The new specimens described were collected from three localities in Linxia Basin, Gansu, China: Houshan (LX 0008); Shuanggongbei (LX 0009); Niugou (LX 200204) (S Fig. 1).
The specimens in the present research are described as following: Order Perissodactyla Owen, 1848; Family Equidae Gray, 1821; Genus Hipparion de Christol, 1832; Subgenus Hippotherium (Kaup, 1833); Hipparion (Hippotherium) weihoense Liu et al., 1978. Synonym including: Hipparion chiai Liu et al., 1978; Hipparion prostylum Bernor et al., 1985; Hipparion chiai Qiu et al., 1987; Hipparion cf. chiai Deng and Wang, 2004; Hippotherium primigenium Zouhri and Bensalmia, 2005; Hippotherium weihoense Liu, 2012; Hippotherium chiai Liu, 2012; Hipparion aff. brachypus Bernor et al., 2016; Hipparion chiai Li et al., 2017; Hipparion chiai Li et al., 2019. Holotype. A nearly complete skull, the posterior part of the orbit lost, IVPP V 3113.1. Type locality. Shuijiazui, Lantian, Shaanxi (63702.L4). Referred specimen. Specimens firstly described in the present research are collected from Linxia Basin, Gansu, China (S Fig. 1). IVPP specimens collected from Niugou Locality including: IVPP V 24396, skull of a very old individual; IVPP V 24397, right humerus; IVPP V 24398.1–2, left Mc IIIs; V24398.3–4, right McIIIs; IVPP V 24399.1–2, right femurs; IVPP V 24400.1–2, right tibias; IVPP V V24401.1–3, left Mt IIIs; IVPP V V24401.4–8, right Mt IIIs. HMV 1308, skull of a sub-adult individual; HMV 1429, skull of a juvenile individual; HMV 0476, skull of a sub–adult individule, Houshan Locality; HMV 1963, skull of an adult individual, Shuanggongbei Locality; HMV 1964, skull of an adult individual, Shuanggongbei Locality; HMV 0806, skull of an adult individual; Revised diagnosis. large hipparion horse. Moderately robust muzzle. Shallow nasal notch. Preorbital bar (POB) very long. Preorbital fossa (POF) strong, subtriangular–shaped with distinct anterior margin. Proportion of basal cranial relatively low. Cement on cheek tooth thick. Anterostyle/anterostylid short. Plications in pre– and postfossettes strong and complex at middle wear stage. Pli caballine usually single to double, sometimes complex. Protocone usually elongated at middle wear stage. Hypoconal groove deep at middle wear stage. Protostylid usually present. Metaconid rounded, and metastylid subtriangular with a pointed labial horn. Linguaflexid U–shaped. Pli caballinid usually present on premolar at middle wear stage. Limbs slender with high proportion of distal elements. Distribution. Lantian and Fugu, Shaanxi; Qaidam, Qinghai; and Linxia Basin, Gansu, China; Maragheh, Iran. Age. Late Miocene, Bahean.
Attribution and revision
The newly described specimens have a characteristic cranial and dentition diagnosis combination, including medium to large size, shallow nasal notch, long POB, developed POF with a posterior pocket, complex fossettes with long and strong folds, and elongated protocone with flat labial margin. All of these features are identical to the primitive hipparion species in China, Hipparion (Hippotherium) weihoense.
Liu et al.10 reported a large Hipparion species discovered from Lantian, Shaanxi and erected the new species Hipparion weihoense. In the same text, they identified a smaller skull fragment and some teeth with similar features and stratigraphic position to H. weihoense as another new species Hipparion chiai. Qiu et al.9 reviewed hipparion fossils from China and accepted the validity of both species. They ascribed these two species to subgenus Hippotherium, and regarded them as the most primitive hipparion horse in the Old World. Liu8 described a series of specimens from Lantian, Shaanxi and identified parts of these specimens as H. weihoense and H. chiai. Based on the reported specimens from China, there is actually no clear boundary between the cranial features of these two species. Bernor et al.12 identified the hipparion specimens from Maragheh, Iran as Hipparion prostylum. H. prostylum was first reported in France. Based on observation of the specimens of H. prostylum from the type locality (Luberon, France), now housed in the NMNH, Paris, we determined that the upper cheek teeth from Luberon have rounded protocone, single pli caballine, and simple and robust folds in fossettes. This combination of features, based on personal observation on specimens, is more similar to the subgenus Cremohipparion, such as Hipparion forstenae and Hipparion gracile. Later Bernor et al.13 ascribed these Maragheh specimens as Hipparion aff. brachypus. H. brachypus is a huge-built form with deep nasal notch and very robust metapodials14, which are difficult to correspond to the Maragheh form. Morphologically, specimens from Maragheh, Iran are very similar to H. weihoense in China. Based on measurements of skulls (S Table. 1) the POF positions of these specimens are stably distributed; the difference is ontogenetic. The dimensions of the POF are more variable, but another hipparion species in Eurasia also has variable POFs: Hipparion dermatorhinum, found in Baode, Shanxi, China9. The measurements of Bernor et al.6 show highly variable POF of Hipparion primigenum. Moreover, POF height is easily influenced by compressional deformation. In the sketches of the skulls of H. weihoense (S Fig. 6), most individuals have clearly or nearly subtriangular POFs; some POFs with significantly different shapes show clear evidence of deformation. The dentition was regarded as another important feature to distinguish these two species in previous research. Liu et al.10 argued that H. chiai had a simpler fossette on the upper cheek tooth than that of H. weihoense. Based on recent ontogenetic sequence analysis, the fossette characters of hipparion largely depend on wear stage15, 16. Liu et al.10 also indicated that some large individuals of H. chiai had similar features on upper cheek teeth to those of H. weihoense. They did not discuss the features of lower cheek teeth. In our sketches of reported upper and lower cheek specimens of both H. weihoense and H. chiai, there is no significant difference between these two taxa (S Fig. 7–8). Therefore, H. weihoense, H. chiai, and H. prostylum from Maragheh, Iran should be ascribed to the same species. Following the priority principle, all of these specimens belong to H. weihoense.
We used characters from Woodburne4 as our basis, changed a repeated character for the skull, character 36, into a new one on tooth character, added 21 tooth characters from Liu8, and added two new characters on the postcranial and size. We partly followed the taxa in the matrix of Woodburne4 and used Parahippus leonensis, Merychippus primus, and Merychippus insignis as outgroup taxa. Woodburne4 erected five new species of the North American genus Cormohipparion. However, these species overlapped in character, geographic distribution, and age, and are better regarded as synonyms of the type species, Cormohipparion occidentale. We only accept the validity of three known species of this genus: Cormohipparion goorisi, Cormohipparion quinni, and Cormohipparion occidentale. We also added the Eurasian species Hipparion (Hippotherium) weihoense, Hipparion (Hippotherium) africanum, and Hipparion (Hippotherium) catalaunicum, and the subgenera Cremohipparion, Baryhipparion, Sivalhippus, Plesiohipparion, and Proboscidipparion. These 16 taxa and 62 characters constitute a new matrix. One MPT (most parsimonious tree) was obtained. We determined that all Eurasian hipparion subgenera form a monophyly, which fully supports the argument of Qiu et al.9 on systematic palaeontology, classifying all Chinese hipparion horses as one genus. All other subgenera were derived from the subgenus Hippotherium. This interpretation also supports the phylogenetic relation between Cormohipparion and Old World hipparions proposed by MacFadden17. Consequently, subgenus Hippotherium should be regarded as the basal group of Eurasian hipparion horses representing the Hipparion Datum.
The well–preserved postcranial specimens from the Niugou locality indicate the locomotive ability of H. weihoense. A strong medial trochlear ridge (MTR) of the femur can fasten the medial patellar ligament, or parapatellar cartilage, and the patella when the knee joint is hyperextended18, forming a passive stay–apparatus to immobilise musculature in the knee extensors during long periods of standing. The femur MTR of H. weihoense is greatly enlarged relative to the lateral trochlear ridge, notably larger than in Hipparion primigenium, but similar to Hipparion zandaense from the Pliocene of the Zanda Basin. The ratio between the maximum depth of the MTR and the maximum length of the femur is 0.27 in H. primigenium6, whereas it is 0.32 in H. weihoense and 0.3 in H. zandaense11. Gracile limb bone is an indicator of cursorial ability, which is most clearly exhibited in the metapodials of ungulates19. The gracility of the metapodial shaft is represented by diminished breadth relative to length. In Fig. 2, above the zero line are the comparatively larger measurements, and below it are the smaller ones. The ratios between the maximum length and the minimum breadth indicate that H. weihoense, H. zandaense, and C. occidentale have relatively slender metapodials (measurement 3 is smaller or slightly larger than measurement 1), but H. primigenium has very robust metapodials (measurement 3 is notably larger than measurement 1), and the subgenus Proboscidipparion (Hipparion sinense and Hipparion pater) and H. houfenense from the North China Plain also show increased robustness. Typically, metapodial robustness of horses has been considered a marker of evolutionary steps. Robust metapodials indicate primitive steps, whereas slender one indicates the opposite20, 21. Our results supplement this hypothesis with a trend of increased robustness with increased body size, and with metapodial robustness considerably influenced by environmental change. A high proportion of distal elements will lengthen the whole limb to keep its centre of mass situated proximally and to reduce its inertia, which allows for a long, rapid stride, as speed is the product of stride length and stride frequency 22. Lengths of the distal elements of hindlimbs, Mt III, and the first hind phalange relative to proximal elements of H. weihoense and C. occidentale are significantly longer than those of H. primigenium, which indicates the stronger running ability of the former two. Both the advanced H. houfenense and H. sinense have these characteristics (Fig. 3). Consequently, H. weihoense was able to run fast and stand persistently, which is beneficial in open habitats. The running abilities of H. primigenium and H. africanum were weaker and more suited to slower movement in closed habitats6, 23, and their locomotive function stands in contrast to the inferred ecosystem and behaviour of H. weihoense.