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Group Title: Cranium of Dinohippus mexicanus (Mammalia Equidae) from the early Pliocene (latest Hemphillian) of central Mexico (FLMNH Bulletin v.43, no.5)
Title: Cranium of Dinohippus mexicanus (Mammalia Equidae) from the early Pliocene (latest Hemphillian) of central Mexico
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Permanent Link: http://ufdc.ufl.edu/UF00099072/00001
 Material Information
Title: Cranium of Dinohippus mexicanus (Mammalia Equidae) from the early Pliocene (latest Hemphillian) of central Mexico and the origin of Equus
Physical Description: p. 163-185 : ill., map ; 28 cm.
Language: English
Creator: McFadden, Bruce J.
Carranza Castañeda, Oscar
Donor: unknown ( endowment )
Publisher: Florida Museum of Natural History
Place of Publication: Gainesville, Fla.
Publication Date: 2002
Copyright Date: 2002
 Subjects
Subject: Dinohippus mexicanus   ( lcsh )
Equidae, Fossil   ( lcsh )
Horses -- Evolution   ( lcsh )
Paleontology -- Pliocene   ( lcsh )
Paleontology -- Mexico   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 181-183).
Language: Summary also in Spanish.
General Note: Caption title.
General Note: "Publication date: August 7, 2002"--P. 2 of cover.
General Note: Bulletin of the Florida Museum of Natural History, volume 45, number 5, pp. 163-185
Statement of Responsibility: Bruce J. McFadden and Oscar Carranza-Castañeda.
 Record Information
Bibliographic ID: UF00099072
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: oclc - 50502095
issn - 0071-6154 ;

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-FLORIDA
-MUSEUM
OF NATURAL HISTORY


BULLETIN


CRANIUM OF DINOHIPPUS MEXICANUS
(MAMMALIA: EQUIDAE)
FROM THE EARLY PLIOCENE (LATEST HEMPHILLIAN)
OF CENTRAL MEXICO, AND THE ORIGIN OF EQUUS

Bruce J. MacFadden and Oscar Carranza-Castafieda


Vol. 43, No. 5, pp. 163-185


2002


UNIVERSITY OF FLORIDA


GAINESVILLE








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CRANIUM OF DINOHIPPUS MEXICANUS (MAMMALIA: EQUIDAE)
FROM THE EARLY PLIOCENE (LATEST HEMPHILLIAN)
OF CENTRAL MEXICO, AND THE ORIGIN OF EQUUS

Bruce J. MacFadden' and Oscar Carranza-Castafieda-



ABSTRACT
A newly discovered skull of Dinohippus is described from the latest Hemphillian (early Pliocene) Rancho El Ocote locality of
Guanajuato, Mexico, which is dated at 4.8 Ma. This cranium is referred to D. mexicanus, the senior synonym for the latest
Hemphillian species otherwise known from several localities in northern Mexico and the southern United States. Although
crushed, this is the most complete skull known for this extinct species. With the diagnostic configuration of the dorsal preorbital
fossa, distinctive dental pattern, and moderate tooth curvature, this cranium demonstrates a morphology similar to, although
slightly more primitive than, that of closely related and slightly more derived Blancan Equus, such as E. simplicidens. As
represented by occurrences in central Mexico and southern California, D. mexicanus coexisted with Equus during the middle
Blancan from about 4.5 to 3 million years ago. Despite traditional interpretations of anagenetic speciation, the current study
demonstrates that primitive species of Equus originated from D. mexicanus by cladogenesis.

RESUME
En este trabajo, se describe el reciente descubrimiento de uno craneo de Dinohippus de la localidad de Rancho El Ocote,
Henfiliano tardio (Plioceno temprano) del estado de Guanajuato. que ha sido fechado en 4.8 Ma. Este crAneo es referido a
Dinohippus mexicanus, senior sin6nimo de las especie del Henfiliano tardio. que se conoce en diferentes localidades del norte
de M6xico y sur de los Estados Unidos. El crineo es mas complete que se conoce de esta especie D. mexicanus. Con la
configuraci6n diagn6stica de la fosa preorbital dorsal, los diferencias en el patr6n oclusal y la moderada curvatura de los
molariformes, este crineo demuestra una morfologia similar a Dinohippus, ligeramente mas primitive que aquellos que mas
relacionados con Equus del Blancano, como E. simplicidens. Los registros en el centro de M6xico y el sur de California,
indican que D. mexicanus coexisti6 con Equus durante el Blanceano medio desde 4.5 hasta 3 Ma. A pesar de los interpretaciones
de especiaci6n anag6nica, el present studio demuestra que los species primitivas de Equus, se originaron de D. mexicanus
por cladog6nesis.

Key words: Dinohippus, mexicanus, Equus, Hemphillian, Blancan, Mexico




TABLE OF CONTENTS

Introduction ....... ........... ............... .................. 164
A cknow ledgm ents ......................................................................... ................. . 164
Materials, Methods, Terminology, and Abbreviations........................................ 164
System atic Paleontology .......................................................... ................... 165
D discussion .......................................................................... ... ................ . .. 16 8
Nomenclature, geographic distribution, and age of Dinohippus mexicanus.... 168
Craniofacial morphology and transitional dental characters ........................ 170
O rig in o f E q u us ................ ............................................................................. 17 5
Sum m ary and Conclusions ..... ................................ ........................ 181
L literature C ited ................................................................................ ................... 18 1
Appendix. Practical method for determining the radius of tooth curvature ......... 184





SAssociate Director of Exhibits and Public Programs and Curator of Vertebrate Paleontology, Florida Museum of Natural History, University of Florida,
Gainesville FL 32611-2710, email: bmacfaddi flmnh.ufl.edu.
2Professor of Paleontology, Unidad de Ciencias de la Tierra, Universidad Nacional Aut6noma de Mexico, Campus Juriquilla. Queritaro, C.P. 76230, M6xico.
B. J. MacFADDEN and O. CARRANZA-CASTANEDA. 2002. Cranium of Dinohippus mexicanu.s (Mammalia: Equidae) from the early Pliocene (latest Hemphillian)
of central Mexico, and the origin of Equsi. Bull. Florida Mus. Nat. Hist. 43(5):163-185. [End ofVol 43.]






BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


INTRODUCTION
The latest Hemphillian (early Pliocene) about 5 million
years ago was a very interesting time in equid evolution
when four to six sympatric species coexisted at many
localities in North America. Several studies have asserted
that the latest Hemphillian species Dinohippus mexicanus
(Lance) 1950 is of prime importance in understanding
the origin of the modern genus Equus. In fact, D.
mexicanus is hypothesized to be the closest sister-species
of primitive species of Equus, such as E. simplicidens
(Cope) 1893 (e.g., Bennett 1980; MacFadden 1984;
Prado and Alberdi 1996; Kelly 1998).
The species Dinohippus mexicanus originally was
described as Pliohippus mexicanus based on a large
collection of latest Hemphillian horses from the Yep6mera
Local Fauna of Chihuahua, Mexico (Lance 1950). This
horse has since been found at other localities in Mexico
and the United States, and although these occurrences
have sometimes been given new species names, they all
seem close to, or conspecific with, D. mexicanus
(MacFadden 1984; Carranza-Castafieda 1992). Over the
past several decades, intensive geological and
paleontological excavations have yielded an excellent
assemblage of latest Hemphillian fossil mammals from
the state of Guanajuato, Mexico. One of the specific
localities in this region, Rancho El Ocote, has a rich and
diagnostic fauna, including abundantly represented
horses, particularly D. mexicanus and another
monodactyl species Astrohippus stockii (Lance) 1950,
but also including the rarer tridactyl Neohipparion
eurvys le (Cope) 1893 and Nannippus aztecus Mooser
1968 [= N. minor (Sellards) 1916; Carranza-Castafieda
and Ferrusquia-Villafranca 1978; also see Hulbert 1990,
19921. Despite many years of field work at Rancho El
Ocote, the large collection of horses from this locality
has until now consisted of isolated teeth, dentitions, and
postcranial remains. The facial morphology of
Dinohippus mexicanus, which is of fundamental
importance to understanding Neogene equid systematics,
was previously unknown from Rancho El Ocote, poorly
represented from Yep6mera, and unknown from other
latest Hemphillian localities (Lance 1950; MacFadden
1984. 1986; Azzaroli 1988).
During a field trip in 1997, the authors visited Rancho
El Ocote and collected a nearly complete, although
crushed, skull of Dinohippus mexicanus, one of only
two known from this species as it is broadly defined (e.g.,
MacFadden, 1984). This new skull is significant because
D. mexicanus is hypothesized to be the closest relative


of primitive species of Equus (Bennett 1980; MacFadden
1984; Prado and Alberdi 1996; Kelly 1998), and the
evolution of craniofacial morphology is critical to
understanding the systematics of Neogene equids (see
review in MacFadden 1992). The new skull from Rancho
El Ocote therefore elucidates previous phylogenetic
hypotheses concerning the origin of Equus. In addition
to the skull of D. mexicanus described here from Rancho
El Ocote, new teeth of equine horses collected from an
early Blancan locality in Jalisco, Mexico, elucidate the
mode of speciation that gave rise to primitive Equus.

ACKNOWLEDGMENTS
This project was supported by NSF EAR 99-02898. We
thank Gerardo Alvarez for specimen preparation, Antonio
Altamira-Gallardo and Erika Simons for photography,
Merald Clark for preparing the line drawings, Dr. Sam
McLeod, Dra. Marfa del Carmen Perrilliat, and Dr.
Richard Tedford for access to, respectively, the LACM,
IGM, and AMNH collections, and Richard C. Hulbert
Jr., Everett H. Lindsay, Charlotte M. Porter, and Fred G.
Thompson for helpful comments that improved the
manuscript. This is University of Florida Contribution
to Paleobiology number 520.

MATERIALS, METHODS, TERMINOLOGY,
AND ABBREVIATIONS
The following vertebrate paleontology collections were
examined during this study and are abbreviated in the
text as follows:
AMNH, American Museum of Natural History, New
York.
F:AM, Frick:American Mammals, part of the AMNH.
IGM, Instituto de Geologia Museum, Ciudad
Universitaria, Universidad Nacional Aut6noma de
M6xico.
LACM, Natural History Museum of Los Angeles County.
UF, University of Florida.
UMNH, Utah Museum of Natural History, University
of Utah.
All measurements are in millimeters, and are reported
to the nearest tenth mm (teeth) or mm (cranial
measurements). Statistical calculations were done using
Microsoft ExcelTM.
The following abbreviations and/or codes are used
in the text:
A; adult wear stage, occlusal pattern moderately worn.
APL; greatest anteroposterior tooth length, excluding
cement.







MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


Figure 1. Method for determining the radius of curvature,
CURV, of upper cheek teeth of fossil horses (modified from
Skinner and Taylor [1967]; also see Appendix). Reproduced
courtesy of the American Museum of Natural History.





C; upper canine.
CURV, curvature of mesostyle, as measured in the radius
of circle inscribed (Fig. 1, taken from Skinner and
Taylor 1967; also see Appendix).
DPOF; dorsal preorbital fossa.
I; upper incisor.
J; juvenile wear stage; tooth unworn or little worn; this
is used to measure individuals that demonstrate maxi-
mum potential crown height (see MSTHT, below).
L; left side.
L.F.; local fauna; a geographically and temporally
restricted fossil assemblage.
M; m; molar (upper, lower).
Ma; megannum, millions of years ago, in reference to a
point in time.
MSTHT; mesostyle crown height.
0; old age, tooth heavily worn; these individuals were
removed from pooled analyses representing
characteristic dental measurements (Table 1).
P, p; premolar (upper, lower).
R; right side.
TRN; greatest transverse width, excluding cement.


SYSTEMATIC PALEONTOLOGY
Class Mammalia Linnaeus, 1758
Order Perissodactyla Owen, 1848
Family Equidae Gray, 1821
Genus tDinohippus Quinn, 1955
tDinohippus mexicanus (Lance) 1950
(Figs. 2-11; Tables 1-2)
Synonomy of Rancho El Ocote referred sample:
large Pliohippus sp. Arellano 1951, p. 613
Hippotigris ocotensis Mooser 1958, p. 360
Protohippus muelleri Mooser 1965, p. 157
Dinohippus muelleri Mooser 1973, p. 258
Equus (Dolichohippus) mesamericanus Mooser 1973,
p. 261
Pliohippus mexicanus Carranza-Castafieda and
Ferrusquia-Villafranca 1978, p. 165
Dinohippus ocotensis Dalquest and Mooser 1980, p. 8
Dinohippus mexicanus Carranza-Castehieda 1992, p. 186
Holotype.-"Pliohippus" mexicanus, LACM-CIT
(California Institute of Technology) 3697, partial L
maxilla with P2-M3 and part of zygomatic arch, from
CIT locality 286, Yep6mera L. F., Chihuahua, Mexico
(Lance 1950).
Referred specimen.-IGM 7596, cranium with R
13. P2-M3, L 12-13, P2-M3; also cast UF 206861.
Locality, Age, and Collector.-Rancho El Ocote,
Ravine de La Caretta, IGM locality GTO 2b,
Guanajuato, Mexico, latest Hemphillian (early Pliocene),
ca. 4.8 Ma. Collected by the authors on 29 May 1997.
Specific Diagnosis.-Medium-sized monodactyl
equine horse, basilar skull length ca. 430 mm; mean
moderately worn ontogenyy = A) M1-M2 APL = 24.5
mm and TRN = 24.0 mm (Table 1). Nasal notch retracted
to a position lying dorsal to the P2. Moderately hypsodont
with mean unworn ontogenyy = J) M1 2 MSTHT = 69.0
mm (Table 1). Cheek teeth moderately curved
transversely, with mean CURV = 73 mm. DPOF
moderately well developed dorsally, ventrally, and
posteriorly, although characteristically lacking a distinct
rim. Long preorbital bar between DPOF and orbit. Malar
fossa very poorly developed, or absent. Cheek tooth enamel
pattern generally simple. Upper cheek tooth protocones
oval and moderately elongated, especially posterior to
the connection with the protoloph, and pre- and
postfossettes crescentic in occlusal cross section with few
plications. Hypoconal groove well developed and persists
until late wear. Lower cheek tooth metaconids and
metastylids have rounded borders and are well-separated,
ectoflexid moderately deep in the premolars and deep in







BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


Table 1. Comparison of dental measurements from Dinohippus interpolatus (from Miami = Coffee Ranch Quarry, Texas, in
AMNH collection), Dinohippus mexicanus (pooled sample from Florida, Texas, and Mexico in F:AM, IGM, LACM and UF
collections), and primitive Equus (pooled sample from Florida, Idaho, Nebraska, Texas, and Mexico in AMNH, F:AM, IGM,
LACM, UF, and UMNH collections). Tooth measurements are taken on Ml or M2. Measurements are reported in the
following sequence for each entry: N, number of specimens measured; x, mean; s, standard deviation; range, i.e., observed
minimum to maximum. See text for abbreviations.

Character Dinohippus interpolatus Dinohippus mexicanus primitive Equus ANOVA Prob' Different?'

APL2 12, 25.8, 0.8, 24.6-27.3 21, 24.5, 1.1, 22.5-26.7 12, 27.2, 1.4, 24.8-29.0 <0.001 Yes
TRN2 12, 25.1, 1.1, 22.8-26.4 20, 24.0, 1.6, 20.7-27.8 12, 28.2, 1.9, 25.1-31.3 <0.001 Yes
MSTHT3 9, 69.0, 4.9, 62.1-78.1 9, 69.0, 6.0, 60.1-77.4 5, 90.3, 5.5, 86.3-99.8 <0.001 Yes
CURV 5, 60, 0, 60-60 6, 73, 6.1, 65-80 11, 115.5, 25.4, 90-185 <0.001 Yes
TRL 3, 159.6, 1.1, 158.4-160.6 3, 157.0, 6.2, 151.0-163.3 34, 188.5, 7.9, 168.8-204.34 -

'ANOVA probability level for the three species and whether or not these samples are statistically different.
2Adult wear stage (A), i.e., juveniles (J) and old age (0) individuals removed from pooled sample.
3Juvenile wear stage (J) in which tooth crowns are unworn or little worn, to indicate maximum MSTHT.
4Data for Equus taken from MacFadden (1989) for sample of Equus simplicidens from Idaho.


the molars, and pli caballinids poorly developed or absent
(also see Lance 1950; MacFadden 1984).
Dinohippus mexicanus differs from contem-
poraneous Astrohippus stockii because of its larger size,
lack of a ventral malarr) fossa, and details of the enamel
pattern (e.g., shape of the protocone and less flared
metaconids and metastylids). D. mexicanus differs from
more primitive species within this genus (such as D.
interpolatus) in less transversely curved cheek teeth and
possibly slightly less defined DPOF. D. mexicanus differs
from primitive species of Equus, such as E. simplicidens,
in smaller size, shorter crown heights, more transversely
curved upper cheek teeth, less elongated protocones with
rounded enamel, and less expanded metaconids and
metastylids with rounded enamel borders.
Specimen Description.-With a mean cheek tooth
row length of 157.9 mm and a mean M12 APL of 23.3
mm (Table 2), IGM 7596 represents a moderately large


equine horse. As evidenced by tooth measurements, IGM
7596 falls at the lower end of the observed range for the
current, geographically broader concept of the species
Dinohippus mexicanus (Table 1). The occipital condyles
are not preserved, but the basioccipital region just anterior
to the condyles indicates an approximate basilar skull
length (i.e., tip of foramen magnum to anterior-most
portion of symphysis; Osborn 1912) slightly greater than
430 mm (Fig. 2). The skull is badly crushed but preserves
many important characters of this species, in particular
the morphology of the facial region and dorsal preorbital
fossa (DPOF). The nasal notch is retracted to a position
that lies dorsal to the posterior half of P2. The infraorbital
foramen appears to lie dorsal to P3. An apparent
depression directly anterior to the DPOF seems to have
resulted from crushing during fossilization. The DPOF
is best preserved on the left side (Fig. 3). It is positioned
high on the cheek -75 mm above the dorsal part of the


Table 2. Dental measurements (excluding cement) of Dinohippus mexicanus, IGM 7596 (UF 206861)
from Rancho El Ocote (GTO 2b), Guanajuato, Mexico.
Measurement 13 P2 P3 P4 Mi M2 M3 P2-M3
R anteroposterior length (APL) 18.3 34.3 26.3 25.8 22.5 22.6 23.8 152.4*
R transverse width (TRN) 10.6 24.9 26.0 25.0 24.2 23.8 21.6
L anteroposterior length (APL) 18.5 35.0 26.0 24.8 25.0 163.3
L transverse width (TRN) 9.9 27.0 26.8 25.5 21.0
*Postmortem gap between M2 and M3 (3.4 mm) subtracted from 155.8 mm to yield 152.4 mm, the latter of which is taken
as the actual P2-M3 APL.










z




N
>

F..











t 1









Figure 2. Ventral view of cranium of Dinohippus mexicanus, IGM 7596 (= UF 206861, cast), from Rancho El Ocote, latest Hemphillian of Guanajuato, Mexico.







BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


/


\-


0 5 10
cm
Figure 3. Left lateral, reconstructed view of cranium of Dinohippus mexicanus, IGM 7596 (= UF 206861, cast) from Rancho
El Ocote, latest Hemphillian of Guanajuato, Mexico.


tooth row (above Ml). The DPOF has well-developed
edges, including the dorsal margin (also see Kelly 1998
for the significance of this morphology). Posterior to the
crushed region that lies anterior to the DPOF, the
anteroposterior length of the DPOF is greater than -65
mm, the dorsoventral height is -25 mm, and this fossa is
-10 mm deep in the center (also see Eisenmann et al.
1988 for measurement conventions), although this depth
may be accentuated by crushing. The posterior margin
of the DPOF is situated far forward of the orbit, i.e.,
there is a long preorbital bar of ~-80 mm. The sutures of
individual bones are not well preserved, but it appears
that the DPOF lies on the nasal and maxillary bones
anterior to the lacrimal bone. The region of the malar
facial fossa, located ventrally, is not preserved on the
left side, but there is an indication of a faint depression
on the right side (not illustrated). As represented on the
right side, a strong transverse malar crest is located on
the ventral region of the face. The muzzle region is very
broad (74 mm) and robust. There is a slight postcanine
constriction (so that the corresponding transverse width
just posterior to the canine is 56 mm).
Although the R & L II and 12 are not preserved, as
inferred from the alvoeli and R & L 13, the shape of the
incisor series is moderately curved (Fig. 2), i.e., it is
neither very rounded, as in some browsing mammals,
nor is it very linear, as in such extinct grazing horses as
Calippus. Neither of the canines is preserved. The post-
canine diastema (R = 54.0 mm, L is not preserved) is
much larger than the precanine diastema (R = 15.8 mm,


L is not preserved). The dPI is absent. The cheek teeth
represent a mature adult in late middle wear. The
perimeter of the tooth crowns is covered with thick
cement. Although direct measurements of CURV cannot
be taken, as represented by the alveolus for the left Ml
and adjoining P4 and M2, the cheek teeth are moderately
curved in IGM 7596 (see Table 1 for CURV for other
specimens of Dinohippus mexicanus, D. interpolatus,
and primitive Equus). The enamel forming the exterior
of the tooth and internal fossettes is relatively thick (Fig.
4). There are prominent parastyles and mesostyles on
the ectoloph. The fossette borders are relatively simple
(in contrast to Equus or some advanced hipparionines)
with no, or one, plication on the anterior border of the
prefossette and posterior border of the postfossette. In
contrast, the posterior half of the prefossette and anterior
half of the postfossette contain one or two plications. The
hypoconal groove is moderately developed. Because the
protocone is at a relatively advanced stage of wear, the
pattern exposed on the occlusal surface is only moderately
distinct. The characteristic advanced protocone seen in
equines, which consists of a wooden-shoe shape with
angular posterior border, is particularly evidenced in M2
and M3.
DISCUSSION
Nomenclature, geographic distribution, and age of
Dinohippus mexicanus.-The genus Dinohippus was
proposed by Quinn (1955) to encompass those species
previously referred to Pliohippus that lack a well-
developed ventral preorbital fossa. Pliohippus sensu







MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


0 5
cm


Figure 4. Occlusal view of right (reversed) P2-M3 of Dinohippus mexicanus, IGM 7596 (= UF 206861. cast) from Rancho El
Ocote, latest Hemphillian of Guanajuato, Mexico.


strict is retained for horses with very complex facial
pits, including those with well-developed dorsal and
ventral fossae. As currently envisioned, Dinohippus is
first known from middle Miocene (?Barstovian, but
certainly Clarendonian) localities from California (D.
leardi (Drescher) 1941 from Black Hawk Ranch; Kelly
1998) and specimens in the AMNH/F:AM collections
from the mid-continent (e.g., Webb 1969; Skinner and
Johnson 1984). Thereafter, Dinohippus is relatively
widespread until the end of the Hemphillian (MacFadden
1992; also see discussion of Blancan localities below).
Several species names have been applied to large
monodactyl horses within the concept of advanced, i.e.,
late Hemphillian, Dinohippus Quinn 1955. During the
early late Hemphillian, as exemplified by localities in
the Texas panhandle (e.g., Coffee Ranch = Frick Miami
Quarry), Dinohippus has traditionally been assigned to
the species D. interpolatus (Cope) 1893 (Matthew and
Stirton 1930, as Pliohippus) and D. leidyanus (Osborn)
1918 from the Snake Creek Formation and equivalent
units in Nebraska (Osbom 1918; Matthew 1924a; Stirton
1940, as Pliohippus). Kelly (1998) describes Dinohippus
interpolatus from Oakdale, California. In addition to
these localities, large, essentially undescribed, early-late
Hemphillian collections of Dinohippus, either referable
to D. interpolatus or D. leidyanus, are represented in
the AMNH/F:AM collections from elsewhere in North
America, including large quarry samples from Edson,
Kansas, Optima (= Frick Guymon), Oklahoma, and
Redington, Arizona, as well as smaller collections from


South Dakota, New Mexico, and Nevada (MacFadden
pers. observ. 2001). All these samples are early-late
Hemphillian, ca. 6 Ma (Tedford et al. 1987). Early-late
(ca. 6 Ma) and latest (ca. 5 Ma) Hemphillian mammalian
faunas in North America contain different equid species
assemblages. The typical early late Hemphillian equid
fauna consists of Dinohippus interpolatus or D.
leidyanus, Astrohippus ansae (Matthew and Stirton)
1930, Neohipparion eury style (or N. gidleyi), and
Nannippus lenticularis (Cope) 1892 (sensu Hulbert
1988, 1993, = Hipparion lenticulare of Matthew and
Stirton, 1930). In contrast, typical latest Hemphillian
faunas contain Dinohippus mexicanus, Astrohippus
stockii, Neohipparion eurst'yle, and Nannippus aztecus
(= N. minor, see Hulbert 1990).
Lance (1950) originally described the species
Pliohippus mexicanus from the Yep6mera L.F. of
Chihuahua, Mexico. Quinn (1955) erected the genus
Dinohippus for larger "pliohippine" horses from the late
Miocene and early Pliocene of North America that lack
the complex facial fossae seen in Pliohippus sensu strict.
As his study was mostly confined to earlier Miocene
faunas from the Texas Gulf Coastal Plain, he did not
address the generic allocation of "Pliohippus" mexicanus
within the genus Dinohippus. Mooser (1973) followed
Quinn's (1955) generic designation, although not
accepting Lance's (1950) species for the large latest
Hemphillian horse from Rancho El Ocote. Mooser first
allocated the Rancho El Ocote Dinohippus to Hippotigris
ocotensis (Mooser 1958), then Protohippus muelleri






BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


(Mooser 1964), and finally, following Quinn (1955), to
the genus Dinohippus as D. muelleri. Dalquest and
Mooser (1980) assert that D. ocotensis from Rancho El
Ocote differs from D. mexicanus from Yep6mera in
having more elongated, angular, and deeply grooved
protocones, and "in having an anterior extension or spur
extending in advance of the isthmus" (p. 10). The exact
significance of this latter character is ambiguous; it seems
to imply the difference in development of the pli caballinid
in the lower cheek teeth. Carranza-Castafieda and
Ferrusquia-Villafranca (1978) and Carranza-Castafieda
(1992) did not accept the validity of Dinohippus muelleri
or D. ocotensis from Rancho El Ocote and other
equivalent-aged localities in Guanajuato and referred
these latest Hemphillian horses to the species mexicanus
(first as Pliohippus, and more recently to Dinohippus).
Other workers (e.g., Bennett 1980; MacFadden 1984;
Prado and Alberdi 1996; Kelly 1998) have mostly
included mexicanus in Dinohippus. Statistical analyses
of measured dental characters of early late and latest
Hemphillian Dinohippus seem to represent continuous
variation within two morphologically similar, closely
related species, i.e., D. interpolatus and D. mexicanus
(the latter sensu lato, i.e., encompassing a pooled sample
from Ocote, Yep6mera, and Florida; Fig. 5). We therefore
assert that the differences used by Dalquest and Mooser
(1980) to distinguish D. ocotensis and D. mexicanus
represent individual variation that can be seen within
different wear stages within a population. Thus, the latest
Hemphillian species of large monodactyl horse is
referable to D. mexicanus and it was widely distributed
throughout southern North America.
With the revised, geographically more inclusive,
concept presented here, Dinohippus mexicanus is known
from several latest Hemphillian fossil localities in North
America (Fig. 6). In central Mexico, D. mexicanus occurs
at Rancho El Ocote (including Arroyo de Carretta, GTO
2b, location of IGM 7596), as well as several other
localities in Guanajuato, including Rinconada (IGM
locality GTO 43; Carranza-Castafieda 1992), Arroyo
Tepalcates (GTO 52), and Rancho San Martin (GTO
42). In Chihuahua, D. mexicanus is known from the
extensive LACM collection from Yep6mera (Lance
1950). D. mexicanus also occurs from the Bone Valley
deposits collected from open-pit phosphate mines in
central Florida (MacFadden 1986). In the Texas
panhandle, D. mexicanus occurs at the Christian Ranch
L.F. and Rentfro Pit 1 locality (Schultz 1977; Tedford et
al. 1987; MacFadden pers. observ. 2001). As far as is


known, D. mexicanus does not occur in the western U.S.
at any localities in California, Oregon, or Washington. It
also does not occur in the northern Great Plains of the
U.S. and Canada (north of the Texas panhandle). Whether
this limited geographic distribution is a result of a paucity
of latest Hemphillian sites (see Tedford et al. 1987), or
represents the actual biogeographic distribution of D.
mexicanus, cannot be determined at the present time.

Craniofacial morphology and transitional dental
characters.-In the context of the present paper, many
essential cranial characters pertain to the development
of preorbital pits, or fossae, in the cheek region on either
the maxillary or nasal bones. The function of these
preorbital fossae, which do not occur in modem Equus
and lack any direct functional analog, is not certain. They
have been purported to be sexually dimorphic and some
workers have argued that these structures are not of
taxonomic utility. We do not intend to rehash this
argument here as it has been amply addressed in the
literature (see review in MacFadden 1992). Suffice it to
say that, as also discussed in MacFadden (1984) for the
Yep6mera horses, the preorbital facial fossa (in this case
the DPOF) is of fundamental importance in understanding
the morphological differences between, and phylogenetic
interrelationships of, late Cenozoic equids. As far as is
known, Pliohippus and Astrohippus have very complex
facial fossa that consist of multiple pits located both
dorsally and ventrally on the cheek region (e.g., Bennett
1980; MacFadden 1984; Kelly 1998). With the exception
of a weakly developed DPOF in Equus grevyi (Skinner
and Hibbard 1972), extant Equus lacks any preorbital
fossae. With the addition of the new specimen (IGM
7596) from Rancho El Ocote described here, the evolution
of preorbital facial fossa in Hemphillian Dinohippus and
primitive Blancan Equus can be further clarified.
In overall morphology of the preorbital fossae,
Dinohippus interpolatus and D. leidyanus are generally
similar (also see Kelly 1998). Although the type of
"Pliohippus" interpolatus Cope 1893 is based on a single
M2 from near Goodnight, Texas (Osbom 1918), large
samples referred to this species are described from Coffee
Ranch, Texas (Matthew and Stirton 1930), and there is
a similarly large, essentially undescribed, collection from
the equivalent Frick Miami Quarry at the AMNH. As
exemplified by F:AM 116171 and 116172 (the latter
being the type of Pliohippus bakeri Azzaroli 1988
(Azzaroli 1988, plate 1), a concept not followed here),
the preorbital facial morphology of D. interpolatus can






MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


32-


30-


28-
E

26-
z
I-
24-


22-
Z


Lu,'


A
0


O
0 O O0
D^o [:] EO

[y D E


22


D[ D. interpolatus


26
APL (mm)


D D. mexicanus E IGM 7596


28 30


O primitive Equus


100- B



90-


E
S80-

E
I-


70- E
D C


60- 0
24 26


n


28 30
APL (mm)


32 34 36


Figure 5. Bivariate plots of dental characters for Dinohippus interpolatus, Dinohippus mexicanus, and primitive Equus. (A)
Anteroposterior Ml or M2 length versus transverse Ml or M2 width, middle wear (juveniles and old age individuals removed).
One outlier of Equus from Sand Draw, Nebraska (APL = 33.1 mm, TRN =28.7 mm) is removed from the plot. (B) Anteroposterior
Ml or M2 length versus unworn to little worn (juvenile) Ml or M2 crown height.


OO


;] D






BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


Figure 6. Selected late Hemphillian and early Blancan localities from North America with occurrences of Dinohippus and
Equus, as discussed in this paper. Christian-Rentfro refers to the Christian Ranch L. F. and Rentfro Pit I locality in the Texas
panhandle (Schultz 1977; Tedford et al. 1987).


be characterized as having a poorly developed ventral
malarr) fossa and a moderately well developed DPOF.
The ventral fossa is essentially a faint, exceedingly
shallow depression lying principally on the maxillary
bone, but with its posterior-most extent developed on the
maxillary-malar suture. The DPOF is a well defined oval-
shaped depression that lies on the nasal and maxillary
bones. Although the general configuration of the DPOF
is consistently developed in different individuals, there
is some variation in the development of the margins of
the DPOF For example, in F:AM 116171 and 128143
of D. interpolatus from Miami Quarry, the DPOF


margins are well defined dorsally, ventrally, and
posteriorly, and these margins are delinated by a curving
of the bone into the pit. In the other well preserved
specimen (F:AM 116172; see Azzaroli 1988, plate 1)
from Miami Quarry, the general configurations of the
fossae are similar, although the dorsal and posterior
margins of this structure are delineated by a distinct rim.
In both specimens, as also seen in other species of
Dinohippus, the anterior part of the DPOF is weakly
defined.
The other early late Hemphillian species of
Dinohippus was originally described as Pliohippus





MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


leidyanus Osborn 1918 from the Snake Creek Fauna of
Nebraska. The holotype, AMNH 17224, consists of a
beautifully preserved cranium and mandible of a subadult
female (with the M3 almost fully erupted but not in wear,
Fig. 7A). Another specimen from the Snake Creek Fauna,
AMNH 18972 (not illustrated here), represents an adult
male in which the malar fossa is essentially absent.
Because the ventral region of the holotype is distorted
by post-mortem crushing, it is not possible to see any
original morphology of the malar preorbital fossa. In these
specimens the DPOF is well developed and its general
morphology is similar to that for D. interpolatus, with
well developed dorsal, posterior, and ventral margins.
Given the description above, the two species of early
late Hemphillian Dinohippus, i.e., D. leidyanus and D.
interpolatus, are very similar in facial morphology, as
they also are in such other characters as size and dental
pattern. In addition to those described above from Coffee
Ranch/Miami and Snake Creek, excellent quarry samples
exist in the AMNH for early late Hemphillian
Dinohippus, e.g., from Optima = Guymon, Oklahoma,
and Edson, Kansas. These large quarry samples could
be used in the future to further assess the intrapopulational
variation in the DPOF (as Skinner and MacFadden 1977
did for Cormohipparion), as well as those characters
that could be used to differentiate the closely related D.
interpolatus from D. leidyanus.
MacFadden (1984) described a partial skull of Dino-
hippus mexicanus (LACM 275/3732; Fig. 7B) from the
latest Hemphillian of Yep6mera (sublocality 275) that
preserves the preorbital region. The Yep6mera fossil
horizons occur within a normally magnetized zone
correlated to the early Gilbert magnetic chron. Lindsay
et al. (1984) present an extrapolated age of ca. 4.6 Ma
for the locality, placing it in the latest Hemphillian, an
age that is roughly the same as for the Guanajuato sites
that contain the same mammal faunas, including the other
skull of D. mexicanus (IGM 7596). Both skulls have
well-preserved preorbital regions in which the DPOF is
well defined dorsally, ventrally, and posteriorly, whereas
the anterior region is confluent with the cheek region.
The malar region is not preserved in IGM 7596, but in
LACM 275/3733 this region is essentially smooth and the
malar fossa is absent. The overall morphology of the
preorbital region in D. mexicanus is therefore similar to
that demonstrated in D. interpolatus and D. leidyanus.
The late Pliocene (Blancan) species of primitive
Equus have been given many names, and there currently
is not universal agreement as to their nomenclature and


distribution. The resolution of the nomenclature of
"primitive Equus" species is outside the intended scope
of this paper. We therefore refer to it here as primitive
Equus, but note that it embodies the concept of, or is
close to, Equus simplicidens, as it was first known from
the Texas panhandle. Primitive Equus, which has been
referred variously to E. simplicidens, E. shoshonensis
Gidley 1930, and Plesippus idahoensis (Merriam) 1918
(Gazin 1936; Skinner and Hibbard 1972; Repenning et
al. 1995), is also known from the extensive collection of
skeletons from the Horse Quarries at Hagerman, Idaho.
One skull of E. simplicidens, AMNH 22077, from
Crawfish Draw, Mt. Blanco, is the type locality for this
species. This nearly complete skull preserves the
preorbital region, including the DPOF and malar fossa.
In this specimen the malar fossa is weakly developed as
a shallow depression on the maxillary and malar bones.
The DPOF has well-developed dorsal and posterior
margins, whereas the ventral and anterior parts of this
structure are confluent with the adjacent cheek region.
By far the most comprehensive sample known for
primitive Equus is from Hagerman, as exemplified by
four adult specimens in the AMNH/F:AM (32555
[female], 32553 [male], 32551 [female], and 32556
[male]). In all these specimens the configuration of the
facial fossae is similar. The malar fossa is usually poorly
developed and represented by a small pit on the maxillary
bone just anterior to the maxillary/zygomatic suture. The
DPOF is present, although it seems slightly less well
defined than in Dinohippus. In particular, the ventral
and anterior margins are not distinct, thus these parts of
the fossa are confluent with the adjoining part of the
cheek region. In the Hagerman sample, the mean length
of the DPOF is 95 mm and the mean height is 36 mm
(see Eisenmann et al. 1988, p. 11, for exact location of
measurements, i.e., B33 and B35). The relative size of
the DPOF in primitive Equus is therefore similar to that
of Dinohippus mexicanus. Hence the morphological
changes in the development of the DPOF from
Dinohippus to Equus involve the weakening of the
anterior and ventral margins, and not a relative reduction
in size.
Several important characters related to size and
dental morphology distinguish Dinohippus mexicanus
from primitive Equus, such as E. simplicidens. Primitive
Equus is a larger horse, with a mean basilar skull length
of 540 mm (N = 4; specimens AMNH 20077, 32551,
and F:AM 32553, 32555), whereas D. mexicanus (IGM
7596) has a basilar length of 430 mm. Although there is


































0 5 10







B D






z
Figure 7. Comparison of lateral views of crania of (A) Dinohippus leidyanus, AMNH 17224, holotype, modified from Osborn (1918), (B) Dinohippus C
mexicanus, LACM-CIT 275/3723, from Yep6mera (from MacFadden 1984), (C) Dinohippus mexicanus, IGM 7596 (= UF 206861, cast), from
Rancho El Ocote, and (D) Equus simplicidens, F:AM 32555, from Hagerman, Idaho.





MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


some overlap of individual specimens (Fig. 5A), the Ml
and M2 APL and TRN of primitive Equus are statistically
larger than in D. mexicanus (Table 1). Primitive Equus
also has significantly higher crowns, with juvenile, little
worn M or M2s having a mean MSTHT of 90.3 mm,
whereas D. mexicanus has the corresponding mean
MSTHT of 69.0 mm (and D. interpolatus also has a
mean Ml or M2 MSTHT of 69.0 mm; Table 1, Fig.
5B). The upper cheek tooth crowns are most highly
curved in D. interpolatus (mean CURV = 60 mm),
moderately curved in D. mexicanus (mean CURV = 75
mm), and less curved in primitive Equus (mean CURV
= 115.5 mm; Table 1). This represents a morphocline
from the more primitive, highly curved crowns found in
Dinohippus during the early late Hemphillian on the one
hand to the relatively straight crowns found in primitive
Equus during the Blancan on the other hand.
The protocone also shows a morphocline from
Dinohippus to Equus (also see Kelly 1998). In relatively
primitive monodactyl horses (pliohippines and
dinohippines, i.e., excluding Equus), the protocone is
small and connected to the protoloph (Fig. 8A). In
"typical" or middle wear of D. mexicanus, the protocone
is characteristically elongated posterior to the connection
to the protoloph and has the advanced "wooden-shoe"
shape in which the lingual margin of the protocone is
concave (Figs. 4, 8B; also see Dalquest 1988). In more
advanced wear this wooden-shoe shape is weakened and
the lingual margin of the protocone is anteroposteriorly
straight. In both primitive and advanced Equus, the
anterior margin of the protocone is greatly expanded
towards the anterior of the tooth (Fig. 8C). In the lower
cheek teeth the metaconids and metastylids of D.
mexicanus are typically rounded, whereas in primitive
Equus they typically have angular borders (also see
discussion below).

Origin of Equus.-The fossil evidence documenting the
origin of primitive Equus during the Pliocene has both
fascinated and perplexed paleontologists since the second
half of the nineteenth century. Marsh (1879) not only
produced his orthogenetically arranged chart of equid
morphological and presumed evolutionary stages, but
also recognized that Pliohippus was a "near ally of the
modern horse" (p. 504). Gidley (1907) revised the
Miocene and Pliocene Equidae from North America and
allocated Cope's Equus simplicidens to Pliohippus
because he believed that this species had more of an
overall resemblance to that genus than to Equus. Based
on some then-recently excavated equid skeletal material


from Mt. Blanco, Texas, Matthew (1924b) proposed the
generic name Plesippus, and also assigned Cope's
material of Pliohippus simplicidens from the same
locality to this new genus. This Pliocene equid genus
was purported to be morphologically similar to (e.g., in
tooth curvature), although more advanced than,
Pliohippus, but also shared similarities (e.g., the great
reduction in the facial fossa) with Equus sensu strict.
Matthew (1926) arranged all North American fossil
Equidae into nine morphological levels, or grades, starting
with Hyracotherium ("Eohippus") and ending with
Equus. As evidenced from the resulting phylogenetic chart
(Matthew 1926, p. 167), Plesippus is depicted as having
been descended anagenetically (i.e., with no temporal
overlap) from Pliohippus in the early Pliocene, and Equus
from Plesippus at the beginning of the Pleistocene.
Stirton (1940, 1942) did not follow Matthew's
designation of Plesippus as a valid genus. Rather, he
considered it a subgenus within Equus. Interestingly, in
these same papers, Stirton mentions the possibility that
primitive Equus was descended polyphyletically from two
or more species of Pliohippus. Stirton (1940, p. 194)
noted: "More careful work, however, needs to be done to
trace the descent of the different species from the Lower
to Middle Pliocene forms." Based on several characters
of the upper and lower cheek tooth morphology, Dalquest
(1978, 1988) and Bennett (1980; reproduced as Fig. 9)
also supported the notion of a polyphyletic origin for the
genus Equus from Astrohippus on the one hand (giving
rise to Asinus) and Dinohippus ("Pliohippus") on the
other hand (giving rise to zebras and caballines).
In a significant departure from the accepted dogma
of the time, Quinn (1955) hypothesized that living equids
are represented by three extant genera, Hippotigris
(zebras), Asinus (asses), and Equus sensu strict (horses),
and that these were descended from a common ancestor,
Eoequus wilsoni Quinn 1955, from the middle Miocene
Hemingfordian Cold Spring Fauna of the Texas Gulf
Coastal Plain. This radial vertical taxonomy has not met
with subsequent acceptance. A legacy of Quinn's (1955)
work, however, was his creation of the new genus
Dinohippus for pliohippine horses lacking the complex
facial fossa seen in Pliohippus. Interestingly, and also at
odds with subsequent studies, Quinn (1955, Fig. 3)
indicated that Dinohippus became extinct in the early
Hemphillian and is not involved in the ancestry of Equus.
Skinner (in Skinner and Hibbard 1972) allocated the
Pliocene species simplicidens to Equus, and included it
in the subgenus Dolichohippus, which has otherwise been
used to denote Grevy's zebra Equus (Dolichohippus)





BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


Dinohippus
interpolatus


Dinohippus
mexicanus


Figure 8. Comparison of protocone shapes and hypsodonty (taken from Dalquest 1988, Fig. 4; scale
not indicated, but presumed to be at, or close to, original scale). Occlusal (top) and lateral (bottom)
views of (A) Dinohippus interpolatus from the Coffee Ranch (= Frick Miami) L. F., late Hemphillian
of Texas, (B) Dinohippus mexicanus from the Rancho El Ocote L. F., latest Hemphillian of Guanajuato,
Mexico, and (C) Equus sp. from the Cedazo L. F., Pleistocene of Aguascalientes, Mexico.


grevyi. Skinner's belief was that the Blancan Equus
species E. simplicidens was widespread throughout North
America, including Mt. Blanco (the type locality),
Hagerman, Idaho (previously referred to as E.
shoshoensis by Gazin 1936), Kansas, and numerous
localities in Nebraska.
Bennett (1980) was the first worker to produce a
cladogram of the interrelationships of Equus and its
closest sister taxon Dinohippus. In the pre-computer
days, this cladogram was "hand drawn," and therefore
did not benefit from the rigorous phylogenetic character
analysis that has developed since her study. In this
scheme, the species D. mexicanus was the closest sister-
species of Equus "shoshoensis." MacFadden (1984)


discussed the interrelationships among Dinohippus,
Astrohippus, and Equus. In contrast to what previous
workers had said about the polyphyletic origins of Equus,
MacFadden (1984) indicated that, based on the very
complex facial fossa morphology, Astrohippus is not
closely related to Equus. If this were the case, as has
been previously asserted based on dental characters, then
Astrohippus with its complex DPOF and malar fossa,
would have to undergo a considerable morphological
transformation in order to be closely related to the origin
of Equus. MacFadden (1984; also 1986) states that
Dinohippus is close to Equus simplicidens, in particular
the advanced species D. mexicanus. Azzaroli (1988)
analyzed the morphology of late Hemphillian monodactyl


Equus
sp.






MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


horses from Coffee Ranch. Although we do not agree
with the designation of a new species, Pliohippus bakeri,
from this important locality (also see Kelly 1998), several
conclusions of Azzaroli (1988) are relevant here,
including: (1) the taxonomic importance of the facial pits
in understanding the interrelationships of late Cenozoic
equine horses, and (2) the close relationship between
Dinohippus (including D. leidyanus [= interpolatus] and
D. mexicanus) and Equus. Although the possible
polyphyletic origin of Equus is still arguable, the current
consensus is that Dinohippus, in particular D. mexicanus,
is the closest outgroup to some segment of primitive
Equus that existed in North America during the Blancan.
Within the past decade, several phylogenetic studies
that include the interrelationships of Dinohippus and
Equus have been presented for Neogene equids from
North America (Evander 1989; Hulbert 1989; Prado and
Alberdi 1996; Hulbert 1996; Kelley 1998). Of these, the
study by Kelly (1998) presents the most comprehensive
cladistic analysis using 40 cranial, dental, and post-
cranial characters for the Tribe Equini, including the
species of Dinohippus, i.e., D. leardi, D. interpolatus,
D. leidyanus, D. mexicanus, and Equus simplicidens.
The synapomorphies that Kelly (1998) uses to justify


his cladogram (Fig. 10) are mostly confirmed, or further
demonstrated, by the cranium of D. mexicanus IGM 7596
described here. In particular, the depth of the nasal notch
(character/state 1.2), configuration of the DPOF (10.2,
5.0, 9.0, 6.0), deep hypoconal groove (26.0), oval
protocone (18.2), and protocone never connected to
hypocone (21.0) are all characters that are seen in the
Dinohippus species morphocline, including D. mexicanus
from Rancho El Ocote.
While we are in general agreement with the obser-
vations presented by Kelly (1998), the development of
the malar fossa deserves some comment here. Kelly
(1998) indicates that the malar fossa is present and well
separated from the DPOF (8.1) in D. leardi and D.
leidvanus, but this feature is absent in D. leidyanus and
D. mexicanus (8.0). Studies of the populations referred
to D. interpolatus and D. leidvanus at the AMNH indicate
that there may be more variation in the development of
the malar fossa and its separation from the DPOF than
is coded in Kelly's (1998) cladistic analysis. Other than
this, Kelly's (1998) study presents a solid basis for
understanding the morphological and inferred phylogetic
transformations seen in the species of Dinohippus.
Kelly (1998) also discusses the synapomorphies that


Table 3. Synapomorphies used to support nodes in cladogram presented by Kelly (1998, Fig. 10 here) for Neogene
equine horses from North America and the character states demonstrated in D. mexicanus from Rancho El Ocote.


Node


Ocote D. mexicanus


Dinohippus-Equus clade (Node 8)
Nasal notch dorsal to posterior half of P2 (character 1, state 2) 1.2
Preorbital bar long (character 10, state 2) 10.2
TRL > 160 mm (character 37, state 4) *
Dinohippus interpolatus (Node 9)
Shallow DPOF (character 5, state 0) 5.0
DPOF posterior margin without rim, no pocket (character 9, state 0) 9.0
Hypoconal groove open to base of crown (character 26, state 0) 26.0
Presence of intertubercular crest (INT) on humerus (character 40, state 1) ?

Dinohippus leidvanus (Node 10)
Dorsal margin DPOF rounded (character 6, state 0) 6.0
Malar fossa absent (character 8, state 0) 8.0
Protocone never connects to hypocone (character 21, state 0) 21.0
Dinohippus mexicanus-Equus simplicidens (Node 11)
Protocone oval (character 18, state 2) 18.2

*With a mean P2-M3 TRLof 157.9 mm for IGM 7596, this is transitional between character/state 37.3 (TRL between
126-160 mm) and 37.4 (TRL > 160 mm).






BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


Figure 9. Origin of Equus from Dinohippus, from Bennett (1980) and reproduced with permission of
the Society of Systematic Biologists.


justify the Dinohippus mexicanus-Equus simplicidens
(= primitive Equus here) node (his 11) with 18.2, the
shared presence of an oval protocone. In addition to this,
our present study indicates that a decrease in CURV can
also be used to justify this dichotomy. Kelly (1998) does
not discuss the synapomorphies that separate Equus from
more primitive Dinohippus, but numerous characters are
presented in Bennett (1980), Hulbert (1989), and Prado
and Alberdi (1996). The results of the present study
indicate, or further confirm previous studies, that the
following synapomorphies separate Equus from
Dinohippus:
1. DPOF more poorly defined;
2. Increased overall size (e.g., as represented by basilar
length, tooth row length, or upper molar dimensions;
3. Increased relative hypsodonty;
4. Reduced CURV; and
5. More flared protocone; metaconids and metastylids
with angular enamel borders.
The question arises as to the mode of speciation that
occurred between Dinohippus mexicanus and primitive
Equus, such as Equus simplicidens. Early workers (e.g.,
Matthew 1926) indicated descent through grades,
suggesting anagenesis. Dalquest (1988) believed that
horse evolution in the Hemphillian and early Blancan


was gradual and probably accelerated in the latest
Blancan and Pleistocene. He further asserted that this
increased rate of evolution provided an example of
punctuated equilibria. Hulbert (1996) depicts Equus as
originating from Dinohippus by anagenesis, i.e., phyletic
speciation during the Pliocene. In fact, recognition and
calibration of the exact evolutionary transition between
Dinohippus and primitive Equus has been difficult to
resolve because of the lack of superposed latest
Hemphillian/early Blancan sites (Lindsay et al. 1984).
Nevertheless, there are two localities that span this
transition and potentially document evidence of the mode
of speciation between D. mexicanus and E. simplicidens.
One of these was previously reported (Downs and Miller
1994) and the other is a new locality from central Mexico
that has transitional morphology represented by isolated
teeth of advanced equine horses pertaining to Dinohippus
and/or Equus.
Downs and Miller (1994) describe late Cenozoic
horses from the well-calibrated sequence in the Anza-
Borrego desert of southern California. Although the
presence of Dinohippus is not surprising from the late
Hemphillian localities, they also describe a specimen that
they tentatively refer to cf. Dinohippus sp. from overlying
levels. If this assignment to Dinohippus is correct, then







MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equis


this occurrence extends the range of this species well
into the Blancan, with a local range for this species from
about 4 to 2.7 million years, indicating temporal overlap
with the known range of Equus simplicidens (sensu
Downs and Miller 1994) at Anza-Borrego.
Newly collected specimens further suggest an
extension for the genus Dinohippus into the Blancan.
These come from the previously unreported Jal-Teco 7
locality from Jalisco in central Mexico (Fig. 6), which is
currently being worked by the IGM. This locality also
has an occurrence of Pliocene equids that are of relevance
to an understanding of the origin of Equus. One level,
Las Gravas, within the continuous Jal-Teco 7 sequence,
which spans late Hemphillian to Rancholabrean, overlies
an ash dated at 4.8 Ma (unpublished data), contains


glyptodonts, capybaras, and two types of equids, and is
interpreted to be early Blancan age. There are several
isolated equid teeth collected in situ from Las Gravas
that represent two distinct morphologies. The more
primitive morphology, which is referable to Dinohippus
mexicanus, includes shorter crowned lower dentitions
(Fig. 11, left) with relatively rounded metaconids and
metastylids. The more advanced morphology, which is
referable to primitive Equus, e.g., E. cf. simplicidens,
includes relatively more hypsodont teeth with greatly
expanded metaconids and metastylids with angular
borders (Fig. 11, right). There are two possible
explanations for this very interesting co-occurrence of
ancestral and descendant species. These two
morphologies could represent: (1) different individuals













Increased size, increased hypsodonty,
reduced CURV, angular and flared
S / protocones, metaconids, and metastylids


Node 10: 6.0, 8.0, 21.0; reduced CURV


SNode 9: 5.0, 9.0,26.0,40.1


Node 8:1.2, 10.2, 37.4


Figure 10. Portion of strict consensus cladogram presented by Kelly (1998; Fig. 10) representing Dinohippus and Equus, with
coded synapomorphous character states justifying each node, as follows: (8) 1.2, nasal notch deep, i.e., dorsal to posterior half
of P2 or deeper; 10.2, preorbital bar long; 37.4, mean TRL > 160mm; (9) 5.0, shallow DPOF, 9.0; posterior DPOF margin
without pronounced rim and no pocket; 26.0, hypoconal groove open to near base of crown; 40.1, forearm intertubercular crest
(INT) only moderately developed; (10) 6.0, DPOF with rounded dorsal margin; 8.0, malar fossa absent; 21.0, protocone never
connects to hypocone; (11) 18.2, protocone elongate-oval. In addition to these characters, the present study indicates the
following for Equus relative to D. mexicanus: reduction in CURV; and for Equus (node not analyzed in Kelly 1998): DPOF
more poorly defined, increased size, increased hypsodonty, further reduced CURV, angular and flared protocones, metaconids,
and metastylids.







BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


cm

Figure 11. Comparison of occlusal (top) and external (bottom) views of lower molars of Dinohippus mexicanus (IGM 7597,
left m3) with Equus simplicidens (IGM 7598, right ml or m2) from the same locality, Jal-Teco7, Las Gravas, early Blancan,
Jalisco, Mexico.


within the same population of one of the two species, or
(2) two sympatric, sibling species soon after the
cladogenesis resulting in primitive Equus. In either case,
this example, along with that from the Anza-Borrego
desert described above, indicate that primitive Equus


originated from Dinohippus mexicanus via cladogenesis
and that there was a time during the Blancan in which
these two sister-species co-existed. This pattern has not
been previously recognized because of the lack of
suitable, well calibrated Blancan localities in North







MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


America or the possible biogeographically restricted
range of sympatry, or both.
In addition to the mode of speciation, it is interesting
to attempt to reconstruct the diets of Dinohippus in order
to better understand the origin of Equus. Although high-
crowned, based on evidence from carbon isotopes and
enamel microwear (MacFadden et al. 1999), Dinohippus
had a variety of diets depending upon the local ecology.
In western North America, Dinohippus (i.e., D.
interpolatus and D. leidyanus) was principally a C4
grazer, whereas in Florida the slightly more advanced,
but closely related, species D. mexicanus had a mixed
diet with a considerable proportion of C3 plant foods,
perhaps representing browse. Ongoing studies
(MacFadden et al. in progress) of the carbon isotopes of
Dinohippus mexicanus from relevant late Hemphillian
localities in Mexico, including Yep6mera and Rancho El
Ocote, will further resolve geographical patterns of diets
in the known southern range of this important species.

SUMMARY AND CONCLUSIONS
The discovery of the new skull of Dinohippus mexicanus
described here adds to knowledge of the previously poorly
represented facial morphology of this important late
Cenozoic equid species. D. mexicanus is morphologically
transitional in the distinctive facial morphology and
dentition with respect to more primitive late Hemphillian
D. interpolatus and D. leidvanus on the one hand and
primitive Blancan Equus on the other hand. While classic
interpretations of the evolution from advanced
pliohippines (Dinohippus in the more recent literature)
to primitive Equus mostly have advocated anagenesis,
the co-occurrence of Dinohippus and Equus in the
Blancan indicates cladogenesis. Dinohippus mexicanus
was widespread in the southern U.S. and Mexico during
the latest Hemphillian and it also ranged into the early
Blancan during this time, although its latter distribution
may have been more restricted. Ongoing studies from
central Mexico will further resolve the calibration and
paleoecology of the Dinohippus/Equus transition.

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BULLETIN of the FLORIDA MUSEUM OF NATURAL HISTORY Vol. 43 (5)


APPENDIX
Practical method for determining the radius
of tooth curvature


As is now well established, the evolution of transverse
curvature of upper cheek teeth (CURV; Fig. 1) is of
principal importance in distinguishing late Cenozoic
horses and their phylogenetic interrelationships, in this
case, the transition from D. interpolatus (and D.
leidyanus), to D. mexicanus, to primitive Equus. As
described by Skinner and Taylor (1967), a practical
method for measuring this character was developed with
a characteristically simple, decidedly "low-tech" method
by M.F. and S.M. Skinner of the AMNH. Oftentimes,
simpler is better and intuitively obvious to the practical-
minded, and such is the case here. The Skinners produced
both: (1) a series of stiff cards (made of oak-tag), each
having one curve (or in some cases two or three curves


for the smaller curvatures) cut out with radii of curvatures
varying from 10 to 310 mm (Fig. Al, left), and (2) a
glass plate with the equivalent (to those of the cards)
curves defined by increasing radii of curvature (Fig. Al,
right). In either of these cases, the researcher can take a
particular tooth and slide it up or down along the glass
plate, or determine the best fit from the cards, so that the
curvature can be determined. Also, the oak-tag cards can
be used for teeth still in maxilla and crania. The method
works wonderfully, and produces measured data that can
be quantified and statistically analyzed, as was done
above (Table 2). Both the "prototypes" described here
are available for use on the second "Horse" Floor in the
Frick Collection at the AMNH.







MacFADDEN and CARRANZA-CASTANEDA: Cranium of Dinohippus mexicanus and the origin of Equus


0cm


Figure A 1. Card cutouts (left) and glass plate (right) used to measure radius of curvature (CURV) of the upper cheek teeth of
fossil horses.








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