 by
9.8 mm. It is heavily worn anteriorly. In occlusal view the posterior
side is slightly convex. The lingual side of the tooth is considerably
wider than the labial side.
HEMME HILLS, CALIFORNIA. An essentially complete left fibula
was collected from the Hemphillian Hemme Hills Site, UCMP V-5053,
Contra Costa County, California. This fibula is small, the greatest
length measuring 235.7. The shaft is distinctly curved distally and
has a prominent area for attachment for the peroneus brevis muscle,
as in Megalonyx.
MT. EDEN, CALIFORNIA. Frick (1921) described a mature inter-
mediate upper molariform tooth from the Mt. Eden fauna. Its dimen-
sions are 18.8 by 12.3 mm. It is slightly larger than the corresponding
tooth of Pliometanastes protistus, but is an inadequate basis for a
generic assignment to Mc'galrnyx.
MULHOLLAND, CALIFORNIA. Four postcranial elements are de-
scribed by Stirton (1939) from Mulholland, site 2, of California.
They include a left tibia, a third and a fourth metatarsal and a median
phalanx. The tibia from Mulholland has a prominent astragalar spine
and it must be assumed that the astragalus possessed a deep notch to
receive it. The astragalus of Pliometanastes lacks a prominent V-
shaped notch and presumably a prominent spine on the tibia as well,
but until the variability of this character can be assessed, a generic
assignment on this basis in unwarranted.
RATTLESNAKE CREEK, OREGON. Sinclair (1906) described a mega-
lonychid claw core for which Ameghino (1912) proposed the name
Sinclairia oregoniana. The claw cannot be distinguished from those
of Megalonyx or Pliometanastes, so that the name is a nomen dubium.


Vol. 12




1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 287

Sinclair's supposition that this specimen was from Miocene Mascall
deposits seems unlikely; a more probable interpretation is that it
came from the Pliocene Rattlesnake beds of the same area.
OTIS BASIN, OREGON. A very small proximal phalanx, UO 23275,
measuring only 24.7 mm wide, 30.1 long and 22.1 deep, was collected
from the Hemphillian fauna of Otis Basin (Shotwell, 1963).
KREBS RANCH, OREGON.- A proximal third manual phalanx, UO
8074, and two large and a small second phalanx, UO 8057, 8058 and
8060 cannot be identified generically but further attest to the ubi-
quity of megalonychids in the Hemphillian of North America.
WESTEND BLOWOUT, OREGON. Three elements of a moderate
sized megalonychid, a third metacarpal, UO 9812, a proximal third
manual phalanx, UO 9814, and a partial second phalanx, 9813, occur
in this late Hemphillian site. This material and that from the previous
locality were alluded to by Shotwell (1958).
RELATIONSHIPS
The fundamental subdivisions of sloth phylogeny become evident
by Santacrucian time in South America. Among megalonychoids the
megatheriids (including nothrotheres) reduce and then lose their
anterior sets of caniniform teeth, at the same time expanding their
symphyseal regions into great predental scoops. The true megalony-
chids, by contrast, develop a secant, self-sharpening pair of canini-
forms, in front of which only a modest predental "spout" is retained.
Other major differences, such as the twisted astragalus, subsequently
develop between the megatheriids and the megalonychids. Many
such differences are still reflected in the contrasts between Bradypus
and Choloepus, the two genera of living tree sloths.
Eucholoeps represents the Santacrucian megalonychid stock (Scott,
1903-04). Its arrangement of shearing caniniforms and moderate sym-
physeal spout have been retained by most later megalonychids: by
Pliomorphus and Ortotherium in South America, by Acratocnus,
Miocnus, Syj 'ociius, Paulocnus and most genera in the West Indies,
and by Pliometanastes in North America. The exceptional genera
in which this system has been considerably modified are Megalocnus
in the West Indies, and Mcgaloiny. in North America.
Another primitive feature observed in Eucholeops is the nearly
rectangular shape of its molariform teeth. This pattern is retained
in Miocnus, Synocnus and Acratocnus of the West Indies and in
?Pliometanastes galushai of North America. Most of the late genera
develop subtrapezoidal molariform teeth.





BULLETIN FLORIDA STATE MUSEUM


Important transitional genera from Eucholeops to later forms are
Pliomorphus Ameghino and Ortotherium Ameghino from the late
Miocene of South America. In both genera the caniniform teeth
remain subtriangular, the symphyseal spout is moderately shortened,
and the molariforms subrectangular. The West Indian and North
American megalonychid radiations stemmed from some Miocene
South American sloths closely allied to Pliomorphus. Paulocnus and
Pliometanastes also lie very near that common stock.
Although the West Indian sloths are known only from Pleistocene
deposits, their evident diversity implies a much longer occupation
of the region, possibly from late Miocene time. It is not yet clear
whether this diversity represents one, two or even more independent
invasions by South American pioneer stocks. In the most recent
study of the group de Paulo Couto (1967) avoids explicit discussion
of this question; however, his classificatory headings, separating the
megalocninae from the ortotheriinae, might imply a double invasion.
The simplest interpretation, pending further evidence, remains a
single late Miocene invasion of the West Indies followed by an
insular radiation.
We have compared the various West Indian genera with Pliomet-
anastes in search of some common ancestral pattern. As indicated
below, however, the comparisons are relatively remote.
Megalocnus is the most specialized of the West Indian genera.
Some of the major differences from Pliometanastes are as follows:
rodent-like specialization of the anterior teeth, palate greatly depres-
sed in relation to the basicranial axis, mandible lacking a pre-dental
spout and being considerably more robust posterior to the diastema,
pectoral and deltoid crests of the humerus wholly separate, and tuber
calcis of the calcaneum lacking the degree of posterior expansion
observed in calcanea of Pliometanastes.
Mesocnus differs from Pliometanastes in its meniscoid caniniform,
relatively shallower and more slender mandibular symphysis, and
the absence of an entepicondylar foramen on the humerus. Mteg1 -c-
nus and Mesocnus, however, resemble Pliometanastes and Megalo ilx
in their obliquely set, subtrapezoidal cheek teeth.
Microcnus has a shorter, even smoother cranial roof than Plio-
metanastes. And it has a shorter narrower symphyseal spout. Its
most remarkable specializations are its very small size and the arboreal
modifications of its skeleton, particularly evident in the distinct neck
of the astragalus. These bradypodid-like features distinguish Microc-
nus from all other megalonychid ground sloths.


Vol. 12




1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 289

The genera Acratocnus, Miocnus and Synocnus have subtriangular
caniniform teeth that seem at first to ally them with Pliometanastes
(de Paulo Couto, 1967) but the symphysis in each of these genera
is fundamentally different from the Florida genus: the symphyseal
spout is shallower, narrower and it is abruptly inflected anterior to
the caniniform; the caniniform teeth are set well anterior to the
posterior edge of the symphysis, are separated laterally from the
symphysial groove by a distinct ridge, and are inclined strongly
anteriorly, whereas in Pliometanastes they are nearly vertical. These
genera also have more rectangular, transversely implanted teeth than
Pliometanastes, Megalonyx, Megalocnus and Mesocnus in which the
teeth are set obliquely and are subtrapezoidal in shape. Postcranial
differences separating Acratocnus from Pliometanastes include the
long neck of the calcaneum (Anthony, 1916), the relatively small
tuber calcis with little posterior expansion, and the long slender
proportions of the limb elements.
Perhaps the closest comparison to Pliometanastes may be made
with Paulocnus petrifactus from eastern Curacao (Hooijer, 1962,
1964). It is about the same size as the Florida genus and similarly
lacks a sagittal crest. The lower caniniform tooth approaches the
nearly vertical orientation of that tooth in Pliometanastes, and the
symphyseal spout is relatively broader than in the other West Indian
forms. Also the distance between the caniniform and the next tooth
is relatively short in Paulocnus as in Pliometanastes. However in
Paulocnus, the lower caniniforms are not flattened lingually, the
symphyseal spout is shorter, and the symphysis extends farther
posteriorly, underlying M1. Comparison of limb elements is rendered
exceedingly difficult by the poor preservation of the Curacao material.
However, it is clear that the neck of the calcaneum is more con-
stricted in Paulocnus than in Pliometanastes.
From these comparisons we conclude that of all the West Indian
genera Paulocnus most resembles Pliometanastes. These two genera
probably resemble the ancestral Mio-Pliocene stock that lived in
northern South America. Curacao lies so close to the mainland of
South America that Paulocnus is unlikely to bear any immediate
relationship to the radiation(s) that occurred in the Greater Antilles.
These other West Indian forms appear to be divided into two broad
groups: (1) Mesocnus and Megalocnus, both with subtrapezoidal
molariform teeth; and (2), the remaining genera with more rectang-
ular molariform teeth. Possibly these stem from two separate inva-




BULLETIN FLORIDA STATE MUSEUM


sions; more probably they represent a complex radiation from a
single stock allied to Paulocnus and Pliometanastes.
Pliometanastes is clearly the more primitive of the North American
megalonychids, but it does not follow that this genus is directly
ancestral to Megalonyx. Evidently, Megalonyx already existed along
with Pliometanastes in the Hemphillian. By late Hemphillian time
the genus had attained its essential progressive features. Although
M. mathisi presumably represents a primitive stage within the evolu-
tion of Megalonyx, it occurs too late in the Hemphillian to be part
of the original transition. The common ancestor of the North
American genera must have been pre-Hemphillian. It presumably
ranged south of the United States at that time.
The course of development of the caniniform of Megalonyx has an
important bearing on this phylogenetic reconstruction. Scott (1903-
04) attempted to recognize an early phylogenetic line with Mega-
lonyx-like canines in his Megalonychotherium from the Santa Cruz
beds. However, its teeth are in fact no different from those of
Eucholeops; we, therefore, support Kraglievich's (1925) view that
these two genera are synonymous. Moreover the new records of
Megalonyx suggest that it developed the characteristic canines during
or just prior to the Hemphillian in North America. Pliomorphus shows
some striking similarities to Megalonhiyx in the configuration of the
skull, particularly the basicranial region. The major differences are
found in the low dorsoventrally constricted nasal region of Pliomorphus
and its triangular caniniform teeth.
In order to understand the significance of the long, ovate, blunt-
worn caniniforms of Megalonyx, it is necessary to analyze their
masticatory function. In his excellent study of mastication in tree
sloths, Sicher (1944) emphasized the pattern of dental attrition
facets as the key to a functional analysis. In Bradypus the flat glossy
facets occupy the posterior slopes of the lower molariforms and the
anterior slopes of the uppers, indicating that the principal masticatory
stroke has an anterior component. In Choloepus the opposite occurs:
the principal stroke has a posterior component, even though a lesser
protractive closing phase also occurs when the caniniforms are used
for cutting. This different emphasis in the two Recent sloth genera
is reflected in the zygomatic and mandibular construction (Sicher,
1944).
In Megalonyx, Pliometanastes, and Paulocnus the heavy attrition
facets occupy the posterior edges of the lower and the anterior edges
of the upper molariform teeth. The opposite edges of the teeth are


290


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1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 291

more or less flat. The principal masticatory stroke presumably had
an anterior component as in Bradypus. The same muscular complex
could also have served to engage the self-sharpening triangular
caniniform teeth in Pliometanastes and Paulocnus (Fig. 21).
The transition from a sharp triangular to a blunt ovate caniniform
in Megalonyx is represented by M. mathisi. Placement of the
caniniforms more directly above one another would produce blunt
horizontal occlusion rather than sharp vertical occlusion, with no
major change in jaw movements. The adaptive value of this change
may be associated with a shift from tropical to more temperate forests
where the more sclerophyllous leaves were more readily stripped off
by the broad blunt caniniform teeth. After this pattern became
established, the caniniforms broadened considerably as evidenced by
the change from M11cgalonyx curvidens to M. leptostomus to later
species. As this trend continued, the vertical shearing surface of the
anterior caniniforms lost its effectiveness. A longer more horizontal
occlusal surface between the canines, however, came to function as
an additional grinding mill.



\u /KJPower shear-up and forward
S power grind-up and back



CHOLOEPUS



stopper



Power stroke -
up and forward




MEGALONYX
FIGURE 21. Diagram of dental wear surfaces in Choloepus and Megalonyx.
Shear surfaces solid, grinding surfaces dashed.





W
z
LJ
0 Nuevo Leon Greater Antillean
Megalonyx megalonychid Paulocnus megalonychids nothrotheres megatheres





LU. .\ : *..







Pliomorphus Ortotherium Complex






Eucholoeops and allies Hapolops ond allies Planops and allies

Ld Complex with emphasis on Complex with emphasis on reduction
Z shearing caniniforms or loss of caniniforms
........ 0.. ... ..
o gal ny x:::: ::::o::::::.:.:.......:...:::
............... ..........


FIGURE 22. Phylogenetic diagram of the relationships of megalonychid sloths.





1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS 293

Accompanying enlargement of the caniniforms in Megalonyx, the
symphyseal spout was greatly reduced and the bone between and
behind the upper and lower caniniforms considerably thickened.
M. mathisi represents a primitive stage in this respect as in the little
enlarged caniniform teeth. Analogous modifications evidently occur-
red in Megalocnus. No other changes in the jaw of Megalonyx have
been observed. The articulation and musculature of the jaw and the
occlusion of the molariform teeth appear to remain the same in
Pliometanastes and Megalonyx.
Reduction of the symphyseal spout and buttressing of the al-
veolar bone behind the caniniforms probably represent structural
changes to support heavier wear on the caniniforms. Moreover, the
grasping function of the spout may have been supplanted in part by
the modified caniniforms. Futhermore the caniniforms of Megalonyx
project little above the level of the cheek teeth and thus allow the
tongue greater freedom of movement.
Suggested relationships between known megalonychids are illus-
trated in figure 22.






BULLETIN FLORIDA STATE MUSEUM


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Vol. 12






1968 HIRSCHFELD/WEBB: PLIO-PLEISTOCENE SLOTHS


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