The third phalanx is the skeletal element that interacts most closely with the substrate in locomotion. In comparison with other equids, caballoid horses have relatively broad third phalanges. Broad third phalanges are thought to be an adaptation to soft substrates, providing a larger contact area and broader distribution of weight (Eisenmann, 1984; Forsten, 1988; Kuzmina, 1997; Foronova, 2006). Although this characteristic has been analyzed for Middle Pleistocene equids (van Asperen, 2010), this interspecific pattern does not seem to be reflected in variation at an intraspecific level.

Study of Late Glacial (LG) horses (Bignon and Eisenmann, 2006) demonstrates the homogeneity in proportions of the third phalanges at a local scale between different areas, according to the authors p. 169 "related to the global climatic context and the local ecology, namely the food exploitation by horses of river banks and marshes vegetation." New data coming from LGM and LG levels of the site of Coulet des Roches identify slightly narrower proportions than contemporaneous populations, showing that horses at this site frequented less heavy or harder ground habitats (Crégut-Bonnoure et al., 2018) as in the case of the horse from Portugal, E. f. antunesi (Cardoso and Eisenmann, 1989). In contrast, the Late Pleistocene horse E. f. latipes (Gromova, 1949) from the Paleolithic sites of Kostenki, Russian Valley, Don River (Kuzmina, 1997) and Mezin, Desna basin, Ukraine (Belan, 1985) show extremely wide third phalanges.

### Diet and Habitat

Regardless of climatic conditions, Middle Pleistocene horse mesowear has the characteristics of a grazer (e.g., Rivals et al., 2008; Rivals and Lister, 2016; Uzunidis et al., 2017). In grazers, the abrasive nature of the foodstuffs consumed, as well as the relatively high amount of grit found on plants growing at ground level, produce an abrasion-dominated wear pattern with round or blunt cusps with low relief (Williams and Kay, 2001). Middle Pleistocene horses often have some of the highest mesowear scores, and therefore the most abrasive diets, among the species represented in the faunal assemblages (e.g., Rivals et al., 2008). However, occasionally horse populations show variation in mesowear, ranging from grazer to mixed feeder with a significant amount of browse in the diet (e.g., Rivals et al., 2009, 2015). Interglacial horse assemblages are often characterized by a higher amount of variation in diet than glacial assemblages (Rivals et al., 2009).

In contrast to their generally grazer-like mesowear signature, Middle Pleistocene horse microwear is more variable, ranging over the entire spectrum from grazer to browser (e.g., Rivals et al., 2009, 2015; Rivals and Lister, 2016). The same assemblage can have a grazer-like mesowear pattern but a browser-like microwear pattern, possibly due to seasonal variation in diet (Uzunidis et al., 2017). One problem with a browsing microwear signature is that it is not possible to distinguish between browsers eating low herbaceous browse (forbs) or trees and shrubs (Rivals and Lister, 2016). Przewalski horses, for example, consume large amounts of dicotyledons (Schulz and Kaiser, 2013).

Animals that feed on different kinds of foodstuffs tend to have different ratios of carbon and nitrogen isotopic values in their tooth and bone collagen. Carbon isotope values relate to the different photosynthetic pathways used by plants, known as the C3, C4, and CAM pathways. The European Middle Pleistocene is strongly dominated by C3 plants. However, within C3 plants, there is further fractionation between woody and herbaceous plants depending on the openness of the landscape, known as the canopy effect (Heaton, 1999; Drucker et al., 2008). As a result, differences in feeding behavior can lead to different δ 13C values in herbivores (Feranec, 2007b). Nitrogen isotopic values vary primarily with the trophic level at which the species feeds (Bocherens and Drucker, 2003).

Mean δ 13C values for Middle Pleistocene horses tend to reflect grazing in open forest or grassland (e.g., García García et al., 2009; Feranec et al., 2010; Pushkina et al., 2014). However, δ 13C values reflecting foraging in more closed habitats also occur, mainly in temperate assemblages such as those from Steinheim (MIS 11?) and Neumark-Nord (MIS 5e; e.g., Britton et al., 2012; Pushkina et al., 2014). Pleistocene equids tend to have slightly lower δ 13C values than bovids, which may be due to differences in digestive physiology (Britton et al., 2012). There is also variation in horse δ 15N values, to the degree that at some sites, horses appear at a higher trophic level than bovids, their closest competitors for food resources (ibid.). This could be due to the horses consistently selecting lower-quality forage and the way in which their non-ruminant digestive system processes this forage. However, it is more likely that equids and bovids flexibly adapted their feeding strategies to resource availability and the density of herbivores feeding on the same plants (Menard et al., 2002; Britton et al., 2012).

### EUROPEAN WILD ASS *Equus hydruntinus*

Equus hydruntinus, also known as the European wild ass, was first described by Regalia (1907) at the start of the twentieth century from the Upper Paleolithic cave site of Romanelli (Italy), to be precise, in the Upper Complex (≪ Terre brune ≫) attributed to the Epigravettian (Sardella et al., 2018). An exhaustive study of its anatomy and its geographical distribution was made by Stehlin and Graziosi (1935). Many authors have designated Equus altidens (or Equus granatensis Eisenmann, 1999) as the probable ancestor of Equus hydruntinus (Musil, 1969; Forsten, 1986, 1990, 1999a; Azzaroli, 1992; Eisenmann, 1992; Alberdi et al., 1998; Alberdi and Palombo, 2013). This scenario is based in particular on assemblages of intermediate size and chronology (Petralona, Cullar de Baza-1, Venosa) (Van der Made et al., 2017). Affinities with E. tabeti (Aïn Hanech) and E. cf. tabeti (Oubeidiyeh) (Guerrero-Alba and Palmqvist, 1997; Eisenmann, 1999) have also been identified. In contrast, current thinking favors a new arrival from Asia over a local evolution in Europe (Eisenmann, 1992; Palombo and Alberdi, 2017). According to this model, Equus hydruntinus would have entered to occupy the ecological niche left vacant by Equus altidens (Palombo and Alberdi, 2017). At the maximum extent of its geographical expansion in the Late Pleistocene, the distribution of Equus hydruntinus extended from Europe to the south-west of Asia (Middle East, Caucasus) and probably into North Africa (Uerpmann, 1987; Wilms, 1989; Orlando et al., 2006). During the Holocene, the contraction of the steppe, the significant fragmentation of its geographical distribution and distinct subpopulations, and human exploitation progressively caused its disappearance between the Neolithic and the Iron Age (Spassov and Iliev, 2002; Crees and Turvey, 2014), more precisely, according to palaeogenetic information, during the Bronze Age (Bennett et al., 2017). As this hypothesis is subject to certain limitations (cf. Nores et al., 2015), it cannot be excluded that the enigmatic ≪ zebro ≫ mentioned in numerous Medieval manuscripts from the Iberian Peninsula corresponds to Equus hydruntinus (Antunes, 2006).

### Systematics and Phylogeny

It was not until the start of the twenty-first century that the enigma of the phylogeny of Equus hydruntinus was significantly resolved through the contribution of studies of ancient DNA. Hitherto, the systematic position of this equid had long been disputed. The species was variously included in the subgenus ≪ Asinus ≫ (Stehlin and Graziosi, 1935; Gromova, 1949), or ≪ Hemionus ≫ (Azzaroli, 1992), placed close to the zebras (Davis, 1980; Bonifay, 1991), seen as the last representative of the Plio-Pleistocene group of the ≪ stenonids ≫ (Forsten, 1986, 1990, 1999a; Forsten and Ziegler, 1995; Alberdi and Palombo, 2013), or even considered as belonging to a separate ≪ Hydruntinus ≫ genus (Radulesco and Samson, 1965). These multiple hypotheses regarding its affiliation are related to the mosaic of characteristics found in the skeleton and dentition of this species: gracility and cursorial proportions of the limbs as in the hemiones, very short protocones as in the ≪ stenonids≫, a deep ectoflexid as in the zebras (Eisenmann and Mashkour, 1999). It was not until the start of the twenty-first century, with the significant study of the morphology of two crania from the site of Kabazi II (Crimea), that the proximity of Equus hydruntinus to the hemiones became clearer (Burke et al., 2003). The rise of palaeogenetics allowed this affiliation to be confirmed by many mitochondrial sequences (Orlando et al., 2006, 2009). The last palaeogenetic studies to date do not definitively support the classification of E. hydruntinus as a distinct species and places it among the Asiatic wild asses (Bennett et al., 2017).

From a morphological and metrical point of view, many characteristics indeed distinguish between Equus hydruntinus and Equus hemionus. The principal differences can be found in the cranium and teeth. The crania from the site of Kabazi II, and above all the complete cranium from the Emine-Bair-Khosar cave, reveal the main differences with E. hemionus: the length of the palate is greater, the muzzle is proportionally very short and relatively wide, the naso-incisival incision is clearly much shorter (Burke et al., 2003; Orlando et al., 2006; Eisenmann et al., 2008; van Asperen et al., 2012) (**Figure 11**). The species is often referred to as microdont, meaning that the teeth are relatively small in proportion to the postcranial skeleton, but it is also characterized by a high degree of hypsodonty (Stehlin and Graziosi, 1935; Eisenmann and Baryshnikov, 1995; Eisenmann et al., 2008). The mesio-distal length of the M3 is always shorter than that of the M2 (Bonifay, 1991; Boulbes, 2009). The variation in the protoconal index within the tooth row is also different from that of hemiones, with notably an IP which increases from the P4 to the M1 (Eisenmann and Patou, 1980; Cardoso, 1995; Boulbes, 2009). The relative proportions of the limb bones are close to those of hemiones, however, the MC III is proportionally shorter than the MT III (Eisenmann et al., 2008). The frequency of the presence of the small facet for the trapezoid varies between different species of hemiones, but it is more often present than in Equus hydruntinus. The metacarpals and metatarsals of E. hydruntinus have somewhat more robust diaphyses (which are also thicker) (Eisenmann et al., 2008; van Asperen et al., 2012). On the other hand, relatively to the length of the bone (variable 1), the epiphyses (5 to 14) are less developed (**Figure 12**). Finally, the distal keel (12) shows proportionally less protrusion. Other morphological criteria for the different skeletal elements show the ≪ primitive ≫ character state (Bonifay, 1966, 1991; Prat, 1968; Boulbes and Rillardon, 2010).

# Biochronology

The appearance of Equus hydruntinus in the European fossil record is not synchronous across all geographical provinces, which can be partly explained by its rareness.

It seems that the oldest remains of this species in Europe can be found in Spain, in the Middle Pleistocene levels of the section of Vallparadís (E. cf. hydruntinus, level 11, EVT3), which is dated to a little later than 0.6 Ma on the basis of the small mammal fauna (Aurell-Garrido et al., 2010; Madurell-Malapeira et al., 2010; Martínez et al., 2014). In the complex of the Sierra de Atapuerca, Equus cf. hydruntinus was identified at Gran Dolina from level TD 10, correlated with MIS 11-9 (Saladié et al., 2018), in particular from the top of this level (TD 10.2 et TD 10.1), and at Galeria in levels GIIb and GIIIa (Rodríguez et al., 2011; Van der Made, 2013).

In Italy, the first appearance of Equus hydruntinus forms one of the chronological markers of the Middle Aurelian faunal assemblages (Vitinia FU, MIS 8.5 and 7) (Marra et al., 2014, 2018). In France, de caves of Lunel-Viel have long been considered to harbor the oldest remains of the species (Bonifay, 1991). Other fossils from this country could be even older, in particular those

FIGURE 11 | Skull of Equus hydruntinus from the site Emine-Bair-Khosar, Crimea; catalog no. Ba2 384 (photo E. van Asperen; scale in cm; reprinted from van Asperen et al., 2012 © Palaeontological Association February 2012). (A) Lateral view; (B) Occlusal view.

FIGURE 12 | Simpson's ratio diagram of metacarpals from subspecies of extant Hemiones and Equus hydruntinus. Reference: Equus africanus (Eisenmann, 2019). Measurement system (Eisenmann, 2019): 1-Maximum length, 3-Breadth at the middle of the diaphysis, 4-Depth of the diaphysis at the same level, 5-Proximal articular breadth, 6-Proximal articular depth, 7-Diameter of the facet for capitatum, 8-Diameter of the facet for hamatum, 10-Distal supra-articular breadth, 11-Distal articular breadth, 12-Depth of the sagittal crest, 13-Minimum depth of the medial condyle, 14-Maximum depth of the medial condyle. Data: Hemiones, E. hydruntinus from Roterberg and Romanelli (Eisenmann, 2019); E. hydruntinus minor from Lunel-Viel, E. hydruntinus davidi from Arquet (Boulbes, pers. data); Equus hydruntinus from Emine-Bair-Khosar (van Asperen et al., 2012).

from the loess anciens inférieurs at Achenheim (Forsten, 1996, ex E. wernerti ≫) and an isolated tooth from the site of Orgnac 3 (Aouraghe, 1992). Equus hydruntinus is absent from certain very rich fossiliferous sequences, such as those from the Middle Complex of La Caune de l'Arago (MIS 14 to 12) (Moigne et al., 2006). The species is mentioned with caution for the Somme basin in the upper unit (Marne Blanche) of the quarry Carpentier d'Abbeville, recently dated and correlated with MIS 15 (Antoine et al., 2016).

In Germany, Equus hydruntinus is present at Schöningen 13II-4, correlated with the Reinsdorf Interglacial (MIS 9) (Van Kolfschoten et al., 2015) and Steinheim (Forsten, 1999b), the chronological position of which is subject to debate (MIS 11?). In the British Isles, the first appearance of the species is reported during the course of MIS 11 (Pettitt and White, 2012), especially from a number of isolated teeth from the site of Swanscombe (Hoxnian=Holsteinian) (Schreve, 1997; Bridgland and Schreve, 2004).

In the light of these finds and the review of the literature, it seems that the chronological range of E. hydruntinus extends in Europe from the start of the late Middle Pleistocene until the Holocene (**Figure 1**). Its presence in levels dated toward 0.6 Ma in the North of Spain (and perhaps in the North of France), and in contemporary sites in Transcaucasia (Baryshnikov, 2002) and Acheulean sites from central Syria and the site of C-Spring Azrak in Jordan (Clutton-Brock, 1989; Reynaud Savioz and Morel, 2005) could suggest a first incursion of the species during the MIS 15 interglacial followed later by a second wave of migration from MIS 11-9.

# Sub-species or Ecomorphotypes of *Equus hydruntinus*

Several forms of Equus hydruntinus have been described in the palaeontological literature in addition to the type first described by Stehlin and Graziosi (1935) from Romanelli. The name of the subspecies Equus hydruntinus minor indicates the small body size of the population from Lunel-Viel (Bonifay, 1991). It is possible that this form recurred over the course of the chronological range of E. hydruntinus (Boulbes and Rillardon, 2010). The description of this form was based on abundant material, including a cranium, but its affiliation with Equus hydruntinus has been questioned by certain authors (Azzaroli, 1990; Van der Made et al., 2017). Equus hydruntinus danubiensis, in contrast, represents a large-bodied form described by Samson (1975) from Romania. Eisenmann et al. (2008) also include the fossils of a gracile equid from Petralona (E. petraloniensis, Tsoukala, 1991; Tsoukala and Guérin, 2016), dated to 0.4 Ma, within the variation of the species: "E. hydruntinus petraloniensis." According to certain authors, the slender equid from Venosa, considered to be one of the last representatives of Equus altidens (Alberdi and Palombo, 2013), could already represent Equus hydruntinus (Forsten, 1999a).

Another form, Equus hydruntinus davidi, defined by Alimen (1946) based on a single tooth from the Mousterian site of Saint-Agneau (Charente), has a longer protocone. This taxon was subsequently used by Prat (1968) to characterize the elongation of this dental character observed in many sites in the southwest of France post-dating the ≪ first Würmian stage ≫. Following Prat's work, Equus hydruntinus davidi has also been identified from other sites in the south of France (Eisenmann and Patou, 1980; Crégut-Bonnoure and Granier, 1982; Guadelli, 1995; Boulbes, 2009). Differences between teeth from Ramandils cave (MIS 5, Rusch et al., 2019) and Crouzade cave (MIS 3, Saos

et al., 2019) is for example demonstrative (**Figure 13**). Another pattern (connected to the variation of protocone) is the reduction of the depth of vestibular groove on lower molars (Boulbes, 2009). These trends (fixed after MIS 3) could reflect a response to environmental pressures (genetic assimilation), the increase in the length of the protocone favored by the consumption of more abrasive food in cold and steppe climatic context (Boulbes, 2009). Guadelli and Delpech (2000) propose that the geographical barrier posed by the Alps could have fostered the independent evolution of western populations compared to those in the Italy-Balkans province, where populations with dental proportions similar to the type from the Romanelli caves persisted. However, isolated observations show that the morphotype with a long protocone could also have been present in Italy (Conti et al., 2010). The upper teeth of the enigmatic Equus graziosii (Azzaroli, 1979) (Maspino, Italy, Late Pleistocene) resemble those of the morph E. hydruntinus davidi (**Figure 13F**). This species is related to asses (Azzaroli, 1979; Eisenmann, 2006a); proximity with E. hydruntinus has been already pointed by Caloi (1995). This morphotype was also developed on the other side of the Pyrenees in the north-east of Spain, on the site of Teixoneres, dated to MIS 3 (Álvarez-Lao et al., 2017). Equus hydruntinus davidi was also identified from one late Würmian site in Portugal (Cardoso, 1995). We note here the fact that these late populations from the south of France and the north of Spain are more closely similar to hemiones in their morphological characteristics (convergences). The palaeogenetical study (Bennett et al., 2017) supports the hypothesis that hydruntine population could have gone through a bottleneck during the last two glacial periods and that Europe could have been recolonized from a refugial population, as proposed earlier on morphological grounds (Boulbes, 2009).

# Ecology of *Equus hydruntinus*

Like the hemiones, Equus hydruntinus was adapted to semiarid, steppic conditions and shows a preference for temperate climates, although it could tolerate limited cold (Prat, 1968; Delpech, 1984; Eisenmann, 1984; Azzaroli, 1990; Bonifay, 1991; Burke et al., 2003; Palombo and Alberdi, 2017). According to Abbazzi et al. (1996), Equus hydruntinus is primarily adapted to dry Mediterranean conditions and does not necessarily depend on the presence of open landscapes. However, a preference for open biotopes is corroborated by the Holocene distribution of the species, reduced especially to open landscapes in southern and eastern Europe (Crees and Turvey, 2014). Its relationship with temperature is more difficult to ascertain. The species shows a preference for more temperate climates (Caloi, 1995; Forsten and Dimitrijevic, 2004), but it is also present in cool or cold climatic contexts (Prat, 1968; Boulbes, 2009). van Asperen et al. (2012) remark that the crania of E. hydruntinus can be distinguished from related equids by their relatively short muzzles, which indicates an adaptation to cold climatic conditions.

At Steinheim, E. hydruntinus is characterized by δ 13C values that are clearly higher than those for E. ferus (Pushkina et al., 2014). Variation in the δ 18O and δ 13C values show that E. hydruntinus "occupied an ecological niche between the true horses, which inhabited shrubland, and the bovids of the grasslands, similar to that of the wooly rhinoceros, although with a slightly higher temperature and more closed vegetation" (p. 175, Pushkina et al., 2014). However, E. hydruntinus seems to have been less tolerant of climatic variations than the true horses, forcing it to adapt or, where appropriate, to migrate. The high chronological resolution of numerous well-dated sites indeed allows us to posit several phases of displacement of its geographical range over the course of the second half of the Late Pleistocene (Delpech, 2003). During the coldest phases of the Last Glacial Maximum (LGM) the species retreated to the peninsulas of southern Europe (Wilms, 1989; Crees and Turvey, 2014), a scenario that would also have characterized preceding glacial episodes (Boulbes, 2009). For example, its presence is confirmed at the start of MIS 4 in a relatively arid climate in Calabria (southern Italy), where a Mediterranean influence persisted (Marra and Bellomo, 2008). During cold episodes, Equus hydruntinus is accompanied by species that are very welladapted to such a climate, such as for example M. primigenius and C. antiquitatis in the northeast of Spain (Álvarez-Lao et al., 2017), and Saiga tatarica and Rangifer tarandus in the south of France (Crégut-Bonnoure and Paccard, 1997).

## REMARKS AND CONCLUSION

The main dispersal event concerning Equus in Europe (newcomers, in situ and anagenetic evolutionary processes) first Equus, turnover during the post-Olduvai late Villafranchian, caballoid horse's emergence—are connected to major climatic change and document turnover of large mammal communities as well-demonstrated by Palombo and Alberdi (2017). The difficulty with regard to equid morphology is to discriminate between evolutionary and ecomorphological traits. The problem is even more complex because recent equids are known for their high level of plasticity and intra-specific variation, while differences between species are relatively low. Progressive characters such as elongation of the protocone, reduction of the ectoflexid, cursorial adaptations, and protrusion of the distal keel of the metapodials are variable at specific level and could have ecological significance. Despite this difficulty to draw up a phylogenetical and biochronological framework, it is possible to calibrate certain general patterns of evolution. Thus, emergence of elongated protocones in Europe for example can be dated between 1.2 and 0.7 Ma in supposed independant lineages (E. suessenbornsis, E. apolloniensis, E. altidens, E. mosbachensis) at the time of fundamental changes in the behavior of glacial cycles during the Quaternary glaciations and the expansion of open and dry environments.

The inventory of the different stenonid forms shows that at the time of the emergence of this group more than 2 million years ago, a wide range of morphologies occur—large, small, robust, slender—as well as combinations of these types. In contrast, the morphology of the teeth (more genetically dependent) seems to be less subject to variation. As a consequence, evolutionary models based on trend of size and/or gracilization are hard to demonstrate. Large and very large stenonids form a heterogeneous group and revision is needed to clarify their taxonomy and temporal distribution.

From the late Early Pleistocene to early Middle Pleistocene many fossils are assigned to Equus altidens. However, the morphology of the teeth of the Süssenborn equid seems too derived to be included in the variation of all the slender equids from this period. E. granatensis is well-defined and the primitive pattern of the teeth (very short protocones and especially very deep ectoflexids on molars and sometimes deep on premolars) casts doubt on an evolution from a stenonid stock. The presence of the more robust E. wuesti at around 1.0 Ma with proportions of limbs and teeth close to E. granatensis complicates the problem. Revision of this group is needed to solve the question of whether E. granatensis was an endemic species from the south of Spain, a primitive representative of E. altidens or an ecomorphotype (Palombo and Alberdi, 2017). The detailed study of the Epivillafranchian key sites of Vallparadis (Aurell-Garrido et al., 2010) and Bois de Riquet (Bourguignon et al., 2016) would certainly provide new information.

The large E. suessenbornensis has a wide geographical range from Beringia to Western Europe which is accompanied with little variation in terms of morphology and proportions. In contrast, slender species split in several geographical provinces. Interesting is the succession and regionalization of co-occurrences of equids in the course of Early and Middle Pleistocene: E. suessenbornensis-E. granatensis/E. altidens or E. wuesti in western and central Europe and E. mygdoniensis-E. apolloniensis around the eastern Mediterranean. The latter seems not to be linked to stenonid s. s. or E. suessenbornensis but could represent a step within the lineage of asses soon after their differentiation (Eisenmann and Boulbes, in preparation) in accordance with DNA and palaeogenetic projections (Jonsson et al., 2014; Bennett et al., 2017). These lineages finally were replaced by the more generalist caballoid horse group at the beginning of Middle Pleistocene.

Despite the overall homogeneous morphology of true horses during the Middle Pleistocene, paradoxically the ecomorphological adaptations described above coupled with some evolving traits provide real biochronological details in a given geographical province. Since the exact combination of the environmental factors that influence horse morphology subtly varies between the different glacials and interglacials, each oxygen isotope stage was characterized by a specific morphology, as outlined above. However, some caution needs to be taken in using horse morphology as a biostratigraphic indicator. First, the patterns sketched above were identified in northwest and central European horses or south of France, and cannot be transposed directly on other regions, such as southern or eastern Europe, where climatic conditions may have been significantly different. Second, due to the

relatively homogeneous morphology of Middle Pleistocene horses, assemblages analyzed must be sufficiently large to produce statistically significant results. Although there is clear variation between ecotypes, there is also a significant amount of overlap, so any attempt to determine the affinities of small numbers of specimens must be regarded as preliminary. Finally, due to these overlaps and the small differences between ecotypes, it is advisable to apply a range of analytical methods to new assemblages to identify their morphology. Statistical methods such as principal component analysis and discriminant function analysis can disentangle more subtle variations (van Asperen, 2010, 2012). In this regard, recent developments using geometric morphometric approaches (on enamel patterns in particular) provide good phylogenetic and taxonomic markers (Barrón-Ortiz et al., 2017; Cucchi et al., 2017; Hanot, 2018) that may be complementary to traditional morphometry. Comparisons between the results of different methods can aid in solving the question of the phylogeography and regionalization of horses in Europe. Combining metric/morphological data with studies of tooth wear characteristics (mesowear, microwear) and isotopic analyses of teeth (Saarinen et al., 2016; Uzunidis et al., 2017) is the best way to ensure the robustness of palaeoecological inferences from ecomorphological patterns and their use as a proxy.

Several questions remain to be solved concerning Equus hydruntinus. In the first place, its origin, namely whether it corresponds to a local evolution in Europe or whether it entered Europe in one or more waves of migration from Asia. The detailed study of the oldest fossils (Vallparadis) and comparisons with intermediate forms to E. altidens (Venosa, Petralona) will provide essential information. Secondly, its status as a distinct species or subspecies of Equus hemionus. Paleontology has demonstrated the existence of many distinct morphological characters between Equus hydruntinus and Equus hemionus although most are plesiomorphic. The presence in E. hydruntinus of the mitotype of Equus hemionus (samples from 100 Ka) in association with distinctive morphological characteristics implies that Equus hydruntinus can be seen as an ecomorph or a subspecies of Equus hemionus (Bennett et al., 2017). However, according to the first appearance date of the species (0.6 Ma), it is also not impossible to consider that in the recent history of the taxon (0.1 Ma) gene transfer took place by introgressive hybridization. The sympatric cases noted between the two species in the Eemian, notably in the Binagady site (Azerbaidjan) (Eisenmann and Mashkour, 1999), followed by numerous observations during the Upper Pleistocene of characters convergent with the hemiones could support this hypothesis. The particular form of Equus hydruntinus davidi from southern France and the enigmatic Equus graziosii from Italy require further reflection. Palaeogenetic investigations, not only on the mitochondrial DNA but also the nuclear DNA, will in parallel have to multiply on fossils of various geographical origins. Given the difficulties with their identification, it is also not impossible to rule out the existence of several small, slender equidae in Western Europe including Equus hydruntinus, Equus hemionus and possibly other forms not recognized until now.

# DATA AVAILABILITY

All datasets generated for this study are included in the manuscript and/or the **Supplementary Files**.

# AUTHOR CONTRIBUTIONS

NB and EA wrote sections of the manuscript. Both authors contributed to manuscript revision, read, and approved the submitted version.

# FUNDING

This article was funded by the UMR 7194 HNHP, MNHN-CNRS-UPVD, EPCC-CERP Tautavel.

# ACKNOWLEDGMENTS

The authors are very grateful to the two reviewers who helped to improve the manuscript. Special thanks to the editors G. M. Semprebon, F. Rivals, and R. L. Bernor for inviting us to contribute to this research topic Examining Evolutionary Trends in Equus and its Close Relatives from Five Continents. V. Eisenmann kindly gave us some of her personal data and permission to mention new results about E. apolloniensis and E. wuesti, we are grateful to her. G. D. Koufos gave us some data of equids from Mygdonia Basin, thanks to him. Thanks to H. de Lumley and A. M. Moigne who allowed us (NB) to study the important collections of CERP, at Tautavel. We also thank I. Toro-Moyano and B. Martínez Navarro, for welcoming one of us (NB) a few years ago to study the equid from Venta Micena at the Museo de Prehistoria de Orce. The studies conducted on the sites of Les Ramandils, La Crouzade, and Tournal are undertaken in the framework of a Collective Research Program (PCR) of the French Ministry of Culture Le monde moustérien méditerranéen entre Rhône et Pyrénées, dir. T. Saos. The work on the Denizli Basin in Turkey was done in the context of the First Homo in Turkey research program funded by the CNRS (PICS 2016-2018, dir. A. Vialet and with the collaboration of M. C. Alçiçek). Current excavations carried out in the sites of Caune de l'Arago, dir. C. Perrenoud, Bois de Riquet, dir. L. Bourguignon, l'abri du Maras, dir. M. H. Moncel, Portel-Ouest, dir. G. Becam, La Crouzade, dir. T. Saos, Coulet des Roches, dir. E. Crégut-Bonnoure are supported by the French Ministry of Culture. Research in the site of cueva del Ángel is carried out by the Fundación Instituto de Investigación de Prehistoria y Evolución Humana (FIPEH, dir. C. Barroso Ruiz).

### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo. 2019.00301/full#supplementary-material

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**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Boulbes and van Asperen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# What Is Equus? Reconciling Taxonomy and Phylogenetic Analyses

Christina I. Barrón-Ortiz 1 \*, Leonardo S. Avilla2 , Christopher N. Jass 1,3 , Víctor M. Bravo-Cuevas 4 , Helena Machado2,5 and Dimila Mothé2,6

1 Quaternary Palaeontology Program, Royal Alberta Museum, Edmonton, AB, Canada, 2 Laboratório de Mastozoologia, Departamento de Zoologia, Instituto de Biociências, Universidade Federal do Estado do Rio de Janeiro (UNIRIO), Rio de Janeiro, Brazil, 3 Jackson Museum of Earth History, University of Texas at Austin, Austin, TX, United States, 4 Museo de Paleontología, Área Académica de Biología, Universidad Autónoma del Estado de Hidalgo, Hidalgo, Mexico, 5 University of Oregon, Earth Sciences Department, Eugene, OR, United States, 6 Programa de Pós-graduação em Biodiversidade e Biologia Evolutiva, Centro de Ciências da Saúde, Instituto de Biologia, Campus Ilha do Fundão, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

### Edited by:

Corwin Sullivan, University of Alberta, Canada

### Reviewed by:

Peter Heintzman, Arctic University of Norway, Norway David Marjanovic,´ Museum für Naturkunde, Germany

> \*Correspondence: Christina I. Barrón-Ortiz<EMAIL_ADDRESS>
### Specialty section:

This article was submitted to Paleontology, a section of the journal Frontiers in Ecology and Evolution

Received: 07 March 2019 Accepted: 27 August 2019 Published: 12 September 2019

### Citation:

Barrón-Ortiz CI, Avilla LS, Jass CN, Bravo-Cuevas VM, Machado H and Mothé D (2019) What Is Equus? Reconciling Taxonomy and Phylogenetic Analyses. Front. Ecol. Evol. 7:343. doi: 10.3389/fevo.2019.00343 Interest in the origin and evolution of Equus dates back to over a century, but there is still no consensus on the definition of the genus or its phylogenetic position. We review the placement of Equus within several phylogenetic frameworks and present a phylogenetic analysis of derived Equini, including taxa referred to Equus, Haringtonhippus, Dinohippus, Astrohippus, Hippidion, and Boreohippidion. A new, morphology-based phylogenetic tree was used as an initial hypothesis for discussing what taxa Equus encompasses, using four criteria previously used to define the genus category in mammals: phylogenetic gaps, uniqueness of adaptive zone, crown group definition, and divergence time. According to the phylogenetic gaps criterion, Equus encompasses clade 6 (Ha. francisci = E. francisci, E. conversidens, E. quagga, E. hemionus, E. mexicanus, E. ferus, E. occidentalis, and E. neogeus) based on morphological synapomorphies. Equus is assigned to clade 6, or possibly clade 7, according to the uniqueness of adaptive zone criterion. The crown group criterion places Equus at clade 6. Based on the time-calibrated phylogeny of Equini, the divergence time criterion suggests that Equus encompasses clade 9. This clade comprises all taxa traditionally assigned to Equus analyzed in our study, including the eight taxa listed above as well as E. stenonis, E. idahoensis, and E. simplicidens; the latter two are sometimes referred to the subgenus Plesippus and the former to the subgenus Allohippus. With the exception of the divergence time criterion, the results of our evaluation are congruent in identifying clade 6 as the most suitable position for Equus. The taxonomic implications of delimiting Equus to clade 6 in our phylogenetic tree include elevation of Allohippus and Plesippus to generic rank, assignment of a new genus to "Dinohippus" mexicanus, and synonymy of Haringtonhippus with Equus.

Keywords: phylogeny, taxonomy, Equidae, Equus, Dinohippus, Haringtonhippus, Allohippus, Plesippus

# INTRODUCTION

The origin and evolution of the genus Equus has been of particular interest to paleontologists for over a century (e.g., Marsh, 1879; Gidley, 1907; Matthew, 1924, 1926; Stirton, 1940, 1942; Lance, 1950; Dalquest, 1978, 1988; Bennett, 1980; MacFadden, 1984; Azzaroli, 1988, 1992; Hulbert, 1989; Prado and Alberdi, 1996; Kelly, 1998; MacFadden and Carranza-Castañeda, 2002). There are several evolutionary hypotheses for the genus and the definition of the generic name across those hypotheses has varied. Since the 1980s, discussions concerning the phylogenetic position of the genus have centered primarily on the position of some named species of Dinohippus with respect to Equus (e.g., Bennett, 1980; MacFadden, 1984; Azzaroli, 1988, 1992; Hulbert, 1989; Prado and Alberdi, 1996; Kelly, 1998). In that sense, our understanding of what constitutes the genus has been relatively stable, with discussions of taxonomy restricted to the base of the tree. In contrast, a recent molecular study suggested a taxonomic scheme that departs from previous taxonomies in that E. francisci, a species previously nested within Equus, is placed in a new genus, Haringtonhippus (Heintzman et al., 2017). That interpretation implicitly constrains the definition of the genus to the crown group and has potential implications for the taxonomy of horses traditionally assigned to Equus. In this study, we review the use of the generic name Equus within several phylogenetic frameworks and outline four criteria previously used to define a genus. We then examine the concept and contents of Equus based on a new phylogenetic analysis of derived Equini given those criteria. As is the case with any phylogenetic hypothesis, we acknowledge that the tree we present will be revised as new data and new specimens are studied. Nonetheless, our phylogenetic results offer a working hypothesis for discussing distinct paradigms for understanding higher level taxonomy, particularly in regards to the definition of the genus Equus.

# Contextual Framework

Under a Linnaean taxonomic scheme, Equus is included within the subfamily Equinae and the tribe Equini (MacFadden, 1992). Equus is hypothesized to have originated in North America (e.g., Bennett, 1980; MacFadden, 1984; Azzaroli, 1988, 1992; Hulbert, 1989; Prado and Alberdi, 1996; Kelly, 1998; MacFadden and Carranza-Castañeda, 2002). Some early phylogenetic hypotheses proposed that Equus originated from derived species of Pliohippus, within the subgenus Astrohippus (Osborn, 1918; Stirton, 1940). Later studies separated derived species of Pliohippus into two distinct genera, Astrohippus and Dinohippus (Quinn, 1955), and proposed several hypotheses for the origin of Equus. Some studies suggested that Equus evolved from a species of Dinohippus, such as D. mexicanus or D. leidyanus (e.g., Lance, 1950; Webb, 1969; Bennett, 1980; MacFadden, 1984; Azzaroli, 1988, 1992). Others posited a polyphyletic origin from both Astrohippus and Dinohippus (Dalquest, 1978) or from a separate genus, such as Eoequus (Quinn, 1955), a taxon later considered a junior synonym of Protohippus (Hulbert, 1988).

Morphological phylogenetic analyses conducted since the 1980s support the close phylogenetic affinity of derived members of Dinohippus, such as D. mexicanus, to early representatives of Equus, including E. simplicidens (Bennett, 1980; Hulbert, 1989; Prado and Alberdi, 1996; Kelly, 1998). Some of these studies identified "D." mexicanus as the sister group of Equus (Kelly, 1998). Other studies suggested including "D." mexicanus within Equus (Prado and Alberdi, 1996) or even including "Dinohippus" s.l. within Equus (Hulbert, 1989). Regardless of placement, the debate over the delimitation of "Equus" has been largely along the stem of the equid tree. Branches of Plio-Pleistocene Equini taxa, other than Hippidion, have traditionally been considered species of Equus.

In contrast to previous studies, the recent naming of a new genus (Haringtonhippus; Heintzman et al., 2017) on the basis of molecular data and estimated divergence times for extant and some fossil equids has potentially significant taxonomic implications for branches of the equid tree. Specifically, the study constrained the definition of Equus to the crown group (Heintzman et al., 2017), implicitly excluding many stem-group species from Equus without explicitly assigning them to any other genus. In that context, we saw an opportunity to explore a deeper philosophical question about the criteria for defining genera, and how those criteria bear on the placement of Equus and the validity of taxa traditionally referred to Equus.

## DEFINITION AND DELIMITATION OF MAMMALIAN GENERA

The literature on species concepts and naming of species is extensive (e.g., Mayr, 1940, 1942, 1963; Simpson, 1961; Ghiselin, 1966, 1974; Van Valen, 1976; Paterson, 1978, 1985; Wiley, 1978; Cracraft, 1983, 1997; de Queiroz and Donoghue, 1988; Templeton, 1989; Mayden, 1997, 2002; de Queiroz, 1998, 2007; Groves, 2004). In contrast, discussions on higher Linnaean taxonomic categories (e.g., the genus) are less numerous. Nonetheless, different authors discussed the meaning and relevance of the genus and proposed various criteria for recognizing and delimiting this higher taxonomic category in a consistent way (e.g., Mayr, 1950, 1969; Cain, 1956; Michener, 1957; Inger, 1958; Simpson, 1961; Hennig, 1966; Dubois, 1987, 1988; Groves, 2001, 2004; Vences et al., 2013). We note that some authors have discussed limitations of the Linnaean classification system and proposed to abandon it (e.g., de Queiroz and Gauthier, 1992; Ereshefsky, 2001; Zachos, 2011) or to combine it with different approaches (Kuntner and Agnarsson, 2006). Therefore, new approaches to nomenclature have been advanced in recent decades (e.g., Papavero et al., 2001; Béthoux, 2007; Cantino and de Queiroz, 2010), but they have not been fully integrated across the Tree of Life. As a result, the Linnaean system continues to form the primary framework used to study and communicate about past and present biodiversity (Vences et al., 2013), particularly in regard to the binomial name (genus and species).

The only widely accepted criterion for delimiting a genus or other higher taxonomic categories is monophyly (Hennig, 1966; Mayr, 1969; Groves, 2001, 2004; Vences et al., 2013). Only monophyletic groups that are well-supported should be named as Linnaean taxa (Vences et al., 2013). Other criteria previously applied to delimitation of extant and extinct genera of mammals are: (1) phylogenetic gaps, (2) uniqueness of adaptive zone, (3) crown group definition, and (4) divergence time (Hennig, 1966; Mayr, 1969; Groves, 2001, 2004; Vences et al., 2013). Some authors advocate for the use of two or more of these criteria, in addition to monophyly, to delimit genera and other higher taxa (e.g., Hennig, 1966; Mayr, 1969), and summary of these criteria is as follows.

### Phylogenetic Gaps

The identification of phylogenetic gaps is a criterion that has been used for delimiting genera as well as other higher taxonomic categories (Mayr, 1969). The gaps between taxa and the relative size of those gaps are the result of evolutionary processes (e.g., speciation, extinction, evolutionary and adaptive radiations, and unequal rates of evolution; Mayr, 1969). Specifically, Mayr (1969) defined the genus as "a taxonomic category containing a single species, or a monophyletic group of species, which is separated from other taxa of the same rank [other genera] by a decided gap" (Mayr, 1969, p. 92). In the context of phylogenetic analyses, the gaps between taxa can be measured by the number of synapomorphic traits. In order to facilitate information retrieval and limit redundancy in taxonomic classifications, Mayr (1969) suggested the size of the gap could vary depending on the size of the taxon.

### Uniqueness of Adaptive Zone

Simpson (1944) proposed the concept of adaptive zone as a key component of evolutionary change. An adaptive zone corresponds to a particular mode of life or a unique ecological situation (e.g., Simpson, 1944, 1953; Mayr, 1950, 1969; Wood and Collard, 1999; Vences et al., 2013). Under this concept, the occupation of a new adaptive zone by a taxon results in significant evolutionary change (Simpson, 1944). Some authors suggest that this evolutionary change should be reflected at one or more higher categories in the taxonomic classification (Mayr, 1969). Under this perspective, the difference in the occupation of an adaptive zone contributes to the width and the sharpness of phylogenetic gaps between taxa (Mayr, 1969). Therefore, a genus is considered to consist of a species or group of species of common ancestry that occupy a different adaptive zone from the one occupied by species of another genus (Mayr, 1950, 1969).

# Crown Group Definition

With the advent of phylogenetic systematics the definition of taxa shifted to a nominalist perspective (de Queiroz, 1994). The essentialist perspective starts from the assumption that the taxon exists, then tries to discover its essential traits, and then refers all organisms with these traits to the taxon in question (de Queiroz, 1994). The nominalist perspective instead assumes that the limits of named taxa are arbitrary conventions, and then proceeds to spell these conventions out. Thus, phylogenetic nomenclature defines taxon names explicitly by anchoring them to defined points on the phylogenetic Tree of Life (de Queiroz, 1994). For example, Mammalia has been defined as the last common ancestor of monotremes and therians plus all descendants of that ancestor (Rowe, 1988; Rowe and Gauthier, 1992). This is a crown group definition of Mammalia (de Queiroz, 1994). In the case of Equus, the last common ancestor of all extant species assigned to Equus and all descendants of that ancestor, is the crown group definition of the genus.

### Divergence Time

Some researchers suggest that a taxonomic arrangement above the species level should ideally not only provide information about evolutionary relationships, but should also reflect the approximate divergence times of the different taxonomic ranks (e.g., Hennig, 1966; Avise and Johns, 1999; Groves and Grubb, 2011). In the particular case of the genus, Groves (2001, 2004) and Groves and Grubb (2011) indicated that the cutoff point for assigning generic status to monophyletic groups of species could be placed at about the Miocene-Pliocene boundary (4–7 Ma). This cutoff point is based on a principle of least violence (which aims at preserving as many traditional genera as possible) and a survey of mammals whose fossil record or molecular divergence estimates are well-known (Groves, 2001, 2004). Under the divergence time criterion, species are regarded as distinct genera if they diverged well-before the Miocene-Pliocene boundary. This approach was intended to make the delimitation of genera a more objective endeavor (Groves, 2001, 2004; Groves and Grubb, 2011).

# WHAT IS EQUUS? DELIMITATION OF THE GENUS BASED ON FOUR CRITERIA

The question "What is Equus?" is a philosophical one that ultimately relates to the evolutionary paradigm under which individual researchers are operating and the research questions that are being asked. In this sense, the question we pose in the title of the paper may have different answers depending on the paradigm under consideration. What Equus is, under these varying paradigms, has implications for how we communicate knowledge about the evolutionary and taxonomic history of horses. Perhaps the more valuable question is not "What is Equus?," but rather "How variable is the taxonomic content of Equus in a given phylogenetic tree, under very distinct paradigms for understanding higher-level taxonomy?" Therefore, we conducted a phylogenetic analysis of derived Equini and examined the placement of the name "Equus" within the resultant phylogenetic tree given the four criteria discussed above for delimiting extant and extinct genera of mammals.

### Phylogenetic Analysis

We performed a phylogenetic analysis of derived Equini using a matrix of 32 morphological characters and 21 Equini taxa, including Astrohippus stocki, Boreohippidion galushai, Dinohippus leardi, D. leidyanus, D. interpolatus, D. mexicanus, Equus conversidens, E. ferus, E. hemionus, E. idahoensis, E. mexicanus, E. neogeus, E. occidentalis, E. quagga, E. simplicidens, E. stenonis, Haringtonhippus francisci, Hippidion saldiasi, Hi. principale and two outgroup taxa, Acritohippus stylodontus, and Pliohippus pernix (**Table 1**). Our study sample included holotype and referred specimens (**Table A1**). We gathered data September 2019

TABLE 1 |Taxa and character matrix used in the phylogenetic analysis conducted here.

CH means character followed by the number of the character. The definitions of characters and their states are in the Appendix. from the literature for E. idahoensis (Scott, 2005) and E. stenonis (Athanassiou, 2001; Palombo and Alberdi, 2017); all other specimens in our study were directly examined by us (**Table A1**). We analyzed the holotype and referred specimens of E. conversidens separately from the holotype of Ha. francisci (= E. francisci), as they are considered distinct taxa in some studies (e.g., Lundelius and Stevens, 1970; Dalquest, 1979; Scott, 1996; Azzaroli, 1998; Bravo-Cuevas et al., 2011; Barrón-Ortiz et al., 2017; Priego-Vargas et al., 2017). We note, however, that some researchers considered E. conversidens a senior synonym of Ha. francisci (= E. francisci) (Dalquest and Hughes, 1965), or a nomen dubium (e.g., Winans, 1985, 1989; Heintzman et al., 2017). Equus ferus is represented in our study by a sample of caballine equids of late Pleistocene age, some of which were previously referred to this taxon (Barrón-Ortiz et al., 2017). For E. hemionus and E. quagga we studied the remains of wild animals. The phylogenetic analysis was performed using TNT 1.1 (Goloboff et al., 2008) with the implicit enumeration option (exhaustive search), using equal weighting for the characters, and without a collapsing rule. We treated all characters as unordered in the analysis reported here. Two additional analyses (one without a collapsing rule and another with collapsing rule 1) that differed by including ordering of characters 1, 29, 30, and 31 break down the distinction between clades 12, 13, 14, and 17 (**Figure S1**), but leave the topology of the other clades intact (as shown in **Figures 1**, **2**). We used the strict (= Nelsen) consensus option to calculate the consensus tree.

The phylogenetic analysis resulted in three equally most parsimonious trees of 85 steps, consistency index (CI) and retention index (RI) of 0.57 and 0.80, respectively. The strict consensus tree is shown in **Figure 1**. Of particular relevance to the present study are the phylogenetic relationships among Dinohippus mexicanus, Haringtonhippus francisci, and species of Equus. Our phylogenetic analysis identified Dinohippus as a paraphyletic group, and "D." mexicanus as the sister group to the clade including all species of Equus and Ha. francisci. The sister group relationship of "D." mexicanus and Equus is consistent with some previously proposed phylogenetic hypotheses (Prado and Alberdi, 1996; Kelly, 1998). Our results also suggests that E. idahoensis, E. simplicidens, and E. stenonis lie outside of crown group Equus. This pattern is consistent with the morphometric analysis of Eisenmann and Baylac (2000), but disagrees with the phylogenetic analysis of Bennett (1980).

Haringtonhippus francisci is nested within crown group Equus in our phylogenetic tree, forming a polytomy with E. conversidens and E. quagga (**Figure 1**). Haringtonhippus francisci (= E. francisci) is situated in the crown group in the morphological phylogenetic analysis of Bennett (1980). A closer phylogenetic relationship of Ha. francisci (= E. francisci) to extant Equus than primitive species of the genus, such as E. simplicidens (= E. shoshonensis) and E. stenonis, is also indicated by cranial proportions (Eisenmann and Baylac, 2000). Ancient mitochondrial DNA analyses suggested that the lineage which was later named Haringtonhippus was the sister species to extant and fossil caballine equids (Weinstock et al., 2005; Orlando et al., 2008; Vilstrup et al., 2013; Der Sarkissian et al., 2015; Barrón-Ortiz et al., 2017). In contrast to these results, a recent genomic analysis concluded that Ha. francisci lies outside of crown group Equus (Heintzman et al., 2017), but its relationship to E. simplicidens and E. stenonis was not studied, as molecular data for these species are presently unknown. To evaluate the consistency between our phylogenetic results and the genomic study by Heintzman et al. (2017), we performed a second analysis in which Ha. francisci was constrained to lie outside of crown group Equus. This analysis resulted in eleven equally most parsimonious trees of 91 steps; 6 steps longer than the most parsimonious trees that we obtained in the unconstrained analysis. In eight of the 11 equally most parsimonious trees, Ha. francisci was the sister group of crown Equus, whereas in the three remaining trees Ha. francisci was the sister group of the clade formed by E. idahoensis + crown Equus. The discrepancy between the morphological and genomic analyses with regards to the phylogenetic position of Ha. francisci is notable, and needs to be investigated in further studies.

### Extent of Equus Based on Four Criteria for Delimiting Genera Phylogenetic Gaps

Application of this criterion to our phylogenetic tree (**Figure 1**) suggests that the name Equus best encompasses clade 6 on the basis of the number of morphological synapomorphies (**Figure 2A**). This taxonomic arrangement excludes E. stenonis, E. idahoensis, E. simplicidens, and "Dinohippus" mexicanus from the genus and renders Haringtonhippus as a junior synonym of Equus. There are six synapomorphies for clade 6 (**Table A2**). In contrast, clade 7 (which includes E. stenonis in addition to the taxa in clade 6), and clade 10 (which includes "D." mexicanus, all taxa traditionally assigned to Equus, and Haringtonhippus) each possess five synapomorphies (**Table A2**). Equus idahoensis and E. simplicidens have at times been referred to Plesippus, at either the generic or subgeneric rank (Eisenmann and Baylac, 2000). Similarly, E. stenonis has been referred to Allohippus at the subgeneric rank (Eisenmann and Baylac, 2000), although we note that some researchers consider Allohippus a nomen dubium (Azzaroli, 1992). Under the phylogenetic gaps criterion, Allohippus and Plesippus should be elevated to generic rank. Furthermore, "D." mexicanus should be assigned to a new genus, and Haringtonhippus synonymized with Equus.

### Uniqueness of Adaptive Zone

An adaptive zone is defined in the literature as a particular mode of life or a unique ecological situation (e.g., Simpson, 1944, 1953; Mayr, 1950, 1969; Wood and Collard, 1999; Vences et al., 2013). This criterion has been used in the definition of our own genus, Homo (Wood and Collard, 1999; Collard and Wood, 2007; Holliday, 2012; Antón et al., 2014). Specializations in body size and shape, locomotor behavior, rate, and pattern of development, among other traits, are argued to have allowed Homo to play a unique ecological role relative to other hominins (e.g., Leakey et al., 1964; Tobias, 1991; Wood and Collard, 1999; Collard and Wood, 2007; Holliday, 2012; Antón et al., 2014). Therefore, these traits have been considered important in the delimitation of the genus Homo by some researchers (e.g., Leakey et al., 1964; Tobias, 1991; Wood and Collard, 1999; Collard and Wood, 2007; Antón et al., 2014; Wood, 2014), although we note that a consensus on the definition of the

genus has not been reached (Wood and Collard, 1999; Collard and Wood, 2007; Holliday, 2012; Antón et al., 2014; Wood, 2014).

In this context, the unique mode of life of extant equids could be defined as that of ungulate mammals that are adapted to live in generally open, arid habitats and that can thrive on lowquality, high-fiber foods such as grasses and other coarse and tough vegetation (Janis, 1976, 1988; Mihlbachler et al., 2011; Rubenstein et al., 2016; Schoenecker et al., 2016). Potential morphological adaptations for this mode of life comprise modifications of the locomotory and digestive systems. Possible locomotor adaptations to living in open habitats include the elongation of distal autopodial bones as well as the development of monodactyly and the reduction or loss of metapodials II and IV (Simpson, 1951; Shotwell, 1961; Janis and Wilhelm, 1993); however, we note that alternative explanations for digit reduction have been proposed (Thomason, 1986; Biewener, 1998; McHorse et al., 2017). The enhanced development of the stay-apparatus, which allows the individual to conserve energy while standing, is also potentially an adaptation to living in open habitats (Hermanson and MacFadden, 1992, 1996). Potential adaptations of the digestive system, particularly the dentition, to feeding on low-quality, high-fiber vegetation in open environments include increased crown-height of cheek teeth and incisors (Janis, 1976, 1988; Damuth and Janis, 2011; Mihlbachler et al., 2011; Schoenecker et al., 2016), increased enamel complexity (Famoso and Davis, 2014; including increased implications of the occlusal enamel [(Gromova, 1949; Simpson, 1951; Rensberger et al., 1984; Eisenmann and David, 1990; Kaiser, 2002)]), elongation of the protocones of the upper molars and premolars (Eisenmann, 1982; Guadelli and Prat, 1995), increased separation of the metastylid and metaconid, and enlargement of the metastylid to the point of being equal or subequal in size to the metaconid in the lower molars (MacFadden and Carranza-Castañeda, 2002).

The phylogenetic result reported here indicates that the locomotory and dental traits mentioned above did not appear at the same time. Three of these morphological traits are synapomorphies for Clade 6: oval protocone outline on P2 (character 22, state 1); oblong protocone outline on P3-P4 (character 23, state 5); and a high, well-developed intermediate tubercle, which is key in the development of the stay apparatus (character 30, state 2) (**Table A2**). Three of the remaining morphological traits are synapomorphies for Clade 7: pliprotoloph and/or pli-hypostyle common and persistent in the upper molars (character 18, state 1); oblong protocone outline on M1-2 (character 24, state 3); and metaconid and metastylid persistently well-separated from each other on p3-m3 (character 27, state 2). Metapodials II and IV reduced to less than half the length of metapodial III (character 31, state 1) is a synapomorphy for clades 10 and 16 (**Table A2**). Absence of metapodials II and IV (character 31, state 2) is a synapomorphy for clade 4 (**Table A2**). An oblong protocone outline in the M3 (character 25, state 2), and a metaconid and metastylid of equal or subequal size in the lower molars (character 28, state 1) are synapomorphies of clade 11 (**Table A2**). An unworn molar crown height > 60 mm (character 19, state 1) is a synapomorphy of clade 14 (**Table A2**). Based on the position of the majority of purported, adaptive zone-related characteristics, Equus is assigned in the phylogenetic tree to clade 6, or possibly clade 7, under the adaptive zone criterion (**Figure 2B**). Under this paradigm, Haringtonhippus is considered a synonym of Equus, Plesippus is elevated to generic rank (the same would apply to Allohippus if Equus were delimited to clade 6), and "D." mexicanus should be assigned to a new genus.

### Crown Group Definition

Under this criterion Equus is defined as the most recent common ancestor of all extant species assigned to Equus, and all descendants of that ancestor. In our phylogenetic tree, a crowngroup definition of Equus is constrained to clade 6 (**Figure 2C**). This clade includes the extant taxa E. quagga, E. hemionus, and E. ferus, which represent each lineage of extant Equus (zebrines, hemionines [including asses], and caballines, respectively) as well as four extinct taxa (E. conversidens, E. occidentalis, E. neogeus, and Ha. francisci). This taxonomic arrangement results in the synonymy of Haringtonhippus with Equus and excludes "D." mexicanus and both plesippine and stenonine equids from the genus. The latter two taxa would then be best assigned to Plesippus and Allohippus, respectively.

### Divergence Time

The divergence time criterion states that species should be regarded as distinct genera if they diverged well-before the Miocene-Pliocene boundary (Groves, 2001, 2004; Groves and Grubb, 2011). Application of the time depth criterion to the time-calibrated phylogeny of Equini (**Figure 3**) retains the traditional taxonomic arrangement of Equus. Clade 9, which comprises E. idahoensis, E. simplicidens, and the remaining taxa traditionally assigned to Equus, is identified as having originated within the Pliocene (**Figure 3**). Therefore, under the divergence time criterion, taxa within clade 9 should be assigned to the same genus (i.e., Equus; **Figure 2D**). In contrast, "D." mexicanus is identified to have originated well-before the Miocene-Pliocene boundary (**Figure 3**) and should be assigned to a new genus (**Figure 2D**).

The time-calibrated phylogeny (**Figure 3**) is, of course, based on fossil occurrences. An independent assessment of divergence times for some Equini taxa, including crown group Equus, Haringtonhippus, and Hippidion, is provided by molecular analyses (Der Sarkissian et al., 2015; Heintzman et al., 2017). These analyses place the divergence time between the lineage leading to Haringtonhippus and that leading to crown group Equus at ∼4.1–5.7 Ma (Heintzman et al., 2017). This estimate spans the Miocene-Pliocene boundary, which is currently recognized at 5.33 Ma (Cohen et al., 2013) and is in the upper range of the cutoff interval (4–7 Ma) proposed by Groves (2001, 2004). Molecular estimates of the time of divergence between Hippidion and Haringtonhippus + Equus (∼5.2–7.7 Ma) overlap the lower range of the 4–7 Ma cutoff interval (Der Sarkissian et al., 2015; Heintzman et al., 2017). Based on these estimated divergence times, Haringtonhippus and potentially Hippidion should be synonymized with Equus under the divergence time criterion.

### DISCUSSION

Delimitation of the genus Equus depends on at least two important factors: (1) identifying well-supported phylogenetic hypotheses, and (2) identifying which clade in a given phylogenetic tree should be considered to comprise the genus Equus. Our study infers the first factor and emphasizes the second by evaluating the consistency in the content of "Equus" across four explicit criteria (phylogenetic gaps, uniqueness of adaptive zone, crown group definition, and divergence time) previously used to delimit genera. Ideally,

discussions of what "Equus" is should also consider taxonomic stability, except where evidence suggests that some aspect of a traditional classification is fundamentally flawed from an evolutionary perspective.

Application of the four criteria for delimiting genera results in slightly different positions for the generic name on the phylogenetic tree (**Figure 2**). Under the phylogenetic gaps criterion Equus should be equated with clade 6 based on the number of morphological synapomorphies (**Figure 2A**). The same is true for the crown group criterion, as extant taxa of Equus here analyzed (E. ferus, E. hemionus, and E. quagga) fall within clade 6 (**Figure 2C**). The adaptive-zone criterion places Equus at clade 6 or possibly clade 7, depending on how the characters are evaluated in relation to the adaptive zone (**Figure 2B**). At a minimum, clade 6 is supported by morphological synapomorphies related to the unique mode of life of extant equids, which represent adaptations for living in open, generally arid, habitats characterized by low-quality, highfiber foods such as grasses and other coarse vegetation (Janis, 1976, 1988; Mihlbachler et al., 2011; Rubenstein et al., 2016; Schoenecker et al., 2016). The divergence time criterion suggests that Equus encompasses clade 9 (**Figure 2D**), based on the timecalibrated phylogeny of Equini (**Figure 3**).

With the exception of the divergence time criterion, the results of our evaluation are overall congruent in identifying clade 6 as the most suitable position of the genus Equus. We note that the aim of the divergence time criterion is to provide an operational definition of genera that incorporates time-depth information (Groves, 2001, 2004; Groves and Grubb, 2011). Nevertheless, whether a time interval of 4–7 Ma is a reasonable cutoff point for all taxonomic groups of mammals is debatable, given the disparate evolutionary rates for different groups of mammals (Carroll, 1998). Moreover, the objectivity of this criterion breaks down for clades that diverged very close to the Miocene-Pliocene boundary, as is potentially the case for Haringtonhippus and the crown group of Equus (Heintzman et al., 2017). In such cases, the taxonomist has to decide whether to keep the sister taxa in one genus or split them into separate genera. Another point to consider about the divergence time criterion is that it relies on the availability of either a very dense fossil record, or very precise molecular divergence estimates (often calibrated by a rather dense fossil record). The fossil record of Neogene

equids is particularly dense (MacFadden, 1992). Therefore, the discrepancies between the time-calibrated phylogeny of Equini presented here (**Figure 3**) and the molecular divergence estimates for Haringtonhippus and Hippidion in two recent molecular analyses (Der Sarkissian et al., 2015; Heintzman et al., 2017) are surprising. Resolution of these differences is beyond the scope of the present study, but this is a topic that should be investigated in future studies. Lastly, the divergence time criterion is designed for extant species and is generally difficult to apply to past biodiversity.

The remaining criteria (phylogenetic gaps, adaptive zone, and crown group) consistently support a clade 6 position for the generic name, with less support for a clade 7 position (in the case of the adaptive zone criterion). We found the phylogenetic gaps and crown group criteria for delimiting genera to be more readily evaluated than the adaptive zone criterion. The broad definition of an adaptive zone implies that it consists of a hyperdimensional space, which in turn makes it difficult to consistently define and quantify. Describing the unique mode of life of extant equids as that of ungulate mammals that are adapted to live in open, generally arid, habitats, and that can thrive on low-quality, highfiber foods such as grasses and other coarse and tough vegetation (Janis, 1976, 1988; Mihlbachler et al., 2011; Rubenstein et al., 2016; Schoenecker et al., 2016) seems reasonable, but likely captures only a small portion of the complete adaptive zone they occupy. The adaptive zone criterion may also be criticized because it looks for key (essential) traits that allow a taxon or group of taxa to occupy a unique adaptive zone. Mosaic evolution is a widespread phenomenon in vertebrate taxa, including equids (MacFadden, 1992); therefore, it is not rare for characters to have evolved at different times, raising debates about which one should be considered the "essential" character. This problem is exemplified in the present study, as both clades 6 and 7 possess synapomorphies that are relevant for the unique adaptive zone of extant equids, as defined here. Another difficulty in applying the adaptive zone criterion is that the exact ecology of extinct organisms is often hard to determine. In the particular case of equids, some aspects of their ecology, such as feeding ecology, are more readily inferred than others (e.g., MacFadden et al., 1999; Fortelius and Solounias, 2000; Solounias and Semprebon, 2002; Kaiser and Solounias, 2003; Merceron et al., 2004; Sánchez et al., 2006; Mihlbachler et al., 2011). As more complete ecological information becomes available, it will allow researchers to better define the adaptive zone or particular mode of life of extinct equid species. This is particularly relevant if researchers consider that Equus should have both phylogenetic and adaptive significance.

Under the phylogenetic gaps criterion Equus should be delimited to clade 6 in our phylogenetic tree. This is the clade that has the most synapomorphies, resulting in a "phylogenetic gap." A potential criticism of this criterion is that the positions and relative sizes of gaps among taxa may reflect not only evolutionary processes (e.g., speciation, extinction, evolutionary and adaptive radiations, and unequal rates of evolution; Mayr, 1969), but also gaps in our knowledge. As more fossils are discovered and more specimens are studied, existing gaps will be subdivided. In this regard, our phylogenetic analysis did not include taxa traditionally assigned to Equus of Pliocene and early Pleistocene age other than E. idahoensis, E. simplicidens, and E. stenonis. Inclusion of additional Plio-Pleistocene taxa could potentially make application of this taxonomic criterion more difficult, if it were to "even out" the phylogenetic gaps between clades. Nevertheless, the six currently recognized synapomorphies of clade 6 (**Table A2**), many of which are also relevant to the adaptive zone criterion, strongly suggest to us that Equus should encompass this clade, pending further phylogenetic analyses that include more Plio-Pleistocene equids traditionally assigned to Equus (e.g., E. cumminsi; E. enormis; E. huanghoensis; E. koobiforensis; E. livenzovensis; E. qingyangensis; E. sanmeniensis; E. yunnanensis; Azzaroli, 1992; Azzaroli and Voorhies, 1993; Downs and Miller, 1994; Eisenmann and Deng, 2005; Palombo and Alberdi, 2017).

As noted above, application of the crown group criterion also supports the idea that Equus encompasses all of clade 6 in our phylogenetic tree. These results are at odds with a recent genomic analysis that concluded Haringtonhippus lies outside of crown group Equus (Heintzman et al., 2017). The crown group criterion has been used in the literature to define genera and other higher taxonomic categories (such as Panthera, see King and Wallace, 2014; and Tapirus, see Holanda and Ferrero, 2013). Furthermore, extant taxa generally allow us to make robust inferences within a crown group about traits that generally do not fossilize. However, one criticism of the crown definition of higher taxa is that it is based on the extinction criterion (Lucas, 1992). As a result, there is great emphasis on living taxa and, instead of promoting stability, this criterion could lead to taxonomic confusion. This is especially true for relict groups, such as Homo. In those instances, other criteria for defining genera may be more relevant. In the case of Homo, the adaptive zone criterion has been used by different researchers to delimit the genus (e.g., Wood and Collard, 1999; Collard and Wood, 2007; Holliday, 2012; Antón et al., 2014).

Clade 6 also meets the three primary taxon naming criteria proposed by Vences et al. (2013) to promote economy of change in Linnaean classification schemes and reduce subjective taxonomic instability. These criteria are monophyly, clade stability, and phenotypic diagnosability (Vences et al., 2013). Clade 6 is characterized by six synapomorphies (**Table A2**), discussed above under the phylogenetic gaps criterion, and is a fairly well-supported clade (**Figure 1**). These characteristics identify clade 6 as a stable clade. Furthermore, the six synapomorphies of clade 6 also meet the phenotypic diagnosability criterion, which states that "a taxon to which a Linnaean rank is assigned should be diagnosable and identifiable phenotypically" (Vences et al., 2013, p. 228). The six synapomorphies of clade 6 are visible in both sexes and in many life-history stages of the organism; these are important requirements for this criterion (Vences et al., 2013). Equally important, considering clade 6 as encompassing the genus promotes overall taxonomic stability as traditionally considered in taxonomic treatments. Given that interpretation, our study suggests that Haringtonhippus should be synonymized with Equus, because the former is situated within clade 6.

Outside clade 6, there are some taxonomic considerations for some taxa. First, "Dinohippus" mexicanus would have to be assigned to a new genus. In our study "D." mexicanus is identified as the sister species to the Equus + Allohippus + Plesippus clade. Our results contrast with previous studies that suggested including "D." mexicanus within Equus (Prado and Alberdi, 1996) or even including "Dinohippus" s.l. within Equus (Hulbert, 1989), and also with studies in which "D." mexicanus was identified as the sister species of Equus (Kelly, 1998). Second, by restricting Equus to clade 6, stenonine (i.e., akin to E. stenonis) and plesippine (i.e., akin to E. simplicidens) equids are excluded from Equus and would be recognized as distinct lineages. The recognition of stenonine and plesippine equids as distinct lineages from other Equus taxa was previously suggested by morphometric analyses of cranial proportions (Eisenmann and Baylac, 2000; Eisenmann and Deng, 2005). Based on those analyses, the extant species of Equus and at least some fossil taxa, such as Ha. francisci (= E. francisci) and E. occidentalis, are grouped under the subgenus Equus, whereas stenonine and plesippine equids are referred to the subgenera Allohippus and Plesippus, respectively (Eisenmann and Baylac, 2000). Based on our results, Allohippus and Plesippus should be given full generic status.

The recognition that Ha. francisci (= E. francisci) and extant species of Equus form a distinct clade (clade 6) from stenonine and plesippine equids is consistent with a previous morphometric study (Eisenmann and Baylac, 2000). However, this phylogenetic arrangement is not supported by some phylogenetic analyses (Bennett, 1980; Heintzman et al., 2017). In the morphological study by Bennett (1980), E. stenonis and E. simplicidens (= E. shoshonensis) are nested within the clade that comprises extant zebras. Moreover, Ha. francisci (= E. francisci) was found to be more closely related to E. onager (Bennett, 1980), whereas in our phylogenetic analysis Ha. francisci forms a polytomy with E. conversidens and E. quagga (**Figure 1**). In the genomic study by Heintzman et al. (2017), Ha. francisci lies outside the clade that comprises extant Equus, but its relationship to stenonine and plesippine equids was not tested because molecular data for these species are presently unknown. The discrepancies between those studies and ours emphasize that the definition of Equus remains a work in progress.

From the morphological side, the study of additional characters, including those relating to postcranial and cranial anatomy, will continue to refine phylogenetic hypotheses of horses. The character matrix that we used is undoubtedly biased toward dental and craniofacial characters, and incorporation of internal cranial characters and more postcranial characters may alter the resulting topology. On the molecular side, better taxonomic sampling is needed. Ultimately, using an integrative, total-evidence approach may provide a better understanding of the evolution and systematics of Plio-Pleistocene equids and a more resolved taxonomy.

As new phylogenetic datasets and hypotheses develop, we have no doubt that there will be additional discussions related to defining Equus and how best to reconcile taxonomy and evolutionary history. We view the deeper value of this endeavor to be stimulation of discussion around naming of genera. As long as binomial nomenclature is retained, naming of new genera (or retaining previously named genera) will almost certainly have a cascade effect, impacting how we view and discuss the evolutionary history of horses.

# CONCLUSIONS

Two factors are particularly relevant in the delimitation of the genus Equus: (1) identifying well-supported phylogenetic hypotheses, and (2) identifying which clade in a given phylogenetic tree should be considered to comprise the genus Equus. In this study, we inferred a phylogenetic tree and addressed the second factor by considering the consistency in delimitation of Equus on our strict consensus tree across four explicit criteria previously used in combination with monophyly to delimit genera. In our phylogenetic tree, an Equus that encompasses clade 6 has the strongest support on the basis of the most mutually consistent criteria (i.e., phylogenetic gaps, crown group, and, to a lesser extent, adaptive zone), and results in the most taxonomically stable placement of the genus. As such, our results suggest that Haringtonhippus should be considered a synonym of Equus, Allohippus and Plesippus should be elevated to generic rank, and "Dinohippus" mexicanus assigned to a new genus.

# DATA AVAILABILITY

All datasets generated for this study are included in the manuscript and/or the **Supplementary Files**.

# AUTHOR CONTRIBUTIONS

CB-O, LA, and CJ conceived the study and prepared an earlier version of the manuscript. LA, DM, HM, VB-C, and CB-O collected and analyzed the data. All authors contributed to the final version of the manuscript.

# FUNDING

The Royal Alberta Museum provided funding for open access publication fees. This study was funded in part by a post-doctoral scholarship (202.375/2018) to DM, from the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and a post-doctoral scholarship (248772/2013-9) to LA, from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

# ACKNOWLEDGMENTS

We would like to thank the curators and collection managers who provided access to specimens in their care: T. Deng and S. Wang (Institute of Vertebrate Paleontology and Paleoanthropology), S. Wang (Tianjin Hoangho-Paiho Museum), K. Fedyniak and M. Edwards (Royal Alberta Museum), V. Rhue and S. McLeod (Natural History Museum of Los Angeles County), A. Farrell (La Brea Tar Pits and Museum), P. Holroyd (University of California, Museum of Paleontology), E. Scott (San Bernardino County Museum), S. Lucas and G. Morgan (New Mexico Museum of Natural History and Science), J. C. Sagebiel (Jackson School Museum of Earth History, University of Texas at Austin), J. Meng and J. Galkin (American Museum of Natural History), B. MacFadden and R. Hulbert (Florida Museum of Natural History), J. Arroyo-Cabrales (Instituto Nacional de Antropología e Historia), O. Carranza-Castañeda and J. Alvarado-Ortega (Universidad Nacional Autónoma de México), R. Aguilar and G. Viramontes (Museo de Paleontología de Guadalajara), C. Laurito and A. Valerio (Museo Nacional de Costa Rica), J. L. Román-Carrión (Museo de Historia Natural Gustavo Orcés V), I. Alvarado and R. Sallas (Museo de Historia Natural), O. Aguilar Condemayta (Museo de Historia Natural de la Universidad Nacional de San Antonio Abad del Cusco), N. Guidon (Fundação Museu do Homem Americano), C. Cartelle (Museu de Ciências Naturais da Pontifícia Universidade Católica de Minas Gerais), R. Rocha Machado (Departamento Nacional de Produção Mineral), L. Carvalho, O. Grillo and U. Gomes (Museu Nacional), R. Fariña, A. Rojas, M. Ubilla, and D. Perea (Universidad de la República), A. Rinderknecht (Museo Nacional de Historia Natural y Antropología), A. Kramarz (Museo Argentino de Ciencias Naturales Bernardino Rivadavia), M. Reguero (Museo de La Plata), R. Vezzosi (Museo Provincial de Ciencias Naturales Florentino Ameghino), F. Paredes-Ríos (Museo Nacional de Paleontología y Arqueología de Tarija), I. Martínez (Universidad Internacional SEK), A. Prieto (Instituto de la Patagonia), K. Moreno and R. LaBarca (Laboratorio de Paleoecología de

### REFERENCES

la Universidad Austral de Chile), D. Rubilar-Rogers (Museo Nacional de Historia Natural), G. Peralta (Museo Histórico Municipal de Osorno), P. Brewer and A. Lister (Natural History Museum), P. Tassy (Muséum national d'Histoire naturelle). We thank P. J. Lewis (Sam Houston State University) for assisting in the study of the holotype of Haringtonhippus francisci. We also thank J. C. Sagebiel (Jackson School Museum of Earth History, University of Texas at Austin) for kindly photographing the skull and mandible of the holotype of Haringtonhippus francisci. R. L. Bernor and M. Montellano-Ballesteros provided helpful discussions. We also thank the reviewers, P. Heintzman and D. Marjanovic, and the associate editor, C. Sullivan, for ´ providing constructive criticism and comments that greatly improved the manuscript. The author DM thanks the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for the Post-doctoral scholarship 202.375/2018; the author LA is grateful to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the Post-doctoral scholarship 248772/2013-9. Both scholarships were crucial for studying the specimens included in this manuscript.

### SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo. 2019.00343/full#supplementary-material

Zachos, F. E. (2011). Linnaean ranks, temporal banding, and time-clipping: why not slaughter the sacred cow? Biol. J. Linnean Soc. 103, 732–734. doi: 10.1111/j.1095-8312.2011.01711.x

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Barrón-Ortiz, Avilla, Jass, Bravo-Cuevas, Machado and Mothé. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Species Diversity and Paleoecology of Late Pleistocene Horses From Southern Mexico

Eduardo Jiménez-Hidalgo1 \*, Gerardo Carbot-Chanona2 \*, Rosalía Guerrero-Arenas 1 , Victor Manuel Bravo-Cuevas 3 , Genevieve Safi Holdridge4 and Isabel Israde-Alcántara5

1 Laboratororio de Paleobiología, Instituto de Recursos, Universidad del Mar, Puerto Escondido, Oaxaca, Mexico, 2 Museo de Paleontología "Eliseo Palacios Aguilera", Dirección de Paleontología, Secretaría de Medio Ambiente e Historia Natural, Tuxtla Gutiérrez, Mexico, 3 Centro de Investigaciones Biológicas, Universidad Autónoma del Estado de Hidalgo, Pachuca, Mexico, 4 Department of Geoscience and Center for Urban Network Evolution, Aarhus University, Aarhus, Denmark, 5 Instituto de Investigaciones en Ciencias de la Tierra, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico

### *Edited by:*

Leonardo Santos Avilla, Universidade Federal Do Estado Do Rio de Janeiro, Brazil

### *Reviewed by:*

Manuel F. G. Weinkauf, Université de Genève, Switzerland Eric Scott, California State University, San Bernardino, United States

### *\*Correspondence:*

Eduardo Jiménez-Hidalgo<EMAIL_ADDRESS>Gerardo Carbot-Chanona<EMAIL_ADDRESS>
### *Specialty section:*

This article was submitted to Paleontology, a section of the journal Frontiers in Ecology and Evolution

*Received:* 29 January 2019 *Accepted:* 04 October 2019 *Published:* 18 October 2019

### *Citation:*

Jiménez-Hidalgo E, Carbot-Chanona G, Guerrero-Arenas R, Bravo-Cuevas VM, Holdridge GS and Israde-Alcántara I (2019) Species Diversity and Paleoecology of Late Pleistocene Horses From Southern Mexico. Front. Ecol. Evol. 7:394. doi: 10.3389/fevo.2019.00394 Equids are among the most common mammals found in faunal assemblages of Late Pleistocene age in Mexico. Much of what is known about the Equus species is the result of studies conducted in central and northern Mexico; much less is known about species in lower latitudes of Mexico. Here we describe three species that inhabited Oaxaca and Chiapas states. The fossil localities are in northwestern and central Oaxaca, as well as the central part of Chiapas. In Oaxaca, the largest species, Equus mexicanus, and the medium-sized Equus conversidens are represented by mandibles, skulls, diverse isolated teeth and some postcranial bones, while the smallest species, Haringtonhippus francisci is represented by a skull fragment and few isolated teeth. In Chiapas, E. mexicanus is represented by a mandible and several isolated teeth, E. conversidens by several mandibles and diverse isolated teeth, and H. francisci by isolated teeth and two mandibles. AMS radiocarbon and uranium dating of some of the equid localities in Oaxaca and Chiapas indicate that they were at least present since ∼44,000 Cal BP years, they were common around 30,000 Cal BP years, and were still present at the end of the Pleistocene, around 12,000 years ago. The record of H. francisci from Chiapas is the youngest in North America. A cluster analysis of extended mesowear data and a discriminant analysis showed that Equus conversidens from Chiapas was obligate grazer, whilst the rest of the equids were variable grazers. Geographic distribution of localities in southern Mexico indicates that during the Pleistocene the equid species moved across the Transvolcanic Belt-Sierra Madre del Sur temperate biogeographic corridor and the Tamaulipas-Central America Gulf Lowlands tropical corridor.

Keywords: *Equus*, Pleistocene, Mexico, Oaxaca, Chiapas, paleoecology, taxonomy

# INTRODUCTION

Equids are among the most common mammals in the Pleistocene Mexican faunal assemblages. Based on morphological characters, three or four Pleistocene horse species have been recently identified in Mexico. Alberdi et al. (2014) recognized Equus conversidens, Equus mexicanus, and Equus cedralensis. Priego-Vargas et al. (2017) recognized the previously mentioned species plus Equus francisci (see their table 10). Barrón-Ortiz et al. (2017), based on linear and geometric morphometric analyses of Equus specimens from the western interior of North America recognized three morphological groups of Equus in northeastern Mexico (San Josecito Cave, Nuevo León, and Cedral localities of San Luis Potosí) and they taxonomically identify such groups as E. ferus, E. conversidens and E. cedralensis; additionally, based on mitochondrial DNA analyses they recognized two equid clades [Caballine and New World Stilt-legged horses (NWSL)] and they referred the NWSL clade to E. conversidens. By means of paleogenomic and morphometric analyses, a new genus of NWSL horse for the Pleistocene of North America was erected: Haringtonhippus francisci. This species was reported (as Equus conversidens) from San Josecito Cave, Nuevo Léon, based upon a short mitochondrial DNA sequence (Barrón-Ortiz et al., 2017); this record was subsumed into H. francisci by Heintzman et al. (2017). Finally, Barrón-Ortiz et al. (2019) based on a morphology-based tree, determined that Equus comprises eight species, including H. francisci, and suggested that Haringtonhippus should be considered a synonym of Equus, but the authors do not formally synonymize both taxa.

At present there are 23 main Pleistocene equid localities in Mexico (Sánchez Salinas et al., 2016; Priego-Vargas et al., 2017); of them, just five (21.73%) were previously reported from southern Mexico (Doutt and Black, 1962; Pérez Crespo et al., 2008; Priego-Vargas et al., 2017; Díaz-Sibaja et al., 2018a). In most of these reports, the equids are only mentioned or briefly described.

In Oaxaca, Doutt and Black (1962) reported a molar of E. mexicanus near Yolomécatl (**Figure 1**), but they did not describe it. Many years later, Pérez Crespo et al. (2008) enlisted the Pleistocene mammalian localities from Oaxaca, including the horses (Equus mexicanus and E. excelsus); Jiménez-Hidalgo et al. (2012), briefly described the horses from the Viko vijin local fauna of northwestern Oaxaca (Equus mexicanus and E. conversidens) and Viñas-Vallverdú et al. (2017) enlisted Equus sp. as part of the Chazumba faunal assemblage. Finally, Díaz-Sibaja et al. (2018a) described some postcranial elements that were identified as Equus cf. E. conversidens from central Oaxaca (**Figure 1**).

In Chiapas, Gómez-Pérez and Carbot-Chanona (2012) briefly described a specimen of Equus conversidens from the Villaflores municipality; the next year, Carbot-Chanona and Ovalles-Damián (2013) enlisted the vertebrate fossil record from Chiapas, including E. conversidens. Two years later, Pérez-Crespo et al. (2015) determined the dietary habits of some mammalian taxa from the Pleistocene of central Chiapas (**Figure 1**), including four specimens of E. conversidens. Finally, Bravo-Cuevas and Jiménez-Hidalgo (2018) enlisted the records of Pleistocene mammals from several areas of central and southern Mexico, including the equids.

This meager knowledge of southern Pleistocene horses compared to what is known from central and northern Mexico impedes our understanding of the inter-specific disparity of the recognized Mexican species, and the magnitude of the intraspecific variation along the Mexican territory. Furthermore, at present it is not possible to fully comprehend the dietary plasticity of Mexican species nor their biogeographic patterns during the Pleistocene.

Thus, the aims of this paper are: (1) to describe the equid species that inhabited northwestern and central Oaxaca, and central Chiapas during the Pleistocene; (2) to disclose the probable dietary preferences and body mass of the equids from Chiapas and Oaxaca; and (3) to comment about the geographic distribution of Mexican equids during the Pleistocene and the habitat where they roamed in southern Mexico.

# MATERIALS AND METHODS

### Studied Specimens

The specimens from Oaxaca are housed at Laboratorio de Paleobiología, Universidad del Mar, Campus Puerto Escondido, Oaxaca, Mexico, under the acronym UMPE. Specimens from Chiapas are housed at Colección Paleontológica of the Museo de Paleontología "Eliseo Palacios Aguilera," Tuxtla Gutiérrez, Chiapas, Mexico, under the acronym IHNFG. They were collected at some of the localities shown in **Figure 1**.

Since we evaluated morphological characters of teeth and bones of fossil horses from southern Mexico and to avoid the unnecessary proliferation of names for the Mexican equid species, we follow the taxonomic scheme of Priego-Vargas et al. (2017, table 10) that recognized four Pleistocene Equus species for Mexico: Equus conversidens, E. mexicanus, E. francisci, and E. cedralensis. Since Heintzman et al. (2017) transferred E. francisci to Haringtonhippus, instead of E. francisci, we used the name H. francisci.

We preferred not to synonymize Haringtonhippus to Equus, as suggested by Barrón-Ortiz et al. (2019), given the profound discrepancies between the genomic and morphologic analyses about the phylogenetic position of H. francisci, which reflect that the definition of Equus is a work still in progress (Barrón-Ortiz et al., 2019). Future analyses combining morphology and genomics will shed light about the phylogenetic relationships of H. francisci.

In order to disclose the taxonomic identity of the equid specimens studied, they were compared to those housed at the Colección de Macromamíferos, Museo de Paleontología, Universidad Autónoma del Estado de Hidalgo (UAHMP), the Vertebrate Paleontology Collection of the Natural History Museum of the Los Angeles County (LACM/CIT), and a cast of the holotype of Equus mexicanus (IGM 4009) housed at the Colección Nacional de Paleontología, Instituto de Geología, UNAM. Likewise, anatomical descriptions and illustrations of the Mexican Pleistocene species were used (Hibbard, 1955; Reynoso-Rosales and Montellano-Ballesteros, 1994; Alberdi et al., 2014; Priego-Vargas et al., 2017).

**Abbreviations:** Teeth abbreviations are: C/c, Canines; I/I, Incisors; P/p, Premolars; M/m, Molars upper/lower. The number indicates the position of the tooth in the tooth row. Other abbreviations are: AMS, accelerator mass spectrometry, C14 dating; Cal BP, calendar years before present; l, left; LD, linkage distance; masl, meters above sea level; perBlunt, percentage of blunt cusps; perHigh, percentage of high occlusal relief; perLow, percentage of low occlusal relief; perRound, percentage of rounded cusps; perSharp, percentage of sharp cusps; r, right.

FIGURE 1 | Main Late Pleistocene equid localities from Oaxaca and Chiapas. 1. Chazumba, 2. Concepción Buenavista, 3. Río Sabinal, 4. Río Salado, 5. Cañada del Misterio, 6. Sandage, 7. Río Tejupam; 8. Yolomécatl, 9. Magdalena Peñasco, 10. Güilla, 11. San Dionisio Ocotepec, 12. San Martín de los Cansecos, 13, Nandachuquí, 14. Los Mangos, 15. Gliptodonte, 16. La Simpatía, 17. La Tejería.

## Measurements and Dental Nomenclature

Mandibular and skull measurements are in mm and follow the system of measurements for Equus bones and teeth (Eisenmann, 2009).

Teeth measurements are in mm and were taken following Eisenmann and Mashkour (2000).

Several measurements were taken from a basicranium specimen (**Figure 2**).

All measurements were taken with digital calipers and recorded within 0.01 mm of accuracy.

Dental nomenclature follows Evander (2004), and Reynoso-Rosales and Montellano-Ballesteros (1994). The used curvature index of teeth follows MacFadden and Carranza-Castañeda (2002).

### Paleoecological Aspects Dietary Guilds

### **Mesowear analysis**

An extended mesowear analysis was performed to include P4/p4 to M3/m3 (Kaiser and Fortelius, 2003; Kaiser and Solounias, 2003). We excluded teeth no yet in occlusion or showing initial wear or with damage on the occlusal surface, and those with a persisting crown height of <15 mm (Fortelius and Solounias, 2000; Kaiser and Fortelius, 2003; Kaiser and Solounias, 2003). A total of 112 cheek teeth were available for this study: 31 upper and 29 lower cheek teeth from Oaxaca (E. mexicanus, 13 upper, 16 lower; E. conversidens, 18 upper, 10 lower; H. francisci, three lower), and 39 upper and 13 lower cheek teeth from Chiapas (E. mexicanus, three upper; E. conversidens, 25 upper, eight lower; H. francisci, 11 upper, five lower). A hierarchical cluster analysis with Euclidean distance and complete linkage was performed on the mesowear variables perHigh, perSharp, perRound, perBlunt, of 35 extant species of herbivores with known dietary preferences (Fortelius and Solounias, 2000; Schulz and Kaiser, 2012) and five extinct Equus species (**Table S1**). The mesowear values of Equus grevyi and E. burchelli of Fortelius and Solounias (2000) were replaced by those of E. grevyi and E. quagga (= E. burchelli) of Schulz and Kaiser (2012).

We only coded the less-worn buccal cusp of each tooth, either the paracone or metacone in the P4-M3 and the protoconid or hypoconid in the p4-m3. The scored mesowear variables included the cusp shape (sharp, rounded, or blunt) and the occlusal relief (high or low). A sharp cusp is one that ends in a point and has no rounded area between the mesial and distal facet, a rounded cusp has a distinctly rounded apex but retains facets on the lower slopes and a blunt cusp is nearly flat and there are no facets (see Barrón-Ortiz et al., 2014). The occlusal relief depends on how high the cusp rises above the valley between paracone and metacone or protoconid and hypoconid (see Kaiser and Fortelius, 2003). All the scored data were converted to percentages, thus giving the variables perHigh, perSharp, perRound, perBlunt (**Table S1**). The dataset was analyzed with

R software version 3.6.0 (R Core Team, 2014), using the library "fpc" (flexible procedures for clustering) version 2.2-3. Data were normalized, Euclidean distance and complete linkage were used to generate the clusters. A bootstrap analysis was performed (100 replicates) to identify how reliable were the groups (Zumel and Mount, 2014; the used script is in **Supplementary Material 1**).

### **Discriminant function analysis**

To determine the accuracy of the classification among dietary guilds of the studied species, a discriminant function analysis was performed with the database used for the mesowear analysis, only using the dataset of extant species with known dietary habits as training set. Then, we ran a second analysis to predict the feeding group of the studied specimens. We used the linear discriminant method and common covariance as the data were of the same order of magnitude and were in covariance (Díaz-Sibaja et al., 2018b). Dietary category was the grouping variable (obligate grazer, variable grazer, mixed feeder, and browser). Posterior probabilities were calculated based on Bayes rules. We used the Wilk's lambda test (α = 0.05) to detect statistical differences between the multivariate centroids of the dietary categories. The analyses were carried out with JMP 8.0 (SAS Institute, 2019).

The cross-validation of the discriminant model was performed with the R library "flipMultivariates" version 0.1, using leave-one-out cross-validation.

### Body Mass Estimation

To estimate the probable body mass of the fossil horses from southern Mexico, we used the predictive equations of Janis (1990) for perissodactyls and hyracoids only:

In order to adhere to the equation specifications, the measurements of the teeth were converted to cm, and the estimated body mass to kg.

To detect if the estimated mean body mass of the identified species was statistically different, we used the Mann-Whitney U-test.

The datasets generated for this study can be found in the accompanying tables and in the **Supplementary Materials** and can be freely downloaded.

### STRATIGRAPHY OF FOSSIL LOCALITIES

Stratigraphic information is available for some of the studied fossil localities. In the Sandage locality, municipality of Coixtlahuaca, northwestern Oaxaca (**Figure 1**), an Equus mexicanus maxillary fragment with teeth was discovered within an A horizon of a paleosol. The A horizon is thick (> 1 m), dark, organic rich, having a silty loam texture, a prismatic structure and evidence for slickensides. The enamel of one molar was dated by AMS, offering an age of 29,426–30,123 Cal BP. The paleosol is covered by several meters of alluvial sediments.

Also, in northwestern Oaxaca, in the Mixteca region, there are two fossil localities within the municipality of San Antonio Acutla. In the Cañada del Misterio locality (**Figure 1**), diverse E. conversidens and E. mexicanus specimens were collected from a medium-grained pinkish sand measuring 40 cm thick, which gradually changes from fine-grained sand to a microconglomerate with clasts measuring up to 3.0 cm in diameter. The fossiliferous bed is unconformably underlain by a succession of light brown silty and clayey beds. Organic sediment and a snail shell of the fossiliferous bed were dated by AMS, returning ages of 43,030–44,510 Cal BP and 43,120–44,830 Cal BP, respectively. At another site in Cañada del Misterio, an Equus mandible was collected on the top of an 11.0 cm thick microconglomerate containing clasts of up to 3.0 cm in diameter, which was covered by 25 cm of dark gray fine silt with abundant charcoal fragments. One of these fragments was dated by AMS, giving an age of 29,320–29,765 Cal BP.

In the Llano de Hueso locality, also in the municipality of San Antonio Acutla, an Equus conversidens skull and some E. mexicanus isolated teeth were recovered in the top of a poorly sorted fine- to medium- grained sand, 30 cm of thick, with medium-grained sand of paleochannels. The sand bed gradually becomes siltier toward the top. The E. conversidens skull was dated by means of uranium series, offering an age of 24,650– 27,305 Cal BP (Ordoñez-Regil et al., 2016).

In the Río Salado locality (**Figure 1**), within Santiago Teotongo and San Antonio Acutla municipalities, several isolated teeth and some postcranial bones of E. conversidens, H. francisci, and E. mexicanus has been collected from fine-grained silty sand. A tooth of E. conversidens collected from the top of the stratigraphic sequence was dated by uranium series, giving an age of 11,380–12,620 Cal BP (Ordoñez-Regil et al., 2016).

In the Río Tejupam locality (**Figure 1**), within the Tejupam municipality, an H. francisci basicranium, as well as several E. conversidens and E. mexicanus specimens were collected from fine-grained silty sand and silty clay beds 25 cm thick (Jiménez-Hidalgo et al., 2011). Organic matter from the silty clayey sediments was dated by AMS returning an age of 19,584–20,180 Cal BP. The deposit of silty-clay is unconformably overlain by a 30 cm thick silty clay bed with dissecation marks. A massive finegrained sand that interbeds with silty sediments with abundant charcoal overlays the previous bed. One charcoal piece was dated by AMS and showed an age of 6,880–7,005 Cal BP.

In the Güilla locality of central Oaxaca (**Figure 1**), an E. mexicanus upper teeth series were collected from the bottom of a well-rounded, imbricated alluvial and fluvial conglomerate of andesite and limestone clasts ranging 6.0–8.0 cm in diameter and of 4.0 m in thickness.

In Los Mangos locality of central Chiapas (**Figure 1**), several teeth and some mandibles of H. francisci, E. conversidens, and E. mexicanus were collected from a 75 cm thick deposit of silty sediments which was overlaid with coarse sand lenses in a silty matrix. Organic sediment from the top of the fossiliferous sediment was dated through AMS, showing an age of 12,720– 12,820 Cal BP.

### Associated Fauna

In the Acutla localities the associated fauna consists of gastropods of the families Physidae and Succineidae, Coelocentrum, Polygyra couloni, Mesomphix (Omphalina) lucubratus and Gyraulus parvus, the bivalve Pisidium, the lizard Sceloporus, the ground sloth Nothrotheriops, the lagomorph Sylvilagus floridanus, the rodents Microtus mexicanus, Peromyscus difficilis, and Reithrodontomys cf. R. megalotis, the tapir Tapirus, the cervid Odocoileus virginianus, the bovid Bison antiquus and the proboscideans Cuvieronius hyodon and Mammuthus columbi (Jiménez-Hidalgo et al., 2011, 2013).

In the Río Salado locality, the Equus specimens are associated with Glyptotherium cylindricum and the camels Hemiauchenia macrocephala and Camelops hesternus. The mammals Bison antiquus and M. columbi are also associated with the equids in the Acutla, Río Salado and Río Tejupam localities. Additional taxa from Río Tejupam include the ostracod Candona, the gastropods G. parvus, Planorbella trivolvis, Lymnaeidae and Poligyridae gastropods, the bivalve Pisidium casertanum, the salamander Ambystoma, and the rodents M. mexicanus, Neotoma mexicana, Neotomodon alstoni, Peromyscus difficilis, and the rabbit S. floridanus (Jiménez-Hidalgo et al., 2011, 2013; Guerrero-Arenas and Jiménez-Hidalgo, 2015).

The associated fauna collected from Los Mangos locality include the glyptodont Glyptotherium cylindricum, the deer Odocoileus virginianus, the capybara Neochoerus aesopi and the bovid Bison sp. (Gómez-Pérez and Carbot-Chanona, 2012; Carbot-Chanona et al., in press).

### RESULTS

### Systematic Paleontology

Order Perissodactyla Owen, 1848 Family Equidae Gray, 1821 Tribe Equini Gray, 1821 Equus Linnaeus, 1758 Equus conversidens Owen, 1869

### Referred Material

Chiapas state: locality Los Mangos. IHNFG-5351, rM2. locality Gliptodonte. IHNFG-2683, lP3-M3 teeth series, lM1-M3 teeth series, lm3, lm1, or lm2; IHNFG-2856, left mandible with p2, p4-m3; IHNFG-2685, lP2-M3 and rP2-M3 teeth series, rp2 m1 teeth series; IHNFG-4148, lp2-m3 and rp2-m3 teeth series, IHNFG-4162, lp4; IHNFG-5762, lP4; IHNFG-5803, lm1-m3 teeth series. locality La Tejería 2. IHNFG-5758, lP3-M3 and rP2-M3 teeth series, lp3-m3 teeth series and rm3. locality La Simpatía. IHNFG-5789, rP4; IHNFG-5804, rp3. Oaxaca state: Locality Oax-4 Río Salado. UMPE 014, lM3; UMPE 448, lm3; UMPE 450, lP4-M1, p3; UMPE 452, lM1; UMPE 470, rP4; UMPE 472 rm2; UMPE 484, rm3; locality Oax-5 Llano de Hueso. UMPE 077, skull; UMPE 476, lm3; UMPE 477 rm1; UMPE 479 lm3; UMPE 481, lM2; UMPE 535, rM1. Locality Oax-6 Cañada del Misterio. UMPE 453, rm1; UMPE 454, rm3; UMPE 478, lM3; UMPE 482, lm3; UMPE 508, rP4; UMPE 542, rp3 UMPE 625, P2-M3 and rostral fragment. Locality Oax-7 Río Tejupam. UMPE 008 left maxillary fragment with DP4-M3; UMPE 539 lM1; UMPE 569, rP3; UMPE 620, left mandible with p2-m3 and incisors; UMPE 898, lm3. Locality Oax-8 Magdalena Peñasco. UMPE 455 rM2; UMPE 457, rm2; UMPE 458, rm2. Locality Oax-17 Río Sabinal. UMPE 531, rM2. Locality San Martin de los Cansecos. UMPE 911, cast of lm3.

### DESCRIPTION

### Skull

The specimen UMPE 077 is somewhat dorsoventrally crushed (**Figure 3**). The skull is broken behind the retro-articular processes of the temporal bones. The nasal notch is retracted to a position that lies dorsal to the mesostyle of P2. The rostrum is deep and the facial crest begins at the level of the mesostyle of P4. Compared to extant Equus, the muzzle is short. The infraorbital foramen is located dorsal to the metastyle of P3. Measurements of the skull are in **Table 1**.

The palatine is lateromedially concave and narrow; the major palatine foramina and the rostral margin of the choanae are located at the level of the M2's protocone. The teeth series converge rostrally, especially the right one (**Figure 3**). The mandibular fossa is shallow.

### Mandible

The mandibles are somewhat stout (**Figure 3**), increasing in height from the premolars to the molars (**Table 2**). The mental foramen is ovoid in outline and well-developed, and it is below the diastema, which is short. The incisor's arcade is quadrangular in outline. The symphysis is relatively slender (**Table S2**).

# Upper Teeth

The P2 is triangular in outline, rostro-caudally short (L/B mean ratio of 1.398), the anterior accessory rib is slightly developed, the parastyle is rounded to slightly acute on its rostral portion; the mesostyle is rectangular in outline and the metastyle is slightly developed. The pli protoloph and the pli protoconule are deep; the pli postfossette and pli hypostyle are also deep. The protocone varies from long to rounded. The plicaballine is moderately developed (**Figure 3**).

The P3 and P4 are quadrangular (**Table S2**), with quadrangular or rounded parastyle and mesostyle, the metastyle is slightly developed; there are 2 or 3 folds in the caudal part of the prefossette, the pli protoloph is deep and the pli protoconule could be shallow or deep. The pli postfossette is deep and the pli hypostyle is shallow or deep. The protocone is elongated and slightly curved on its lingual side, and its length represents around 50% the total tooth length (**Table S2**).

The M1 and M2 are quadrangular (**Table S2**), the parastyle is rounded, the mesostyle is quadrangular to rounded and the metastyle is slightly developed; there are three or no folds in the caudal part of the prefossette, the pli protoloph is shallow to absent; the pli protoconule is deep; the pli postfossette generally is deep, but it is absent in one specimen; the pli hypostyle is shallow or absent. The protocone is elongated and usually flat, but in few specimens, it is slightly curved in its lingual part (**Figure 3**), its length represents slightly less than the 50% of the total tooth length. There is no pli caballine in these molars.

The M3 is triangular in outline and small (**Table S2**), its parastyle and mesostyle are rounded, and the metastyle is not present. There are 1–3 folds in the caudal part of the prefossette, the pli protoloph is absent, the pli protoconule can be shallow or deep. The pli postfossette is shallow to absent; the pli hypostyle is deep or absent. The protocone is long, flat, slightly curved or sinuous in its labial side (**Figure 3**). The M3's have a curvature index of 40.

### Lower Teeth

The incisors lack infundibula; the i3 is separated by the canine by a short diastema (**Figure 3**). In males the canine is well-developed and bears a crest that runs all along the crown height.

The p2 is triangular in outline (**Table S3**), the metaconid is rounded and the metastylid has an acute tip, the linguaflexid is "U"-shaped, there is a rounded entoconid; the postflexid is very long. The ectoflexid is shallow and does not penetrate the isthmus. There is a faint pli caballinid.

The p3 and p4 are rectangular, the metaconid is rounded and the metastylid is triangular (**Figure 3**), the linguaflexid is "V"-shaped; the postflexid is long and with a somewhat sinuous labial margin; usually, the ectoflexid is moderately deep and does not penetrate the isthmus; the pli caballinid is slightly developed.

The m1 and m2 are also rectangular (**Table S3**) and the m3 is triangular in outline. They have a rounded metaconid and a triangular metastylid; the linguaflexid is "V" or "U"-shaped (**Figure 3**); the postflexid is long and less sinuous than in the premolars or almost flat; the entoflexid is moderately deep or deep and can or cannot penetrate the isthmus. The pli caballinid can be present or absent.

# DISCUSSION

The studied specimens have a close morphology and dimensions when compared to fossil material of Equus conversidens from central and northern Mexico. In this species the muzzle is short and tapers rostrally, the infraorbital foramen is set high, the facial crest extends anteriorly to a position dorsal to P4, and the teeth series converge rostrally, as in UMPE 077 (**Figure 3**; **Table 1**). The studied upper teeth from Oaxaca and Chiapas are of medium size compared to the other Mexican Pleistocene species (**Table S2**), the P2 is rostro-caudally short and the M3 is small. The mandible is stout and has a similar size to those identified as E. conversidens (**Table 2**); the lower teeth have an elongated protocone that is slightly curved on its lingual side, the premolars and molars have few folds in the inner side of the fossettes, they have V-shaped linguaflexids and moderately deep ectoflexids; the size of the teeth is similar to those of

occlusal view.

TABLE 1 | Skull measurements of the equids from the Pleistocene of southern Mexico and other selected specimens from North America.

\*Data from Eisenmann (2009). Description of each measurement is in *Supplementary Material 2*.

E. conversidens (**Table S3**) and intermediate in size between E. mexicanus, E. cedralensis, and H. francisci (Azzaroli, 1998; Bravo-Cuevas et al., 2011; Alberdi et al., 2014; Priego-Vargas et al., 2017).

The above described combination of characters identifies the studied specimens as Equus conversidens.

In the studied sample, the M3 of E. conversidens has the lowest curvature index (40 vs. 70 and 50 in the other species). If this curvature value is also present in specimens from other localities, it could be useful to identify isolated M3 of this species.

On the other hand, the presence/absence of incisors' infundibula is very variable within horse species, and it also changes with tooth wear (Barrón-Ortiz et al., 2017). Thus, we do not consider it as a useful taxonomic character, but we believe it is important to describe the character state of the sample from southern Mexico.

Equus mexicanus (Hibbard, 1955)

\*Data from Eisenmann (2009). Description of each measurement is in *Supplementary Material 3*.

### Referred Material

Chiapas state: locality Los Mangos. IHNFG-4872, mandible with both series of DP2-M2, the left incisors and both canine roots; IHNFG-4886, lP3 or lP4; IHNFG-5660 rP4. Locality Nandachuqui. IHNFG-694, rP2; IHNFG-691, rM1 or rM2. Locality La Simpatía. IHNFG-5352, rM3. Locality La Tejería. IHNFG-4700, Mx. Oaxaca state: Locality Sandage. UMPE 908, lP4-M3 and rM1-M3 teeth series. Locality Oax-4 Río Salado. UMPE 011, lM1; UMPE 900 rm3; UMPE 447, calcaneum; UMPE 489, left metatarsal III. Locality Oax-5 Llano de Hueso. UMPE lP4; UMPE 474, P3; UMPE 480, Mx; UMPE 490, right mandible with p2-m2; UMPE 903, rP4. Locality Oax-6 Cañada del Misterio. UMPE 546, rp3; UMPE 643, dentary with both p2 m3 teeth series, the canines and incisors; UMPE 897, lM2; UMPE 901, lP2; UMPE 902, rm2. Locality Oax-7 Río Tejupam. UMPE 505, rM2. Locality Oax-17 Río Sabinal. UMPE 521, skull; UMPE 554, left mandible with p3-m3; UMPE 567, lP2; UMPE 611, lm2. Locality Güilla. UMPE 907, lP2-M3 and rP2-Px-M2.

### DESCRIPTION

### Skull

The specimen UMPE 521 is dorsally crushed, so, it is not possible to observe any undistorted feature. It is large, caudally, the skull preserves the left temporal articular tubercle and the retroarticular process (**Figure 4**). The premaxillaries are stout. The nasal notch is retracted to a position that lies dorsal to the mesostyle of P2. Rostrally, the facial crest ends between the P4 and the M1. The muzzle is short (**Table 1**), as in the skull of E. conversidens.

The palatine is wide (**Table 1**); the anterior margin of choanae is immediately behind M2's protocone (**Figure 4**). The upper teeth series seems straight. The mandibular fossa is shallow.

### Mandible

The mandible is somewhat stout, its height increases caudally along the teeth series (**Table 2**). The diastema is short. The incisor's arcade has a "U" outline. The symphysis is somewhat slender. The articular process is very similar to extant Equus.

### Upper Teeth

The P2 is long and triangular in outline (**Table S2**), the anterior accessory rib is slightly developed, the parastyle is acute to slightly rounded, the mesostyle is rectangular in outline (**Figure 4**), there is a deep pli protoloph and pli protoconule; the pli postfossette and pli hypostyle are also deep. The protocone is subrounded with a wide isthmus. The plicaballine is moderately to well-developed.

The P3 and P4 are quadrangular, with quadrangular parastyle and mesostyle, there are 4–5 folds in the caudal part of the prefossette, and the pli protoloph and the pli protoconule are deep. The pli postfossette and pli hypostyle are also deep. The protocone is elongated and slightly curved on its lingual side, its length represents around 2.41 of the total length of these premolars. The plicaballine is single and moderately developed.

The M1 and M2 are quadrangular, the parastyle is rounded to quadrangular, the mesostyle is rectangular in outline, there is 1–3 folds in the caudal part of the prefossette, the pli protoloph is very variable, it can be absent, be moderately deep or deep; the pli protoconule can be shallow to deep. The pli postfossette varies from moderately deep to very deep; the pli hypostyle varies from shallow to deep. The protocone is elongated and can be slightly curved to flat on its lingual side; its length represents around 2.14 of the total teeth length. The pli caballine is faint to absent, only in the M2 UMPE 505 it is well-developed.

The M3 is triangular in outline (**Table S2**); it has a rounded parastyle, a rounded to slightly quadrangular mesostyle and there is no metastyle (**Figure 4**). There are one to four shallow folds in the caudal part of the prefossette, the pli protoloph is shallow, the pli protoconule can be shallow or deep. The pli postfossete is faint to absent, the pli hypostyle is deep. The protocone is long and flat to slightly curved lingually. The M3's have a curvature index of 50.

### Lower Teeth

The incisors lack infundibula; the i3 is separated by the canine by a very short diastema. In females the canine is a very small conical tooth.

The p2 is triangular in outline (**Table S3**), the metaconid is rounded and the metastylid has an acute tip, the linguaflexid is "V"-shaped, there is a rounded entoconid; the postflexid is very long. The ectoflexid is shallow and does not penetrate the isthmus.

The p3 and p4 are rectangular (**Table S3**), the metaconid is rounded and the metastylid is triangular to ovoid with an acute tip in outline, the linguaflexid is variable, can have a "V" shape, or is "U"-shaped; the postflexid is very long and with a sinuous labial margin; the ectoflexid is moderately deep and does not penetrate the isthmus; the pli caballinid can be present or absent.

The deciduous premolars of IHNFG-4872 are large and their morphology is very similar to those of the permanent premolars

(**Figure 4**), the ectoflexid is deep and penetrates the isthmus. Their measurements are in **Table S4**.

The m1 and m2 are almost quadrangular and the m3 is triangular in outline. They have a rounded metaconid and a triangular to slightly rounded metastylid; the linguaflexid is "V" or "U"-shaped, the postflexid is long and less sinuous than in the premolars; the ectoflexid is deep and penetrates the isthmus. The pli caballinid is present to slightly developed.

### Postcranial Bones

The partial left calcaneus UMPE 447 is large, stout and with a quadrangular outline; the sustentaculum is broken. The left metatarsal UMPE 489 is stout and dorsoventrally short (**Figure 4**); its length is 263.1 mm, the proximal width is 55.3 mm, diaphysis width at middle is 34.49 mm; distal width is 52 mm.

# DISCUSSION

The skull UMPE 521 shares with the holotype of Equus mexicanus its large size, a nasal notch that is retracted to a position that lies dorsal to the P2 and a facial crest that ends between the P4 and the M1 (Hibbard, 1955); the mandibles from Chiapas and Oaxaca share their large size (**Table 2**) and robustness with this species (Azzaroli, 1998).

The upper teeth from southern Mexico have a large size with moderately to complexly plicated enamel on fossettes, long, flat to somewhat curved protocones, a deep pli protoloph and pli hypostyle on premolars and a long P2; these features are present in E. mexicanus, as are those of the lower molars, such as a large size, a predominantly triangular metastylid, and deep to moderately deep ectoflexids.

The limbs bones are large and stout, as those of E. mexicanus from Cedral, San Luis Potosí (Alberdi et al., 2014).

The P2s of E. mexicanus and H. francisci are longer than those of E. conversidens; by the same token, the occlusal area of the M3 in these species is comparatively larger than that of E. conversidens (**Table S2**) and its curvature index is higher.

The above described combination of characters allows this sample to be identified as Equus mexicanus.

Haringtonhippus (Heintzman et al., 2017) Haringtonhippus francisci (Hay, 1915)

### Referred Material

Chiapas state: locality Los Mangos. IHNFG-4211, dentary with both p2-m3 teeth series, the canines and incisors; IHNFG-4698, lP2-M3 and rP2-M2 teeth series; IHNFG-4700, rP2; IHNFG-4211, mandible with lp3-m1, rp2-m3, i1-2 and the canines; IHNFG-4699, mandible with lp2-m3, incisive fragments, and canines. Locality Gliptodonte. IHNFG-4155, lP2-M1 teeth series and rP4, rM2; IHNFG-2708, Mx. Oaxaca state: locality Oax-5 Llano de Hueso. UMPE 906, rP4. Locality Oax-6 Cañada del Misterio. UMPE 905, lP3. Locality Oax-7 Río Tejupam. UMPE 561, basicranium. Oax-17 Río Sabinal. UMPE 904, lP4.

### DESCRIPTION

### Skull

The available skull is broken, only the basicranium is preserved (**Figure 5**); the left bones are slightly distorted; the bones seem somewhat slender. The zygomatic arches are missing. The caudal part of the frontal is present, it is flat as in extant Equus; the parietals are somewhat bulbous, and the median sagittal crest is slightly developed. The foramina that are present at the base of the temporal crest are large. Measurements of the basicranium are in **Table 3**.

The external auditory meatus is faintly ovoid in outline; the mastoid process is triangular in outline as in extant Equus.

The nuchal crest is missing, the occipital condyles are very similar to the ones of extant Equus, the jugal processes are dorsoventrally short; the basioccipital bone is slender, as is the preserved portion of the basisphenoid. The retroarticular processes are also dorsoventrally short. The preserved part of the mandibular fossa is deeper than in E. conversidens and E. mexicanus specimens from Oaxaca.

### Mandible

There are two mandible fragments in the sample (**Figure 5**). They are small and somewhat stout; they increase in height from the premolars to molars (**Table 2**). The mental foramen is ovoid in outline and well-developed.

### Upper Teeth

The P2 is triangular in outline and long (**Table S2**), the anterior accessory rib is slightly developed, the parastyle is rounded, the mesostyle is rectangular to slightly rounded and the metastyle is poorly developed (**Figure 5**). The pli protoloph and the pli protoconule are moderately deep; the pli posftossette and pli hypostyle are also moderately deep. The protocone is short. The plicaballine is well-developed in IHNFG-4698, but is absent in IHNFG-4155, which has more worn teeth.

The P3 and P4 are quadrangular, with quadrangular to subrounded parastyle and mesostyle; the metastyle is slightly developed (**Figure 5**). There are 2 or 3 folds in the caudal part of the prefossette, the pli protoloph is deep to shallow, the pli protoconule is deep. The pli posfossette is deep to moderately deep and the pli hypostyle is shallow. The protocone is elongated and slightly curved on its lingual side, and its length represents around 43.3% of the total length of these premolars (**Table S2**); the pli caballine is absent.

The M1 and M2 are quadrangular (**Table S2**), the parastyle is rounded, the mesostyle is quadrangular and the metastyle is slightly developed. There are two or no folds on the caudal portion of the prefossete, the pli protoloph is deep to shallow, the pli protoconule, the pli postfossete and the pli hypostyle are shallow. The protocone is elongated and flat to slightly concave in its lingual part; it represents around 45% of the total tooth length. The pli caballine is absent.

The M3 is triangular in outline, the parastyle and mesostyle are quadrangular in outline, the metastyle is absent. There is 1-fold in the caudal part of the prefossette, the pli protoloph is somewhat deep, the pli protoconule is very deep; the pli postfossette is absent and the pli hypostyle is deep. The protocone is very long and flat. The M3 has a curvature index of 70.

### Lower Teeth

The incisors lack infundibula. In males the canine is welldeveloped and bears a crest that runs all along the crown.

The p2 is triangular in outline (**Table S3**), the metaconid is rounded and the metastylid has a sub-acute tip; the linguaflexid has an open "V" shape, the entoconid is rounded and the postflexid is long. The ectoflexid is moderately deep and does not penetrate the isthmus. There is not pli caballinid.

The p3 and p4 are rectangular (**Table S3**), the metaconid is rounded and the metastylid is triangular, the linguaflexid is predominantly "V"-shaped, but few are "U"-shaped; the postflexid is relatively long and almost flat; the ectoflexid is moderately deep and does not penetrate the isthmus. There is no pli caballinid.

The m1 and m2 also are rectangular (**Table S3**) and the m3 is very elongated and triangular in outline. These molars have a rounded or sub-acute metaconid and a triangular to subrounded metastylid; the linguaflexid is "V" or slightly "U"-shaped; the postflexid is somewhat long and flat or slightly concave; the ectoflexid is deep and penetrate the isthmus. The pli caballinid is absent.

### DISCUSSION

The basicranium from Oaxaca is most similar in size to a skull identified as H. francisci (LACM(CIT) 109/156450) from the Pleistocene of Nevada (Heintzman et al., 2017); UMPE 561 is smaller than those of E. conversidens and E. mexicanus (**Table 3**). The mandible measurements of IHNFG-4211 are like those of the mandible TAMU 2518, which is part of the holotype of H. francisci (**Table 2**).

The upper molars from Chiapas and Oaxaca share with E. cedralensis and H. francisci their small size (**Table S2**), their flat or somewhat curved and elongated protocone, the few plications on the fossettes, the absence of plicaballine and the very elongated protocone of M3. The lower molars are also of small size (**Table S3**) and share with the small equid species a rounded metaconid and a predominantly acute metastylid, and a lack of pli caballinid (Hay, 1915; Lundelius and Stevens, 1970; Alberdi et al., 2014; Priego-Vargas et al., 2017).

Teeth of H. francisci and E. cedralensis have a very similar occlusal pattern and size (Barrón-Ortiz et al., 2017; Priego-Vargas et al., 2017), but according to Alberdi et al. (2014), E. cedralensis does not possess slender metapodials as H. francisci, but shorter and stouter ones. The studied sample of small

TABLE 3 | Basicranium measurements of Pleistocene equid species present in Mexico.

Number of measurements as in *Figure 2*. Measurement 8 is the width between beginning of the sagittal crest to the external part of the temporal articular tubercle. \*Measurements taken from the image of Eisenmann (2009).

teeth was not associated with any metapodial; nonetheless, the basicranium and the mandible are morphologically similar and about the same size as those of H. francisci, and giving that this small species was the first erected (Hay, 1915), we identify the small specimens from southern Mexico as H. francisci.

### PALEOECOLOGICAL ASPECTS OF EQUID SPECIES FROM SOUTHERN MEXICO

### Dietary Guilds

The cluster of the mesowear analysis shows five main groups (**Figure 6**); one containing the hypergrazer Bison bison and Equus conversidens from Hidalgo (LD = 0.86), one with obligate grazers and variable grazer species (LD = 3.3), one with exclusively variable grazers (LD = 3.3), another with variable grazers and mixed feeder species (LD = 2.4), and a cluster with browser species (LD = 2.0). The bootstrap nodal support of these five main clusters range from 0.70 to 0.95, meanwhile the support of the equid species from southern Mexico ranges from 0.62 to 0.97 and those of the Pleistocene equids from other regions of Mexico varies from 0.68 to 0.93 (**Figure 6**).