Bruce J. Macfadden*

Home , Astrohippus, Dinohippus, Eohippus, Hippidion, Hypohippus, Hyracotherium

MacFadden, B.J., 2006, Early Pliocene (latest Hemphillian) horses from the Yepómera Local 33 Fauna, Chihuahua, Mexico, in Carranza-Castañeda, Óscar, and Lindsay, E.H., eds., Advances in late Tertiary vertebrate paleontology in Mexico and the Great American Biotic Interchange: Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias, Publicación Especial 4, p. 33–43.

Early Pliocene (latest Hemphillian) horses from the Yepómera Local Fauna, Chihuahua, Mexico

Bruce J. MacFadden*

ABSTRACT

The latest Hemphillian (Hh4) is characterized by a distinctive equid assemblage, four species of which are widespread in North America. One of the largest collections of Hh4 equids is from Yepómera, located in Chihuahua, Mexico. Although Yepómera is actually a series of sub-localities, the equids are morphologically similar from each of these and therefore can be considered as a local faunal assemblage of four sympatric species. The distinctive dental patterns, size of the cheek teeth, and (in most cases) metapodial dimensions, make these horses readily distinguishable in the field. Yepómera equids include the monodactyl Astrohippus stockii and Dinohippus mexicanus, and tridactyl Neohipparion eurystyle and Nannippus aztecus. Yepómera is the type locality for the species A. stockii and D. mexicanus, both described by Lance in 1950. Neohipparion eurystyle and the genus Neohipparion and A. stockii and the genus Astrohippus become extinct at the end of Hh4. Nannippus aztecus is a sister taxon of later Nannippus and D. mexicanus is the sister taxon of Equus, the equid genera that survived in North America during the Plio-Pleistocene. With hypsodonty indices between 2.6 and 3.7, all of the Yepómera horse species have high-crowned teeth, traditionally interpreted as a grazing adaptation. Carbon isotope evidence indicates that these four equid species were principally grazers, indicating that Yepómera during Hh4 was predominantly a grasslands habitat.

Key words: Horses, latest Hemphillian, Yepómera, Chihuahua, Mexico.

RESUMEN

El Henfiliano terminal (Hh4) se caracteriza por una asociación distintiva de équidos, de los cuales cuatro especies tienen una amplia distribución en América del Norte. Una de las colecciones más grandes de équidos del Hh4 proviene de Yepómera, localizada en Chihua- hua, México. Aunque Yepómera es realmente una serie de sub-localidades, los équidos de cada una de éstas son morfológicamente semejantes y, por lo tanto, pueden ser considerados como una asociación faunística local de cuatro especies estrechamente relacionadas. Los patrones dentales característicos, el tamaño de los dientes de los maxilares y, en la mayoría de los casos, las dimensiones metapodiales hacen a estos caballos fácilmente distinguibles en el campo. Los équidos de Yepómera incluyen los monodáctilos Astrohippus stockii y Dinohippus mexicanus, y los tridáctilos Neohipparion eurystyle y Nannippus aztecus. Ye- pómera es la localidad tipo para las especies A. stockii y D. mexicanus, ambas descritas por Lance en 1950. Tanto el género Neohipparion, incluyendo a N. eurystyle, como el género Astrohi- ppus, incluyendo a A. stockii, desaparecen a fines del Hh4. Los géneros de équidos que so-

*Florida Museum of Natural History, University of Florida, Gainesville FL 32611, USA. E-mail address: [email protected]

Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 34 MacFadden

brevivieron en América del Norte durante el Plioceno-Pleistoceno son Nannippus aztecus, taxón hermano del ulterior Nannippus, y D. Mexicanus, taxón hermano de Equus. Todas las especies de caballo de Yepómera tienen dientes con coronas altas, con índices hipsodontes de entre 2.6 y 3.7, lo que se interpretado tradicionalmente como una adaptación a pacer. La evidencia de isótopos de carbono indica que estas cuatro especies de équidos pastaban principalmente, indicando que Yepómera, durante el Hh4, era predominantemente un há- bitat de praderas.

Palabras clave: Caballos, Henfiliano terminal, Yepómera, Chihuahua, México.

INTRODUCTION

The Hemphillian land mammal age, which spans from 9 to 4.8 Ma (Tedford et al., 2004) during the late Mio- cene and earliest Pliocene, records a pivotal interval in horse evolution. As a result of the earlier Miocene ex- plosive adaptive radiation of the Equidae (MacFadden, 1992), early Hemphillian equid faunas in North Amer- ica can have as many as ten sympatric genera, whereas by the latest Hemphillian, this diversity dropped to 4-6 genera at individual fossil localities (Figure 1). All of these horse taxa are high-crowned (Figure 2), a morphological adaptation traditionally interpreted for grazing. After the Hemphillian (i.e., into the Blancan), equid diversity continues to drop, with either two or three coexisting genera. These later horses included the small three-toed Nannippus, which was widespread,

Figure 2. Upper first molars (M1) of latest Hemphillian (Hh4) horses from North America (i.e., Bone Valley phosphate deposits of Florida). A-F shows that these taxa are high-crowned relative to a short-crowned tooth of Hypohippus (G). The species Neohipparion eurystyle (B), Nannippus aztecus (C), Astrohippus stockii (E) and Figure 1. Changes in fossil horse generic diversity in North Ameri- Dinohippus mexicanus (F) are found at Yepómera. The two others, ca. After the maximum equid diversity during the middle Miocene, Pseudhipparion simpsoni (A) and Cormohipparion emseliei (D) ~15 Ma, fossil horses have demonstrated a progressive decline. were endemic to the southeastern U. S. (modified from MacFadden During the late Hemphillian (Hh3 and Hh4), eight genera of horses et al., 1999). are known, including the four represented at Yepómera, i.e., Astro- hippus, Dinohippus, Neohipparion, and Nannippus. (Of the other Cormohipparion, which survived only in the southeast- four genera absent from Yepómera, two were endemic to the southern United States, and monodactyl Equus, which was eastern U. S. [Pseudhipparion and Cormohipparion] and two are only known from the west [Hippidion and Onohippidium]). Modi- widespread and represents the origins of the modern fied from MacFadden (1992). genus.

During the late Hemphillian (6.5 to 4.8 Ma), two The Caltech collections of Yepómera horses were successive biochronologically distinct equid assem- initially described in two publications. As a result of his blages are recognized in North America: The early late Caltech dissertation project, Lance (1950) published (in Hemphillian (Hh3, sensu Tedford et al., 2004; e.g., Spanish) an extensive description of three of the four as seen at Coffee Ranch, Texas) assemblage includes species represented at Yepómera, which he referred to Dinohippus leidyanus, Astrohippus ansae, Neohippa- Nannippus cf. minor, a species originally described rion eurystyle, and Nannippus ingenuus (Tedford et from the Bone Valley Phosphate mines in Florida (Sel- al., 1987). The latest Hemphillian (Hh4) assemblage lards, 1916) and two new species Pliohippus (Astro- includes Dinohippus mexicanus, Astrohippus stockii, hippus) stockii, and Pliohippus (Pliohippus) mexica- Neohipparion eurystyle, and Nannippus aztecus that nus. Stirton (1955) described what we now recognize were widespread in North America (two additional as the fourth species Neohipparion eurystyle, which he horses, Cormohipparion emseliei and Pseudhipparion actually named as two separate species, N. floresi and simpsoni are also known in some southern and mid- N. arellanoi. With two of the four currently recognized continental Hh4 faunas). The extensive collections of latest Hemphillian (Hh4) widespread species originally Yepómera horses are one of the best examples of this described from Yepómera, this equid fauna is of great distinctive latest Hemphillian equid fauna. importance to this time interval, as well as to the origins of later equids, including Equus. ABBREVIATIONS Not surprisingly, the nomenclature of latest Hemphillian horses has evolved. The subgenus Astro- CRNHT, unworn or little-worn crown height; HI, hippus has been elevated to full generic status (e.g., hypsodonty index, ratio of M12 CRNHT/APL; L, left MacFadden, 1992; McKenna and Bell, 1997), so the side; LACM (CIT), Vertebrate Paleontology, Natural small monodactyl horse from Yepómera is now re- History Museum of Los Angeles County, California In- ferred to as Astrohippus stockii. The distinctive skull stitute of Technology collection; M1, 1st upper molar; morphology (reduced preorbital facial fossa relative to M2, 2nd upper molar; M12 APL, anteroposterior length Pliohippus) present in the species P. (P.) mexicanus re- of either M1 or M2, measured in middle (adult) wear; sulted in subsequent referral to Dinohippus mexicanus M12 TRN, transverse width of either M1 or M2, mea- (e.g., MacFadden, 1984a; 1986; Azzaroli, 1988; Kelly, sured in middle (adult) wear; Ma, megannum, millions 1998; MacFadden and Carranza-Castañeda, 2002), a of years ago on the geological time scale; MC III, 3rd genus named by Quinn (1955). Lance (1950) proposed metacarpal; R, right side; UNAM, Universidad Nacio- a close affinity of the speciesD . (“P.”) mexicanus with nal Autónoma de México. Equus (Figure 3), a hypothesis that has since been fur- ther corroborated (e.g., Bennett 1980; MacFadden, SYNOPSIS OF PREVIOUS RESEARCH 1984a; Azzaroli, 1988; Kelly, 1998; MacFadden and Carranza-Castañeda, 2002). Based on Hulbert (1990), As documented elsewhere in this guidebook, we owe the tiny three-toed horse, originally described as N. much of our knowledge about Yepómera horses to the cf. minor (Lance, 1950), a species nomen of dubious extensive collections made by the California Institute validity, is now more properly referred to N. aztecus of Technology, mostly during the 1940s, under the di- (originally described from the Ocote L. F. of central rection of Chester Stock. These collections were subse- Mexico; Mooser, 1968). Despite Stirton’s (1955) de- quently transferred to the Natural History Museum of scription of two new species of larger three-toed horse Los Angeles County where they are nicely curated and Neohipparion, i.e., N. floresi, and N. arellanoi, several available for study. Smaller collections of Yepómera subsequent studies have indicated synonymy of both of horses are deposited in other U. S. institutions, includ- these species with the older name N. eurystyle (Cope, ing the University of Arizona and Southern Methodist 1893) (e.g., MacFadden, 1984b; Carranza-Castañeda University, as well as in Mexico (UNAM). and Miller, 1998).

Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 36 MacFadden

are no significant morphological differences among the taxa from these separate localities, and as such, they are justifiably treated as a single equid fauna con- sisting of the four taxa described here. These horses are morphologically distinguishable based on size (Figure 4), dental pattern, and to a certain extent (i.e., there is individual character variation that can overlap between species), metapodial characters (Figures 5, 6).

Astrohippus stockii Lance, 1950

The monodactyl Astrohippus stockii is the most com- monly encountered horse at Yepómera. Yepómera is the type locality for this species. Although A. stockii is well represented by dental and postcranial material from Yepómera, cranial material, including the taxo- nomically diagnostic preorbital facial region is fragmentary. Nevertheless, with regard to the facial region, the complex facial fossae (Figure 7) of A. stockii have been used to indicate phylogenetic affinity with Plio- hippus s. s. (a genus with a characteristically complex Figure 3. Lance’s (1950) phylogeny of “pliohippine” and closely re- facial fossae; e.g., MacFadden, 1984a). More recently, lated horses, including the two monodactyl species from Yepómera, however, cladistic analyses have indicated that despite currently referred to Astrohippus stockii and Dinohippus mexica- the similarity in facial fossa, a suite of derived charac- nus. Two important points can be seen in this phylogeny: (1) A. ters for A. stockii instead indicate affinity with dinohip- stockii is interpreted as a terminal species, and (2) D. mexicanus is pine equines (e.g., Kelly, 1998). ancestral to “Plesippus” and Equus. With a mean M12 APL of 18.2 mm, mean M12 TRN of 17.1 mm (N=15; Figure 4), and estimated With regard to the larger significance of Yepómera horses for paleoecology and evolution, Sondaar (1968) 28 presented an interesting study of the functional mor- 26 D. mexicanus phology of the equid manus. He asserted that Neohip- 24 parion eurystyle and Nannippus aztecus had more sta- 22 bility on their tridactyl manus, particularly on sandy and muddy substrates, whereas the highly cursorial 20

M12 TRN (mm) N. eurystyle Astrohippus stockii and Dinohippus mexicanus were 18 A. stockii adapted to more open country. 16 N. aztecus

14 THE YEPÓMERA HORSES 12 12 14 16 18 20 22 24 26 28 M12 APL (mm) The Yepómera horses come from numerous sublocali- ties designated in the LACM collections. So far as I Figure 4. Scatter plot of anteroposterior length versus transverse have been able to determine, despite Stirton’s (1955) width of M1 or M2 in middle (“adult”) wear stage of the four equid interpretation of Neohipparion from Yepómera, there species from Yepómera (MacFadden, unpublished).

Prox. diam. in mm 31 MCIII

24 23

22 21

18 17 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Figure 5. Scatter plot of MC III proximal width (x axis) versus shaft diameter (y axis). In addition to showing relative proportions, this graph illustrates a sense of relative abundance of the four equid species from Yepómera, i.e., Astrohippus stockii (○) and Dinohip- pus mexicanus (●) are most common, Neohipparion eurystyle (x) is less common, and Nannippus aztecus (+; one specimen, lower left corner) is very rare (modified from Sondaar, 1968). body mass of ~100 kg (MacFadden et al., 1999), A. stockii was a small horse. The juvenile or little-worn mean M1 or M2 CRNHT is 66.4 mm (N = 5), with a HI of 3.7 (MacFadden, unpublished). In middle wear stage the dental pattern has distinctive characters both in the upper and lower cheek teeth (Figure 8). The cheek tooth crowns are relatively straight, certainly much less curved than Pliohippus. The uppers have simple enamel plications, e.g., on the fossettes, and an anteroposteriorly elongated and transversely flat- tened protocone. The hypoconal groove, which is Figure 6. Comparisons of R MC III of the four Yepómera equid spe- well developed on other equines, is characteristically cies, proximal articular surface (“a”) and medial (bottom) views; 1, Dinohippus mexicanus, LACM (CIT) 3811; 2, Neohipparion eu- poorly developed in A. stockii, except on the M3. The rystyle, 3814; 3, Astrohippus stockii, 3812; 4, Nannippus aztecus, lower cheek teeth have widely flared metaconids and 3778 (modified from Lance, 1950) metastylids with parallel (anteroposteriorly straight) enamel borders, and shallow ectoflexids lacking pli caballinids. The enamel borders of the protoconids There have been a few different interpretations and hypoconids are likewise relatively straight. The of the phylogenetic position of A. stockii. In an ances- metapodials are smaller than either D. mexicanus or tral-descendant context, Lance (1950) advocated that Neohipparion eurystyle and larger than Nannippus A. stockii was the latest-surviving species within the aztecus (Figures 5, 6). Sondaar (1968) noted that with genus Astrohippus and therefore was the end of plio- the presence of a MC V and trapezium in the manus, hippine horses (Figure 3). In a cladistic context, Kelly A. stockii and D. mexicanus were more primitive than (1998) interprets A. stockii as the sister-taxon for dino- Equus in which these bones are relatively reduced or hippine equines, and thus part of the clade that includes absent. Equus (Figure 9).

Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 38 MacFadden

A fossulatus

orbit P.

2 cm sp. cf. “Parahippus” leonensis Parapliohippus carrizoensis Pliohippus fossulatus Pliohippus Pliohippus mirabilis Pliohippus pernix Pliohippus nobilis Pliohippus tantalus Heteropliohippus hulberti “Pliohippus” tehonensis Astrohippus stockii “Dinohippus” leardi “Dinohippus” interpolatus Dinohippus leidyanus “Dinohippus” mexicanus Equus simplicidens preorbital fossae

2 cm Figure 9. Cladogram representing the phylogenetic interrelation- Figure 7. Left partial cranium of Astrohippus stockii from Yepómera, ships of pliohippine and dinohippine horses, and Equus (from LACM (CIT) 3577 showing (A) complex preorbital facial fossae Kelly, 1998). and (B) corresponding dentition, L M2-M3 (modified from Mac- Fadden, 1984a). (1987) estimated the body mass of this species to have been ~ 245 kg, roughly the size of a modern Burchell’s

fossette zebra (Equus burchelli; Walker 1975). A fragmentary A cranial region (LACM/CIT 273/3723; Figure 10; see previous page) indicates a weakly developed dorsal preorbital fossa, similar to what is known in primitive hypoconal protocone Equus, e.g., E. simplicidens (also see description of groove 2 cm cranium from the Hh4 Ocote L. F; UNAM IGM 7596; B MacFadden and Carranza-Castañeda, 2002). metaconid metastylid The diagnostic characters of the upper dentition (Figure 11) include moderately curved cheek teeth, an

hypoconid protoconid ectoflexid elongated protocone with a well-developed anterior projection and lingual groove (giving this structure a Figure 8. Cheek teeth of Astrohippus stockii from Yepómera, in ‘wooden-shoe’ shape in P3-M2), crescent-shaped fos- middle (adult) wear stage. A, L upper P2-M3, LACM (CIT) 3621; settes with moderately developed plications. In con- B, L p2-m3, LACM (CIT) 3579 (modified from Lance, 1950). trast to A. stockii, D. mexicanus has a relatively well developed hypoconal groove. The lower cheek teeth Dinohippus mexicanus (Lance) 1950 have rounded metaconids and metastylids that are well separated until late wear stages. The ectoflexids are The monodactyl species Dinohippus mexicanus was shallow in the premolars and deeper in the molars. The named from the hypodigm at Yepómera. This is the pli caballinid is weakly developed, mostly during early largest of the four horses from this locality, with a mean wear stages (MacFadden 1984a; not shown in Figure M12 APL of 24.8 mm (N = 10), mean M12 TRN of 11). The metapodials are relatively robust (Figures 5, 24.1 mm (N = 12; Figure 4), and HI of 2.6. MacFadden 6).

dorsal fossa fossette A

lingual groove protocone hypoconal 2 cm groove

B metaconid metastylid

ectoflexid

Figure 11. Cheek teeth of Dinohippus mexicanus from Yepómera, in middle (adult) wear stage. A, L upper P2-M3, LACM (CIT) 3703; B, R (reversed) p2-m3, LACM (CIT) 3698 (modified from Lance, 1950).

the Texas panhandle (in the Panhandle Plains Museum, MacFadden, unpublished), the range of dental variation Figure 10. Partial cranium of Dinohippus mexicanus, LACM 3723, in “D. mexicanus” encompasses what is also known for from Yepómera showing the preorbital region (A) and correspond- E. simplicidens. A similar population-level analysis of ing dentition (B) in sub-adult (early) wear (modified from MacFad- den, 1984a). D. mexicanus has yet to be done at Yepómera.

Following most workers’ interpretation, the spe- Neohipparion eurystyle (Cope) 1893 cies D. mexicanus is of fundamental importance for understanding the origins of Equus (e.g., Lance, 1950; With a mean M12 APL of 20.7 mm (N = 5), mean Bennett, 1980; MacFadden, 1984a; Azzaroli, 1988; M12 TRN of 18.4 mm (N = 5; Figure 4), and estimated Kelly, 1998; MacFadden and Carranza-Castañeda, body mass of ~141 kg (MacFadden et al., 1999), N. 2002). The traditional interpretation of the first half of eurystyle was a medium-sized extinct horse species. the 20th century was that Equus arose from a species of With a HI of 2.6, Neohipparion eurystyle was a mod- Pliohippus (e.g., Matthew, 1926; Stirton, 1940), which erately hypsodont horse. A partial skull referred to N. Lance (1950) asserted to be ‘P.’ mexicanus (Figure 3). eurystyle, LACM (CIT) 3331 (Stirton’s ‘paratype’ of In contrast to the well-developed complex of preorbital N. floresi) from Yepómera shows a relatively smooth facial fossae in Pliohippus, most species of Equus (par- preorbital facial region (Figure 12) with only a poorly ticularly extant species) lack preorbital facial fossae, developed ventral preorbital depression (MacFadden, so once the importance of these structures was advo- 1984b). The dentition of N. eurystyle is distinctive in cated by Morris Skinner and his disciples, the transition dental pattern (Figure 13). In the upper cheek teeth between Pliohippus and Equus became problematic. the protocone is anteroposteriorly elongated and iso- When Quinn (1955) named the genus Dinohippus, this lated (a distinctive ‘hipparion’ character) during most genus was a large pliohippine with a relatively simple wear stages. A pli caballin is frequently developed and preorbital facial region. With the advent of cladistics, sometimes twinned. The fossette borders are relatively the species “P.” mexicanus from Yepómera was refer- complicated, particularly on the posterior border of the able to Dinohippus and D. mexicanus is the closest sis- prefossette and anterior border of the postfossette. The ter-species of primitive Equus, like E. simplicidens (a styles on the ectoloph are very well-developed, e.g., the view embraced by more recent workers, as cited above, mesostyle is oftentimes flared and sometimes twinned. e.g., Figure 9). In the lower cheek teeth, the metaconids and metas- In certain other latest Hemphillian populations tylids have rounded borders and are widely separated. of fossil horses from Nebraska (Voorhies, 2003) and The ectoflexid is shallow and a pli caballinid is usually

Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 40 MacFadden

fossa Nannippus aztecus Mooser 1968

With a mean M12 APL of 14.6 mm (N = 8), mean M12 TRN of 14.2 mm (N = 8; Figure 4), and an estimated body mass of ~60 kg (MacFadden, 1987), N. aztecus was a tiny horse, of a size similar to that of the extant pronghorn Antilocapra americana (Walker, 1975). With a HI of 3.1, N. aztecus was a relatively hypsodont horse. The distribution of N. aztecus at Yepómera is very interesting. It is generally the rarest of the four equid species, except for CIT sublocality 275 where it is relatively more common (MacFadden, unpublished). No cranial material is known for N. aztecus, in fact none is available for this species elsewhere in North America (MacFadden, 1984b; = N. minor). Likewise, no complete upper dentition is known for N. aztecus from Yepómera and consequently the cheek tooth morphology is illustrated from isolated teeth (Figure 14). Figure 12. Partial skull (A) showing the preorbital region and (B) dentition, LP2-M2, of Neohipparion eurystyle, LACM (CIT) 3331 In addition to its small size, diagnostic characters of the from Yepómera (modified from MacFadden, 1984b). upper cheek teeth include a protocone that is isolated during middle wear stage (Figure 14D is an exception), moderately developed hypoconal groove, pli caballin well developed. The enamel borders of the protoconid poorly developed (Figure 14A is an exception), plica- and hypoconid are relatively straight anteroposteriorly. tions relatively complex on the posterior border of the A well-developed protostylid is a distinctive character prefossette and anterior border of the postfossette, and of N. eurystyle. a very well-developed mesostyle. In the lower cheek N. eurystyle was the latest surviving species of the teeth, the metaconids and metastylids are widely sepa- Neohipparion clade which became extinct at the end of rated and have rounded borders, the ectoflexid is deep, the latest Hemphillian (Hh4). A mesostyle B C prefossette D postfossette

pli hypo. prefossette A mesostyle postfossette caballin groove protocone E pli caballin ectoflexid protocone 2 cm F metaconid metastylid B metaconid metastylid

hypoconid 2 cm protoconid protostylid pli caballinid ectoflexid Figure 14. Upper and lower cheek teeth of Nannippus aztecus from Figure 13. Cheek teeth of Neohipparion eurystyle from Yepómera, Yepómera, in middle (adult) wear stage (modified from Lance, in middle (adult) wear stage, LACM (CIT) 3327 (holotype of N. 1950). A, M1, LACM (CIT) 3758; B, M1, LACM (CIT) 3760; C, floresi). A, L upper P2-M3; B, R p2-m3 (modified from Stirton, LM2, LACM (CIT) 3922; D, LP3, LACM (CIT) 3923; E, Lp2-m2, 1955). LACM (CIT) 3752; F, Rp2-m3, LACM (CIT) 3751.

Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias Publicación Especial 4 Early Pliocene (latest Hemphillian) horses from the Yepómera local fauna, Chihuahua, Mexico 41 a pli caballinid is usually poorly developed or absent that these species separated their niche space based on and the borders of the protoconid and hypoconid are different body size (Figure 15), and there is definitive rounded. geochemical evidence of other kinds of character dis- N. aztecus became extinct at the end of the late placement. Carbon isotopes of tooth enamel indicate Hemphillian (Hh4) and probably therefore gave rise to that the four species of Yepómera horses ranged in their subsequent species of Nannippus by anagenesis. In a diets from hypergrazers to primarily grazers with a cladistic context, N. aztecus is interpreted to be the sis- small percentage of isotopically mixed food in their di- ter-taxon of Pliocene Nannippus, i.e., N. beckensis and ets (MacFadden, research in progress). These geochem- N. peninsulatus (MacFadden, 1984b; Hulbert, 1993). ical results can be interpreted as representing primarily an ancient grassland habitat, but there is little direct evi- CONCLUSIONS, SIGNIFICANCE, AND FUTURE dence (e.g., from plant macrofossils or palynology) to RESEARCH confirm this hypothesis based on carbon isotopes. Paleontologists invariably call for more collecting Because of the wonderful collection of Yepómera hors- to better answer questions despite the fact that wonder- es, which number in the thousands of specimens for ful collections exist already from places like Yepómera. the more common species, this locality is ideally suited For example, although D. mexicanus is already superb- (perhaps the best of its kind for Hh4 local faunas) for ly represented at Yepómera, more collecting is needed more in-depth studies of ‘horseology’ (to use a Morris in the upper (overlying) Concha faunal level to better Skinner term) in the future. This can be at the micro- understand the age of those deposits and the possibil- evolutionary level (within these ancient populations), ity of documenting the evolutionary transition between or between/among species from Yepómera. Dinohippus and Equus. Within species, it would be interesting to see We are fortunate that paleontologists during the if specimens assigned to D. mexicanus within the 20th century realized the importance of Yepómera for Yepómera population display the range of dental mor- understanding fossil horses. The wonderful collections phology that encompasses this species plus E. simplici- that have been made from Yepómera will be available in dens, as has been noted from Nebraska (Voorhies, 2003) the future to answer questions like those posed above, and Texas (MacFadden, unpublished). Following this as well as others that we may not have the techniques theme, comparisons of D. mexicanus with those from or expertise to answer today. other, overlying Blancan localities (e.g., Miñaca Mesa in the LACM) might yield additional insights into the Dinohippus-Equus transition. Stirton was definitely one of the most astute stu- dents of fossil horses in the 20th century and, as such, his interpretations of Yepómera horses are not to be taken lightly. It would be worthwhile to revisit his idea of two species of Neohipparion for what we now consider the single species N. eurystyle from Yepómera. Likewise, an in-depth study of differences in relative abundances of the four species at different localities (e.g., why is N. aztecus common at sub-locality 275 but rare elsewhere at Yepómera) might elucidate the population, or meta- population biology of Yepómera equids. From an ecological point of view, it is interesting Figure 15. Reconstruction and relative proportions of the fossil that the four species of Yepómera equids are all relative- horses from Yepómera (by D. P. Willoughby; modified from Lance, ly hypsodont, with HIs between 2.6 and 3.7. It is clear 1950).

Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006 42 MacFadden

ACKNOWLEDGEMENTS ——— 1984b, Systematics and phylogeny of Hipparion, Neohip- parion, Nannippus, and Cormohipparion (Mammalia, Equi- I thank Ev Lindsay for inviting me to write this paper dae) from the Miocene and Pliocene of the New World: Bul- letin of the American Museum of Natural History, no. 179, for the Yepómera field trip. Sam McLeod was very help- p. 1–195. ful assisting me with the study of the LACM Yepómera ——— 1986, Late Hemphillian monodactyl horses (Mammalia, collections during my most recent visit in 2000. Rich- Equidae) from the Bone Valley Formation of Central Flor- ard C. Hulbert, Jr., and Tabitha Cale kindly read and ida: Journal of Paleontology, v. 60, p. 466–475. provided helpful comments on the manuscript. [This ——— 1987, Fossil horses from “Eohippus” (Hyracotherium) to paper was originally published in the “Guidebook to Equus—scaling, Cope’s Law, and the evolution of body size: Paleobiology, v. 12, p. 355–369. Yepómera Hemphillian and Blancan, Chihuahua, Mexi- ——— 1992, Fossil horses—systematics, paleobiology, and evolu- co Fieldtrip”, 16–18 October 2005, Society of Vertebrate tion of the Family Equidae: Cambridge, Cambridge Univer- Paleontology, 65th Annual Meeting, Mesa, Arizona.] sity Press, 369 p. MacFadden, B.J., and Carranza-Castañeda. Óscar, 2002, Cranium LITERATURE CITED of Dinohippus mexicanus (Mammalia, Equidae) from the early Pliocene (latest Hemphillian) of Central Mexico, and Azzaroli, Augusto, 1988, On the equid genera Dinohippus Quinn the origin of Equus: Bulletin of the Florida Museum of Natu- 1955 and Pliohippus Marsh 1874: Bollettino della Società ral History, v. 43, p. 163–185. Paleontologica Italiana, v. 27, p. 61–72. 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Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico 2006