The Earliest Occurrence of Equus in South Asia

ABSTRACT Fossil horses are ubiquitous members of late Neogene and Quaternary ecosystems in Eurasia and Africa. The genus Equus is conventionally thought to be a Pleistocene immigrant into Eurasia from North America, and the concurrent appearance of these monodactyl equids in Eurasia around 2.58 Ma is accepted as the Equus Datum. Here, we report on a specimen of Equus found near the village of Jhil, from the latest Pliocene of the Upper Siwaliks of India. The specimen was recovered from sediments that are lithologically equivalent to the Upper Pliocene Saketi Formation, and have been dated paleomagnetically to lie just below the Gauss–Matuyama boundary, therefore, the latest Pliocene. Our comparative work shows that the anatomy of Jhil specimen is consistent with the Early Pleistocene Siwalik horse, Equus sivalensis. Our finding extends the temporal distribution of this species into the latest Pliocene, and adds to our understanding of the variation in this species of horse. We also argue that along with potential latest Pliocene occurrences of Equus from Europe, and accounting for the Signor–Lipps effect, the Equus Datum should be revised as a latest Pliocene event.


INTRODUCTION
One-toed horses in the genus Equus Linnaeus, 1758 first evolve in the Pliocene of North America (Cirilli et al., 2022;Rook et al., 2019).It is generally accepted that Equus simplicidens Cope, 1892 is the earliest species in the genus (Bernor et al., 2019;Cirilli et al., 2021b; but see Barrón-Ortiz et al., 2019 for an alternative systematic scheme).The earliest records of Equus in Eurasia seem to occur in the Lower Pleistocene, at the Gauss-Matuyama paleomagnetic boundary (Bernor et al., 1992).Sporadic reports of Equus from the Late Pliocene do exist (Kumar et al., 2014;Moigne et al., 2016), but these specimens have either never been figured, so cannot be confirmed (e.g., Moigne et al., 2016), or have been reclassified as hipparionines (Jukar et al., 2019).The cranio-mandibular remains that we describe here were discovered in situ in sediments that have been paleomagnetically dated to just below the Gauss-Matuyama boundary, and therefore the very latest Pliocene.Therefore, these specimens represent one of the earliest records of Equus not only in the Indian Subcontinent, but also in Eurasia.

GEOLOGICAL SETTING
The specimen we describe herein was recovered from Upper Siwalik subgroup deposits in a stream bank in the Jhil-Bankabara area of Sirmaur District, Himachal Pradesh, India (Fig. 1).The specimen was excavated from a pinkish-gray variegated mudstone approximately 8.0 m above the channel bed.The mudstone is situated below a layer of gray sandstone, which is overlain by a layer of buff mudstone.The deposits correlate lithologically with the upper Pliocene Saketi Formation of the Upper Siwalik subgroup (Nanda, 2015).The area around the village of Jhil and along the Somb river and its tributary the Jamni Khol in the Indian State of Himachal Pradesh was mapped using paleomagnetic techniques by Sangode et al. (1996).Based on the transect they surveyed and their paleomagnetic sequence, the site that produced GU/J-001 is located just below the Gauss-Matuyama boundary in the upper Gauss chron, most likely in the uppermost normal polarity section, and stratigraphically below the Lower Pleistocene Pinjor Formation, which corresponds well with the lithology of the site (Fig. 2).While we cannot pinpoint our section to the transect surveyed by Sangode et al. (1996), the general location in relation to their transect allows us to interpret the age of the site (Fig. 2).A comparison of the matrix attached to the specimen with the surrounding matrix shows the presence of similar proportions of feldspar and quartz (Fig. S1), illustrating that it is unlikely to be reworked post deposition.
The Upper Siwalik subgroup, commonly known as the Upper Siwaliks, includes sediments ranging in age from ∼3.6-0.6 Ma, with fossiliferous deposits extending from the Potwar Plateau in Pakistan in the west to Nepal in the east (Barry et al., 2013;Corvinus, 2006;Dennell et al., 2006;Hussain et al., 1992;Ranga Rao, 1993;Ranga Rao et al., 1988).There is general consensus among Siwalik workers about the nomenclature of the Lower Pleistocene deposits-these deposits are classified as the Pinjor Formation.The Upper Pliocene deposits have been classified as various formations.A localized sandstone bed ranging in age from 3.5-3.3Ma on the Potwar Plateau is the type section of the Tatrot Formation (Barry et al., 2013).Fauna from this formation are defined as belonging to the Tatrot Faunal Zone (Barry et al., 2013).Given the ubiquity of these fauna across space, the term Tatrot Formation is often, but inappropriately, used for similarly aged rocks in the Siwalik Hills of India (Nanda, 2015).In the Siwalik Hills around the city of Chandigarh, these upper Pliocene rocks are commonly referred to as the Saketi Formation (Nanda, 2015), or the Dhamla Formation (Azzaroli & Napoleone, 1982).Although Sangode et al. (1996) referred to the upper Pliocene deposits around the village of Jhil along the Somb and Jamni rivers as the Tatrot Formation, we adhere to the local definition of the Tatrot Formation on the Potwar Plateau and follow Nanda (2015) in classifying these rocks the Saketi Formation.

METHODS
The specimens we describe herein were excavated by NS, RSR, and RP.Specimens were removed and cleaned with brushes and dental picks, and later accessioned in the collections of the Department of Geology, Hemwati Nandan Bahuguna Gharwal University in Srinagar, Uttarkhand, India.All measurements were taken using digital calipers accurate to 0.01 mm.AMJ measured the comparative specimens here using digital calipers at various museums.Further comparisons were undertaken using published images, or personal observations.Dental and cranio-mandibular measurements were taken following Bernor et al. (1997).We compared this specimen with other cranial and mandibular remains of Equus from the Lower and Middle Pleistocene of South Asia housed at the NHMUK, PU, WI, GSI, and RGM.
Anatomical Abbreviations--Upper teeth are abbreviated using upper-case P for premolars and upper-case M for molars followed by numbers 1, 2, etc. to denote serial position.Lower teeth are abbreviated using lower-case p for premolars and lower-case m for molars followed by numbers 2, 3, etc. to denote serial position.
Diagnosis-A large species of monodactyl stenonine horse, nasal notch incised to the mesostyle of P3, the posterior portion of the premaxillary in lateral view does not extend far beyond the nasal notch, nasals are broad in dorsal view and have a concave dorsal profile in lateral view, preorbital fossa is weakly defined as a faint depression high dorsally and posteriorly on the maxilla extending to the lachrymal bone, facial crest is strongly developed and extends anteriorly to the level of the P4, infraorbital foramen is oval shaped and ventrally oriented above the P4, frontals are very broad, orbit is large, brain case is relatively small and oriented downwards, anterior palatine foramina originate at the level of the posterior margin of the M3, choanae are broad and oval shaped, with their anterior margin at the level of the M3 mesostyle, the basilar portion of the occipital is elongated, maxillary tooth row often includes a persistent dP1, protocones on P2-M1 are short and triangular in shape with a moderate lingual notch, protocones are more elongated on M2-M3, hypoglyphs are moderately incised, fossettes are only slightly to moderately ornamented, mesostyles tend to be rounded occasionally with a shallow notch, a single pli caballinid is present, mandible is large and robust, ventral border is straight from p4-m2, lower premolars have an elongate metaconid and squared metastylid, prominent preflexid pli is present on p2-m3, single prominent pli caballinid present, linguaflexids are deep and v-shaped, ectoflexids are shallow and do not invade the isthmus, metapodials are elongate and robust.
Locality, Horizon, and Age-Near the village of Jhil, Sirmur District, Himachal Pradesh, India (30°28′27.07″N,77°25′09.72″E).The specimen GU/J-001 was collected from a pinkish-gray variegated mudstone bed, 6.5 m above the channel bed.Lithologically, the section belongs to the upper Pliocene Saketi Formation (Fig. 1).Based on paleomagnetic studies conducted by Sangode et al. (1996), the locality occurs just below the Gauss-Matuyama boundary, and therefore is slightly older than 2.58 Ma; although the exact age cannot be presently determined (Fig. 2).

Skull
The skull (Fig. 3) is represented by the left and right maxillae, premaxillae, nasals, and left and right upper dentition (P1-M3).The left side of the skull (Fig. 3A) is better preserved than the right (Fig. 3B).Much of the right external bone surface has eroded away on the right.The preorbital fossa is very shallow and oval-shaped, and located just above the facial crest (Fig. 3A).The facial crest is prominent and robust and the anterior end terminates above the mesostyle of the P4.The infraorbital foramen is oval shaped and located 70 mm on the maxilla above the P4.Only the medial portion of the left nasal is well preserved, with anterior and posterior ends incomplete (Fig. 3A, C).The nasopremaxillary suture is also obscured, however, the ventral surface of the nasal notch formed by the dorsal surface of the premaxillary bone is well preserved (Fig. 3A).The proximal margin of the nasal notch is located above the mesostyle of the P3 (Fig. 3A).

Maxillary Tooth Row
The left maxilla preserves dP1-M3 whereas the dP1 is absent on the right maxilla (Figs.3D, 5A).Standard dental measurements are provided in Table 1.The dP1 is oval shaped and labiolingually compressed.The P2 is a long, triangle-shaped tooth.Based on the classification scheme of Bernor et al. (1989), the plications are short with a low degree of complexity.A single pli caballinid is present.The mesostyle is divided.The protocone is triangular in shape with a very slight lingual depression or notch.The hypoglyph is moderately incised.The P3 and P4 are largely similar in morphology.Both are roughly square-shaped teeth.Pre and post fossette plications are simple.The hypoglyphs are moderately incised.Mesostyles are rounded.The P3 has a single pli caballinid whereas the P4 has a double pli caballinid.The protocones on both teeth are short and triangular in shape, but with a more deeply incised lingual depression.Protocones get progressively more elongated in the premolar series.The M1 and M2 are similar in morphology.The pre and post fossettes are better developed than the premolars, but plications remain simple.Mesostyles are rounded and single pli caballinids are present on both teeth.Hypoglyphs are also moderately incised.Molar protocones are more elongated, but retain the triangular shape with a lingual depression.The M3 is a triangular tooth, shorter than the P2.Fossettes are simple with few to no plications.Mesostyle is rounded.Unlike the M1 and M2, the hypoglyph is absent on this tooth.The protocone is more elongated.A short metastyle is present at the distal end of the tooth.

Mandibles
The left mandible is better preserved than the right (Fig. 4).Both lack the anterior end and symphysis.The left mandible preserves part of the posterior angle (Fig. 4B, C).The ventral surface is gently curved.The posterior angle flares out below the m2, and forms a gentle curve into the ascending ramus.The coronoid process is not preserved.

Mandibular Cheek Tooth Row
Both mandibles preserve complete tooth rows (p2-m3; Figs.4A, D, 5C, D).Standard dental measurements are  provided in Table 1.The p2 has a relatively short anterostylid, and the metaconid is rounded while the metastylid is pointed.Linguaflexid is v-shaped, typical of a stenonine horse.The ectoflexid is shallow.The anterior preflexid pli is slightly longer than the posterior.The anterior margin of the postflexid is positioned close to the linguaflexid, and has a wavy outline.A single pli caballinid is present.The p3 and p4 are largely similar in morphology.The metaconid is squared while the metastylid is pointed.The ectoflexid is deeper than on the p2.A single pli caballinid is present.The anterior pli of the preflexid is more prominent.The m1 and m2 are similar to the premolars in morphology; however, the ectoflexids are deeper.Both possess single pli caballinids and prominent preflexid plis.The m3 has a rounded metaconid and pointed metastylid.
The parastylid is extended towards the mesial border of the tooth.The hypoconulid is trilobed in the right m3 but bilobed in the left m3.All teeth have a more elongated postflexid than preflexid.

DISCUSSION
We herein present cranio-mandibular remains from a large stenonine horse that was recovered in situ from just below the Gauss-Matuyana boundary, i.e., the very latest Pliocene, in the Upper Siwaliks of India.Dental characters-such as protocones attached to the protoloph, the presence of asymmetric plis on the preflexid, v-shaped linguaflexid, and shallow ectoflexids-are synapomorphies of monodactyl stenonine horses in the genus TABLE 1. Measurements for the upper and lower dentition of GU/J-001, Equus sivalensis, measured on the left upper and lower tooth rows.Abbreviations: MMl = length at occlusal level; MM3 = width at occlusal level, taken across mesostyle-protocone; MM6 = number of plications on the anterior face of the pre fossette; MM7 = number of plications on the posterior face of the pre fossette; MM8 = number of plications on the anterior face of the post fossette; MM9 = number of plications on the posterior face of the post fossette; MM10 = protocone length; MM11 = protocone width.mml = length at occlusal level; mm3 = length of metaconid-metastylid; mm4 = length of the pre fossette; mm5 = length of the post fossette; mm6 = width of tooth across plane of ectoflexid/linguaflexid; mm8 = W3, width across plane of metastylid and enamel band labial to hypoconid; mm9 = crown height as measured from base to occlusal level on mesial face of the tooth.We further compared GU/J-001 with referred skulls of E. sivalensis, GSI C179 (Lydekker 1882:pl.XIV, fig.2; Fig. S2G,  H), RGM.DUB.3222(Hooijer, 1951:figs.1, 2; pl.IX; Fig. S2C,  D), PU A.639 (Sahni & Khan, 1961b:pl.I-III; Fig. S2I, J), and WIF/A443 (Nanda, 2015:pl. 26; Fig. S2E, F), and referred mandibles of E. sivalensis PU B.40 (Sahni & Khan, 1988:fig. 96; Fig.  (Sahni and Khan, 1961b:pl.III-IV; Fig. S3E,  F).GSI C179 is a skull housed at the Indian Museum, Kolkata collected near the village of Padri.The skull is largely complete except the muzzle.The anterior position of the facial crest, morphology of the preorbital fossa, and dental morphology matches GU/J-001.Importantly, this skull preserves the left dP1, which is elongated and oval-shaped as in GU/J-001 (Fig. 6A, D).RGM.DUB.3222 was collected near the village of Jhil, therefore within the vicinity of the locality that produced GU/J-001.Further details about the exact locality or stratigraphy are unavailable.This skull is laterally compressed and distorted, but shows identical dental morphology (Fig. 6A, C), and the placement of the nasal notch (Hooijer, 1951:pl. IX).WIF/A443 is a large incomplete skull collected near the village of Madhowala in the Indian state of Haryana.The teeth are well worn, but exhibit the characteristic protoconal morphology of E. sivalensis, i.e., for the premolar protocones to be less elongated than the molar protocones (Fig. 6A, E).PU A.639 was collected in the Pinjor Formation near the town of Mirzapur in India.The skull belongs to an old individual given the heavily worn dentition (Sahni and Khan, 1961b:pl.I-III; Fig. S2I, J).It was also found with both mandibular rami PU A.601 and PU A.602.This skull is comparable in size to the lectotype of E. sivalensis but smaller than the other referred crania mentioned above.However, the metaconids and metastylids on the lower cheek teeth of PU A.601 are elongated (Fig. 6K).The metaconids are long and rounded as opposed to short and squared as is typical of E. sivalensis, and GU/J-001 (Fig. 6H,  K).This specimen may belong to a smaller species of Equus which is known to exist in the Upper Siwaliks (Dennell et al., 2006;Forsten & Sharapov, 2000;Gaur & Chopra, 1984;Hussain et al., 1992), but has not yet been adequately described (Bernor et al., 2019;Cirilli et al., 2022;Rook et al., 2019).The mandibular dentition of PU B.40 is largely similar to that seen in GU/J-001 and other mandibles attributed to E. sivalensis, such as NHMUK PV M.23107 (Fig. 6H-J).Based on the gross morphology of GU/J-001 and our comparisons with other equid crania and mandibles from the Siwaliks, we are confident that our specimen belongs to E. sivalensis.While there exists some size variation between the various crania and mandibles attributed to E. sivalensis, this variation is consistent with that seen in other stenonine equids such as Equus stenonis Cocchi, 1867 (Cirilli et al., 2021c) and E. simplicidens (Table 2).It is to be noted that the data in Table 2 for E. simplicidens come from the Hagerman Horse Quarry in Idaho as reported by Vera Eisenmann (Table S1), which gives a range of variation for the upper M1 length of 26.5-31.0mm, but a second population from the Broadwater Quarries in Nebraska exhibits a range of 20.5-32.0 mm (Howe, 1970), thus showing that the range of variation in E. sivalensis is within that observed for other stenonine equids.

Element
Traditionally, monodactyl horses in the genus Equus are thought to have dispersed across the Bering Land Bridge into Eurasia at the Plio-Pleistocene boundary at 2.58 Ma, and are often used as biostratigraphic markers for the Pleistocene (Cirilli et al., 2022;Rook et al., 2019).The "Equus Datum" in Eurasia is based on this synchronous occurrence in Europe, South Asia, and China (Cirilli et al., 2022;Lindsay et al., 1980;Rook et al., 2019;Sun & Deng, 2019).However, slightly earlier occurrences of Equus have been reported in Eurasia.Equus livenzovensis Baigusheva 1978 is known from Liventsovka in Russia from a section dated to between 2.6-2.2Ma (Eisenmann, 2022;Forsten, 1998;Titov, 2008).Equus is also known from the 2.6 ± 0.02 Ma Roca-Neyra site in France (Cirilli et al., 2021a;Nomade et al., 2014), 2.7-2.6 Ma site of El Rincòn 1 in Spain (Agustı ́& Oms, 2001;Alberdi et al., 1997;Domingo et al., 2007).In Italy, Equus cf.E. livenzovensis is known from the ∼2.6 Ma Montopoli site and co-occurs with a hipparionine horse (Bartolini-Lucenti et al., 2022;Bernor et al., 2018;Cirilli et al., 2020;Lindsay et al., 1980;Rook & Bernor, 2017).These latest Pliocene occurrences have not yet been reported in China (Sun & Deng, 2019).The oldest occurrence of Equus in the Pakistani Siwaliks is at site DP 68 at the Gauss-Matyuma boundary in the Chambal Section and site 76-32 in the Mangla Samwal Section ∼50.0 m above the Gauss-Matyuma boundary (Opdyke et al., 1979).The new specimen we report here comes from the latest Gauss, and therefore represents a similar latest Pliocene occurrence of Equus as seen in northern and western Eurasia.
Given that the first appearance datum (FAD) or last appearance datum (LAD) of a species in the fossil record is almost never the true first or last appearance given the incompleteness of the record (Signor & Lipps, 1982;Wang & Marshall, 2016), it is not surprising to find Equus in deposits older than the Pleistocene.Relatively recent investigations into the ∼90 m thick fossiliferous Qaranwala Zone in the upper Saketi Formation of India has shown that the ranges of taxa typically associated with the Pleistocene in the Siwaliks such as Sivatherium giganteum Falconer & Cautley, 1836b, Camelus sivalensis Falconer & Cautley, 1836a, Leptobos falconeri Rütimeyer, 1877-1878, and cervids actually extend back into the latest Pliocene (Azzaroli & Napoleone, 1982;Chapon Sao et al., 2016;Moigne et al., 2016;Nanda, 1994;Sahni & Khan, 1964).Based on our findings, Equus disperses into Eurasia during the Late Pliocene, a time characterized by lower sea-levels driven by the closure of the Isthmus of Panama and the onset of northern hemisphere glaciations (De Schepper et al., 2013;Lisiecki & Raymo, 2007;Miller et al., 2005).This climate change also drives the immigration of several open-habitat specialists into Upper Siwalik ecosystems (Jukar, 2018), probably following the spread of grasslands in Eurasia.However, we note here that Equus is not found in typical Late Pliocene faunal assemblages in Europe, South Asia, or China (Nanda, 2015;Rook & Martínez-Navarro, 2010;Sun & Deng, 2019), and therefore represents a very Late Pliocene immigrant, dispersing into Eurasia from North America.

CONCLUSION
Our findings here show that the genus Equus actually appears in Eurasia just before the start of the Pleistocene.Our results not only help clarify the temporal range of E. sivalensis, but also support earlier reports of isolated teeth in latest Pliocene (2.7-2.6 Ma) sediments near the village of Masol (Moigne et al., 2016).The Equus Datum in South Asia therefore is perhaps better defined as an event occurring in the latest Pliocene rather than the earliest Pleistocene.Singh et al.-Earliest Equus in South Asia (e2227236-8)

FIGURE 1 .
FIGURE 1. Location map for the site that produced GU/J-001.A, satellite Google Earth image showing the locality (white star) near the village of Jhil (Map Data: © 2023 Google, Maxar Technologies.);B, stratigraphic section and measured stratigraphic column at the site where the fossil was found.

FIGURE 2 .
FIGURE 2. Magnetostratigraphy of the Somb-Jamni Khol section modified from Sangode et al. (1996).A, satellite Google Earth image showing the location of the transect (Map Data: © 2023 Google, Maxar Technologies.).White star shows the fossil locality near the village of Jhil; B, line drawing of the survey area modified from Sangode et al. (1996) showing the two transect lines and the extent of the Middle Siwalik sediments, Saketi Formation, and Pinjor Formation; C, lithostratigraphic column of the transect modified from Sangode et al. (2016); D, paleomagnetic column as correlated with the lithostratigraphic column modified from Sangode et al. (2016).

FIGURE 3 .
FIGURE 3. Equus sivalensis skull bones.A, left maxilla, nasal, and premaxilla of E. sivalensis, GU/J-001; B, right maxilla of the same; C, left and right preserved skull elements of the same in dorsal view; D, left and right maxillae with complete tooth rows of the same in ventral view.

FIGURE 4 .
FIGURE 4. Equus sivalensis mandibles.A, left mandibular tooth row of E. sivalensis, GU/J-001; B, left mandible of E. sivalensis, GU/J-001 in lateral view; C, left mandible of the same in mesial view; D, right mandibular tooth row of the same; E, right mandible of the same in lateral view; F, right mandible of the same in mesial view.

TABLE 2 .
Mean and ranges of the first upper and lower first molar dimensions of Equus sivalensis, Equus stenonis, and Equus simplicidens.Abbreviations: N = sample size; L = antero-posterior diameter (length); W = transverse diameter (width); L/W = ratio of length to width.All measurements are in mm.Raw data are available in TableS1.