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 spe- cies 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. mexica- nus, 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 south- ern 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. 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 35 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 ori- gins 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