Hippotherium Datum implies Miocene palaeoecological pattern

Here, we report well–preserved skulls and postcranial specimens of genus Hippotherium from the Linxia Basin, Gansu, China. Based on morphological comparison, the species of Hippotherium in China, Hippotherium weihoense and Hippotherium chiai, should be ascribed to the same species, H. weihoense. We also reviewe other Old World hipparion species in the very early Late Miocene and figure out two evolutionary routes: the Hippotherium and Cormohipparion lineages. Analysis of locomotive ability indicates that H. weihoense likely lived in an open habitat, whereas other species of Hippotherium likely lived in closed habitats. This result shows a palaeoecological pattern in the early Late Miocene in Eurasia influenced by a series of geological events as aridification of mid–latitude Asia progressed, whereas Europe and North Africa remained relatively humid. As the genus originated from East Asia, hipparion horses divided rapidly into different groups with differing functional morphology to occupy diverse niches.

www.nature.com/scientificreports/ Recently, a new skull has been retrieved in the Bantu locality (Linxia Basin). The Bantu locality is located in the same strata as the Guonigou locality and is biostratigraphically correlative to the Guonigou Fauna 17 . This skull is covered with surrounding rock and needs to be prepared. But some exposed parts show very typical features of H. weihoense, such as developed subtriangular preorbital fossa (POF) (SI Fig. S6, i). The recent highaccuracy data of magnetostratigraphy performed by Fang et al. 16 provided an updated chronological framework for the Linxia Basin, with an absolute age for the lower boundary of the Guonigou Fauna of 11.5 Ma, which is the earliest record for a Hipparion-fauna in Eurasia. A potential record was likely present at the Guonigou locality, represented by an isolated M3 with features resembling H. weihoense, such as complicated fossette ornamentation and elongated oval protocone, but more proofs are required for certainty.

Systematic palaeontology. Order Perissodactyla Owen, 1848
Family Equidae Gray, 1821 Genus Hippotherium Kaup, 1833 Hippotherium weihoense (Liu et al., 1978).  Attribution and revision. The newly described specimens have a characteristic, diagnostic combination of cranial and dentition morphology, including medium to large size (basal length range 410-440 mm; cheek tooth row length range 130-160 mm, Table S1), shallow nasal notch, long POB (length range 40-50 mm, Table S1), 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 Hippotherium weihoense. Liu et al. 13 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. 8 reviewed hipparion fossils from China and accepted the validity of both species. They ascribed these two species to subgenus Hippotherium, gave an estimated age of 11-10 Ma to them, and regarded them as those among the most primitive hipparion horses in the Old World. Liu 10 described a series of specimens from Lantian, Shaanxi and identified some of these specimens as H. weihoense and others as H. chiai. Based on the reported specimens from China, there is actually no clear boundary between the cranial features of these two species. The only cranial specimen of H. chiai in Lantian is the very fragmentary type skull. Liu et al. 13 argued that H. chiai had an elongated POF. However the facial part of the type skull has been obviously deformed by diagenetic crushing (SI Fig. S6b). In more recent report, Lantian specimens and Fugu ones respectively attributed by Liu 10 and Li et al. 18 into H. chiai also have subtriangular POFs.
The dentition was regarded as another important feature to distinguish these two species in previous research. Liu et al. 13 argued that H. chiai has a simpler fossette ornamentation on the upper cheek tooth than that of H. weihoense. The fossette complication of hipparion largely depends on wear stage. The ontogenetic sequence analysis of Li et al. 19 clearly shows that fossette ornamentation of H. weihoenese (their Hipparion chiai) would be simpler in very early and late stages than those in other stages. Similar analysis by Bernor  www.nature.com/scientificreports/ According to the analyses presented in this study, in accordance with the most recent revision of Old World hipparionines by Bernor et al. 22 , there is no doubt that Hippotherium weihoense, Hippotherium primigenium and Hippotherium catalanicum are typical species of the genus Hippotherium. They share some marked characters such as shallow nasal notch at the level in front of P2, developed subtriangular POF with posterior pocket and clear anterior rim far from orbit, very long POB, complicated fossette ornamentation, elongated protocone, deep ectoflexid, and square angle on metastylid. Bernor et al. 23 identified dental specimens from Vienna Basin Pannonian C as Hippotherium sp., and regarded it as the stratigraphically oldest (basal MN9, ca. 11.4-11.0 Ma) hipparion record in Europe. Based on their figures, this series of dentitions have the same characters as H. primigenium we listed above. So Pannonian C specimens can be treated as the first occurrence of Hippotherium in Europe. Arambourg 24 erected a new species Hipparion africanum based on Vallesian material from Bou Hanifia, Algeria.
Bernor & White 25 performed morphological comparison and "Log10 Ratio" analysis on postcranial elements, based on which they revised H. africanum as "Cormohipparion" africanum, and concluded that "C. " africanum could not be referred to Cormohipparion s.s. but likely derived from Cormohipparion s.s. Bernor et al. 22 considered "C. " africanum as an essential part of the Cormohipparion dispersal event. Bernor et al. 26 14 . Gracile limb bones are an indicator of cursorial ability, which is most clearly exhibited in the metapodials of ungulates 28 . The gracility of the metapodial midshaft is represented by diminished breadth relative to length. In Fig. 2, hipparion species are compared with extant Asiatic wild ass Equus hemionus onager, which is treated as standard, to show the difference in gracility of the metapodials. The ratios between the maximum length and the minimum breadth dictate that H. weihoense, P. zandaense, and C. occidentale have relatively slender metapodials (logarithm of ratio between object and standard on measurement 3 is smaller or slightly larger than that of measurement 1), but H. primigenium has very robust metapodials (logarithm of ratio between object and standard on measurement 3 is notably larger than that of measurement 1), and Proboscidipparion (Proboscidipparion sinense and P. pater) and Plesiohipparion houfenense from the North China Plain also show increased robustness. Typically, metapodial robustness of horses has been considered a marker of evolutionary grade, with slender metapodials as an advanced feature (Deng and Xue, 1999). Our results imply an exception in which metapodial robustness is considerably influenced by environmental change, represented by P. zandaense, which was positioned at a primitive evolutionary stage 29 but has slender limbs, in contrast with P. houfenense. A high proportion of distal elements (fore and hind metapodial and phalanx) 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 30 . Lengths of the distal elements of hindlimbs, Mt III, and the first hind phalange relative to proximal elements (humerus, radius, femur and tibia) of H. weihoense and C. occidentale are significantly longer than those of H. primigenium. Relatively elongated distal elements make the whole limb lengthened, keep the center of mass situated proximally and reduce inertia, which allows for a long, rapid stride leading to high speed 30 . So H. weihoense and C. occidentale would have stronger running ability than H. primigenium. Both the advanced P. houfenense and P. sinense have these characteristics (Fig. 3). Based on the analysis of functional morphology, H. weihoense was able to run fast and stand persistently, which is beneficial in open habitats. The running abilities of H. primigenium and C. africanum were weaker and more suited to slower movement in closed habitats 7,31 , and their locomotor function stands in contrast to the inferred ecosystem and behaviour of H. weihoense. Although no detailed measurements of C. sinapensis were available for analysis, Bernor et al. 26 have stated that the limb proportions were elongate and slender compared to Hippotherium primigenium, which can also been see in the diagrams of Bernor et al. 26 . Therefore, C. sinapensis was a cursorial species, adapt to open habitat, like H. weihoense. The difference between hipparions from Asia and Europe was much influenced by environment in the early Late Miocene (see below).

Discussion
In  37 proposed enhanced aridity in the Asian interior at 9-8 Ma. An et al. 38 argued that the northern part of the Tibetan Plateau had uplifted considerably in the early Late Miocene. They proposed that the northern part of the Tibetan Plateau appeared at 10-7 Ma, and that an important uplift/growth of the plateau also occurred in the same period. These events occurred significantly later than the first occurrence of hipparion horses in Eurasia. However, magnetostratigraphic investigation showed that the onset of eolian red clay deposition predated 11.4 Ma 39 . Dettman et al. 40 16 , which was the earliest record for hipparion occurrence in Eurasia). The cenogram analysis of   44 showed that the metapodial of C. sinapensis was obviously slender compared to H. primigenium. Zhang et al. 45 argued that Tibetan Plateau uplift and Paratethys retreat occurred at the same time. Retreat of the Paratethys would further reduce vapour delivery to Asia. Thus, the habitats of H. weihoense in other localities were likely similar to those of the Linxia Basin. Based on the present study, Cormohipparion africanum and H. primigenium have relatively robust metapodials. In H. primigenium, the proximal elements account for a high proportion of the limbs. All of these are indicators of a closed habitat. Böhme et al. 46 estimated precipitation for Southwest and Central Europe in the Miocene and proposed a dry period during 13-11 Ma, the time frame when the first hipparions arrived in Eurasia. More recent the environments in Europe generally became more humid, based on Fortelius et al. 42,43 , which indicates that the habitats of H. primigenium and C. africanum in Europe and North Africa, were relatively wooded during 11-8 Ma. In the same period, in the habitat of H. weihoense in northwestern China, especially the eastern margin of Tibetan Plateau, high-crowned ungulates were dominant, which indicates adaptation to an abrasive diet, which could be dominated by grasses characteristic of open environments. Böhme et al. 46 proposed the term "washhouse climate" as an analogy for a climate under high precipitation at 10.3-9.8 Ma. This age is consistent with that of Höwenegg (10.3 Ma) 5 and slightly younger than Eppelsheim 5 in Germany, where specimens of H. primigenium were abundant.
The North American species C. occidentale also had very slender Mt III and a high proportion of the distal elements of the hind limbs (Figs. 2, 3). The habitat type at the end of the Middle Miocene and the early Late Miocene was likely open, based on the diversity and abundance of grazers in North America 47,48 . According to Mihlbachler et al. 49 , hypsodont Equinae species first occurred at 16 Ma, and became dominant at 12 Ma. Analysis of representative SEM photomicrographs of tooth microwear by Hayek et al. 50 showed that C. occidentale were most likely grazers. Based on phylogenetic analysis, such as Woodburne 5 , Hippotherium was derived from the grazing C. occidentale. According to magnetostratigraphic data, the earliest species of Hippotherium was in Eurasia. Therefore, the origin of Hippotherium was likely in East Asia (not, as traditionally considered, Europe 1-5 ). Based on North American palaeoecological proxies, a dominantly open habitat existed in North America significantly earlier than in Eurasia. This habitat led to the emergence of grazing Cormohipparion species, which would later give rise to the Eurasian hipparion horses that were adapted to open environments in the earliest phase of their dispersal. Hipparion horses in Eurasia later adapted to a variety of habitats and developed high diversity 8,22 , however retaining pre-adaptations to feeding in open environments, such as hypsodont dentitions. This is a typical example of environmental pre-adaption of late Cenozoic megaherbivores.   (2) the Cormohipparion lineage including C. sinapensis in Turkey and "C. " africanum in North Africa, with first occurrence as 10.8 Ma represented by C. sinapensis. However, due to the scarcity of occurrences of these earliest Old World hipparionines, particularly along the possible dispersal routes via Northern Eurasia, this scenario should be seen as hypothetical and more well-dated occurrences would be needed to further test this hypothesis of the dispersal history and relationship between Cormohipparion and Hippotherium in Eurasia.
Based on the known record, Cormohipparion and Hippotherium both consisted of open-habitat forms in Asia and closed-habitat forms in Europe. The same environmental settings had a similar effect on both genera. According to the absolute age data, the occurrences of Hippotherium are among the earliest of all hipparionines in the Old World. This genus was widespread in Asia and Europe in early Late Miocene. The Hippotherium Datum, the first occurrence of Hippotherium in Old World, was a marked event of climatic, tectonic and biotic significance. H. weihoense currently seems likely to be the best candidate to represent this event in the light of available occurrences and their dating. The apparently later appearance of Cormohipparion in the Old World might result either from the lack of fossil evidence along the long dispersal route from North America to Western Eurasia, or it could represent a second dispersal of North American Cormohipparion (Fig. 4).
In summary, the following constitutes the ecological pattern of the early Late Miocene: the Tibetan Plateau uplifted and the Paratethys Ocean retreated, which aggravated aridification of mid-latitude Asia, including northwestern China and Turkey, and promoted considerable expansion of grassland. In the meantime, Europe and North Africa still had relatively closed habitats. Hippotherium was derived from the North American Cormohipparion and dispersed into Eurasia. They were highly adapted to open environments, widespread over Eurasia in the general environment of aridity. They also responded sensitively to environmental change, showed excellent adaptative ability to humid, wooded habitats in Europe and mostly more arid and open ones in Asia. The hipparionines divided rapidly into different genera with different functional morphologies to occupy diverse niches in the Old World 8,14,22 .

Conclusion
(a) Morphologic comparison indicates previously reported Hipparion weihoense and Hipparion chiai should be referred to the same species, Hippotherium weihoense. This also confirms that the distribution of H. weihoense was throughout northwestern China. (b) According to the current dating of its occurrence in East Asia, Hippotherium weihoense represents the earliest hipparionine in the Old World and thus marks the Hippotherium Datum in the early Late Miocene in Eurasia (Fig. 5).

Data availability
All data applied in the analyses in the present study are publicly available, raw date can be obtained from SI file.