Both morphological and molecular studies support the idea that Rhinocerotoidea and Tapiroidea form a monophyletic group Ceratomorpha. The Ceratomorphs have abundant, diverse fossil records in the Cainozoic; however, extant Ceratomorphs are reduced to five genera and on the brink of extinction. Furthermore, despite a long research history and numerous fossils, the phylogeny and evolutionary history of the Ceratomorpha still remain contentious. Previous phylogenetic analyses have either focused on Tapiroids or Rhinocerotoids without the combination of both groups. Analyses that have contained both Tapiroids and Rhinocerotoids are still limited in taxa and character selection, so that relationships within Ceratomorpha were not well resolved and many controversies still remained.
The Rhinocerotoidea conventionally comprises the Hyracodontidae, Amynodontidae, and Rhinocerotidae, with the Paraceratheres (Giant Rhinos) recently treated as a separated family derived from Hyracodontidae. Rhinocerotoids probably originated from ‘Hyrachyus’ (or Hyrachyidae), which spread from Eurasia to North America in the Middle Eocene, and has usually been considered to be a transitional form from the Tapiroids to Rhinocerotoids. However, the postcranial skeleton of Hyrachyus suggests that ‘Hyrachyus’ arose from Tapiroids more primitive than Heptodon, and could not be an ancestor of Triplopus, which bears a specialised skeleton for fast running. So Hyrachyus could not be ancestry to all Rhinocerotoid groups. The earliest Hyracodontids and Amynodontids are represented by Triplopus and Amynodon/Rostriamynodon, respectively, from the early Uintan North American Land Mammal Age and/or Irdin Manhan Asian Land Mammal Age. Rhinocerotidae also appeared in the early Uintan as represented by its sister group Uintaceras. A recent study reported the earliest unequivocal Rhinocerotoid, Pappaceras meiomenus, from the Early–Middle Eocene of Arshantan Asian Land Mammal Age, which is slightly earlier than any other known Rhinocerotoids and considered to be ancestral to later Giant Rhinos. But Pappaceras is already more derived than ‘Hyrachyus’, and possesses a combination of both Paraceratheriid and Amynodontid characters, suggesting a close relationship between these two families. Except for Pappaceras, unequivocal Rhinocerotoids have not been reported from the early Eocene or Early–Middle Eocene in either North America or Asia, although some relatively small Ceratomorphs have been argued to be Rhinocerotoids, such as Fouchia, Dilophodon, Rhodopagus, and Yimengia.
In a paper published in the journal Communications Biology on 14 September 2020, Bin Bai of the the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the Center for Excellence in Life and Paleoenvironment, Jin Meng, also of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and of the Division of Paleontology at the American Museum of Natural History, and Earth and Environmental Sciences at the City University of New York, Chi Zhang, again of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the Center for Excellence in Life and Paleoenvironment, and Yan-Xin Gong and Yuan-Qing Wang, once again of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the Center for Excellence in Life and Paleoenvironment, and of the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, describe five genera (including a new genus) and six new species that represent earliest members of Rhinocerotoids, Forstercooperiids, and/or Hyrachyids, on the basis of new, diverse Rhinocerotoid materials from the Early Eocene to the Early–Middle Eocene in the Erlian Basin of Inner Mongolia, China.
Bai et al. further resurrect the genus Ephyrachyus, and erect a new species of Ephyrachyus. These new materials are unearthed from the upper part of the Nomogen Formation and the Arshanto Formation, which are considered to be the early Eocene Bumbanian and the early–middle Eocene Arshantan Asian Land Mammal Age, respectively. The Bumbanian is normally comparable with Wasatchian North American Land Mammal Age, and the Arshantan is comparable with Bridgerian plus the early Uintan North American Land Mammal Age based on the Mammal fauna correlation and the recent palaeomagnetic analyses. These new diverse rhinocerotoids bridge the evolutionary gap between the Early Eocene Ceratomorphs and Middle Eocene Uintan/Irdin Manhan Rhinocerotoids, and suggest that divergence of different Rhinocerotoid groups occurred no later than the Late Early Eocene in a relatively close, humid environment.
The first new species described is placed in the genus Yimengia and given the specific name magna, meaning 'large', referring its relatively large size within the genus. This species is described from eight specimens: IVPP V 26234, a right mandible with dp4, m1, and m3; VPP V 26235, an associated right mandible with dp3, talonid of dp4, m1, and a left mandible with m1; V 26236, a right m3; V 26237, an associated left mandible with m1 and a right mandible with broken talonid of m1; V 26238, a left P4 with the ectoloph broken off, a fragmentary upper molar, and a right M3; V 26239, a left m1/2; V 26240, a left mandible with fragmentary m1; V 26241, a right maxilla with M1–2. All are Early Eocene in age, from the upper part of the Nomogen Formation.
The genus Yimengia differs from the genus Rhodopagus in having P4 metaconule contacting the base of the protocone, M1–2 metacone less lingually appressed and more elongated without bulges at the base of the buccal side, M3 with a more distinct metacone, and centrocrista not confluent with the metaloph, p3–4 paraconid and hypoconid relatively lower, cristid obliqua more lingually slanted, p3 metaconid separated from the protoconid, p4 entoconid less distinct, and the lower molars with relatively longer trigonid, more transversely extended protoloph, and more lingually extended cristid obliqua with a relatively higher contact with the protolophid. Differs from Minchenoletes in having a more lingually placed metacone on M1–3, metaloph of M1–3 joining the ectoloph less forward, M3 metacone more reduced, and less distinct hypoconulids on lower molars. Differs from Triplopus (as represented by Triplopus cubitalus) in having the metaconule not forming a loop with the protoloph on P3–4, a shorter metacone on P3–4, parastyles of upper molars less reduced, M3 metacone more distinct and less lingually appressed, cristid obliqua of p3–4 more lingually slanted, and protolophid more transversely extended on the lower molars.
This genus has now been reported from the Early–Middle Eocene; Guanzhuang Formation of Laiwu and Xintai county in Shandong Province; and the Nomogen and Arshanto formations of the Erlian Basin, Inner Mongolia.
Yimengia magna differs from other species of Yimengia by a larger size, a slightly less lingually placed metacone with a weak rib or convexity on the buccal side on the upper molars, and m3 hypoconulid more developed; further differs from Yimengia chaganense by a larger, more buccally placed parastyle on the upper molars, and more distinct ribs on the anterior sides of the metaconid and protoconid on the lower molars; further differs from Yimengia yani, Y. laiwuensis, and Yimengia zdanskyi by a stronger cristid obliqua joining the protolophid in a high position on m1–2.
The second new species described is also placed in the genus Yimengia, and given the specific name chaganense, in reference to to Chaganboerhe, where the holotype was found. This species is described from eight specimens: IVPP V 26242.1, associated left and right maxillae with P4–M3 with ectolophs partially broken off; IVPP V 26242.2, associated juvenile left and right maxillae with DP2–4 and M1; V 26243, a left mandible with p3–4; V 26244, a left M1/2; V 26245.1, a left M1/2; V 26245.2, .3, a left M1/2 and m1; V 26246.1-3, a left M1, a right M2, and a fragmentary M3; V 26247.1-3, an isolated left dp4, a right mandible with dp4 and m1 in the alveolus, and a right mandible with dp4–m1. All from the Late Early Eocene, low and middle parts of the Arshanto Formation.
Yimengia chaganense differs from other species of Yimengia by smaller parastyles on the upper molars; differs from Yimengia magna by a more lingually placed parastyle and a flat, more lingually placed metacone on M1–3, and more reduced m3 hypoconulid; differs from Yimengia yani, Yimengia laiwuensis, and Yimengia zdanskyi by a stronger cristid obliqua joining the protolophid in a high position on m1–2; further differs from Yimengia yani by a flat metacone on P4; further differs from Yimengia laiwuensis by a more distinct metaconid on p3.
These two new species are characterised by small to medium size among early Ceratomorphs, a reduced parastyle and pinched paracone on M1–3, a flat metacone with relatively long postmetacrista on M1–2, M3 metacone short and strongly lingually depressed, cristid obliqua of p3–m3 strong and joining the protolophid in a relatively high position, and absence of m3 hypoconulid. Almost all characters of the new materials are similar to those of Yimengia, which was previously known by three species from the Guanzhang Formation, Shandong Province. However, the type of Yimengia, Yimengia yani, has stronger parastyles on upper molars, a relatively wider M1, a more distinct metacone rib on P4, and lower cristid obliqua on m1–3 than in the new taxa.
Yimengia laiwuensis, and Yimengia zdanskyi, which were originally assigned to Rhodopagus, were known only from the low jaws. Yimengia laiwuensis is different from Erlian specimens in having a less basined trigonid on p3 with a more reduced metaconid, and a relatively lower cristid obliqua on p3–m3. Wang Jingwen further interpreted a left mandible with two molars of Yimengia zdanskyi (PMUM 3004) as m1–2 rather than m2–3. Yimengia zdanskyi is mainly different from Erlian species in having a smaller size, and m1–2 with a more triangular trigonid and more reduced cristid obliqua.
Yimengia is considered to be closely related to Rhodopagus, which is known from later Irdin Manha and Shara Murun faunas, as represented Rhodopagus pygmaeus and Rhodopagus minimus, respectively Although Rhodopagus pygmaeus has been regarded as a junior synonym of Rhodopagus minimus, Bai et al. have treated them as separate species pending a discovery of more complete material of Rhodopagus minimus. ‘Rhodopagus’ minutissimus from the middle Eocene of Andarak in Kyrgyzstan was later considered to be Pataecops minutissimus. Rhodopagus (as represented by its best-known species Rhodopagus pygmaeus) is mainly different from Yimengia magna and Yimengia chaganense in having a straight ectoloph on P2–4 with a relatively higher parastyle that occludes with the corresponding high, nearly straight, buccally aligned paralophid and cristid obliqua on p2–3. Further, after careful observation of abundant, nearly unworn lower molars of Rhodopagus pygmaeus recently unearthed from the ‘Basal White’ of Erden Obo, Bai et al. notice that the ‘long anterior paralophid’ is actually composed of an anterior paralophid on the buccal half and a cingulum on the lingual half that rises from the anterobuccal cingulum and is nearly confluent with the real anterior paralophid. This configuration is usually obliterated and indistinct after wear.
Rhodopagus radinskyi was described from the Late Early Eocene or Early–Middle Eocene Chakpaktas Svita in the Zaysan Basin, Kazakhstan. Rhodopagus radinskyi resembles Yimengia in having a flat and relatively long metacone on M1–3 with postmetacrista slightly buccally deflected, and distinct cingula along the anterior border and lingual side of the M1–3 protocone. However, Rhodopagus radinskyi is much smaller than Yimengia, and shares with Rhodopagus pygmaeus in having (1) a high, straight P3–4 ectoloph, (2) continuous high longitudinal buccal ridges composed of the paralophid and cristid obliqua on p3–4, and (3) strong parastyle on M1–3.
Veragromovia, which was unearthed from the Middle Eocene Zaysan Basin of Kazakhstan, has also been considered to be a junior synonym of Helalete. But the genus was later resurrected and assigned to the Rhodopagidae. M3 of Veragromovia is different from that of Yimengia in having a larger parastyle, a more reduced and slightly buccally deflected metacone, and a complete lingual cingulum.
Lophialetids are common, endemic Tapiroids distributed in the Early and Middle Eocene of Asia. Minchenoletes and Schlosseria have been reported from the Nomogen and Arshanto formations, respectively, and the size of Yimengia magna is intermediate between them. The Early Eocene Yimengia magna strikingly show some similarities with contemporary Minchenoletes and later Schlosseria in having a flat, long metacone on M1–2 and a strong cristid obliqua on m1–3. Yimengia magna is further similar to Minchenoletes in having a pinched paracone on M1–3, and relatively more anteriorly directed cristid obliqua on m1. However, both Minchenoletes and Schlosseria differ from Yimengia by having a more buccally placed metacone on M1–3, M1–3 metaloph joining the ectoloph relatively far forward, more elongated M3 metacone, and more distinct hypoconulids on lower molars.
The conventional Lophialetid, Breviodon minutus (also known as Breviodon acares) from the Arshanto and Irdin Manha formations is similar to Yimengia chaganense in size, but its molar morphology is generally like those in Schlosseria and Lophialetes and in turn differs from Yimengia. Breviodon further differs from Yimengia in lacking p1–2, and thus having the premolar series relatively shorter than the molar series. Another Lophialetid, Parabreviodon, initially assigned to Cf. Breviodon acares and later erected as a new genus, is known by a partial cranium (AMNH FM 81751) from the Arshanto Formation. The upper cheek teeth of Parabreviodon mainly differ from those of Yimengia in being relatively shorter and wider, and in having a more convex metacone on P4–M3, protoloph and metaloph on P4 forming a V-shaped loop, and more buccally placed metacone on M1–3 with larger parastyle and a longer M3 metacone.
Three small Ceratomorphs from North America, Dilophodon, Selenaletes, and Fouchia, are known from Early and Middle Eocene. Yimengia mainly differs from them in the following combined characters: less molarised premolars (compared to Dilophodon), a flatter and more lingually placed metacone on M1–3 with an elongated postmetacrista (compared to Dilophodon and Fouchia), and a stronger cristid obliqua on m1–3 with a high joint on the protolophid.
It is not unexpected to note that Yimengia shows some similarities with the Hyracodontid Triplopus cubitalus in having a relatively small parastyle, a pinched paracone, a lingually situated and relatively long, flat metacone on M1–2, reduced M3 metacone, a strong cristid obliqua anteriorly directed on m1–3, and reduced m3 hypoconulid. However, Triplopus cubitalus differs from Yimengia in having a loop formed by the protoloph and metaloph on P3–4, a smaller parastyle on M1–3, a smaller and more lingually appressed metacone on M3, vertical cristid obliqua on p3–4 with longer paralophid, and more oblique protolophid and relatively higher cristid obliqua on m1–3.
The third new species described by Bai et al. is referred to the genus Triplopus as Triplopus? youjingensis, where ‘youjing’ means ‘oil well’ in pinyin (phonetic transcription) of the Chinese language, referring to the oil company nearby the fossil locality. The species is described from a single specimen, IVPP V 26248, a right mandible with p2–m3, from the Late early Eocene, basal part of the Arshanto Formation at Nuhetingboerhe.
Triplopus? youjingensis is a medium-sized ‘Hyracodontid’ with low crowned teeth; differs from other species of Triplopus by p3–4 with a rudimentary hypolophid, and the parallel protolophid and hypolophid nearly transversely extended on m1–3. Further differs from Triplopus? proficiens by a more anteriorly directed cristid obliqua on m1–3. Further differs from North American Triplopus by a slightly more lingually directed paralophid on m1–2.
The lower jaw of IVPP V 26248 shows some characters associated with Rhinocerotoids: relatively high paraconids on the lower check teeth, a strong cristid obliqua of m1–3 joining the protolophid in a relatively high position, and the lack of an m3 hypoconulid lobe. The strong cristid obliqua on m1–3 in the new specimen differs from the reduced, low cristid obliqua of the lower molars in Hyrachyus. Further, the relatively small size of the new material, the presence of p1, and the anterolingually extended paralophid on m1–2 are suggestive of Triplopus affinity.
In the Erlian Basin, Triplopus? proficiens has been reported from the overlying Irdin Manhan and Ulan Shireh formations, Triplopus? proficiens is more advanced than the new material in having more molarized premolars, more oblique protolophid and hypolophid on m1–3, and the cristid obliqua of p3–m3 more lingually directed. The convex posterior border of m3 in Triplopus? youjingensis is more similar to that of Triplopus? proficiens from the Irdin Manha Formation than to those from the Ulan Shireh Formation which have a straighter posterior border of m3. Triplopus? progressus known from the later Shara Murun Formation can be distinguished by its smaller size (M1–3 length equals 35 mm).
Triplopus? mergenensis from the middle Eocene Mergen locality of Mongolia is distinguished from Triplopus? youjingensis by larger size (m1–3 length = 70 mm), a more prominent hypolophid on p3–4, and a more transversely extended protolophid on m2–3. Triplopus ckhikvadzei from the Zaysan Basin of Kazakhstan is mainly different from Triplopus? youjingensis in having a larger size (m1–3 length is 57.5 mm), and in lacking p1. The p2–4 of Triplopus ckhikvadzei is very similar to that of Triplopus? proficiens, and in turn different from that of Triplopus? youjingensis.
Compared with North American Triplopus, Triplopus? youjingensis is considerably larger than Triplopus cubitalis, slightly larger than Triplopus obliquidens, and smaller than Triplopus rhinocerinus. In morphology, Triplopus? youjingensis is mainly different from North American Triplopus by the relatively lower crown height, more transversely extended protolophid and hypolophid on m1–3, and somewhat more lingually directed paralophid on m1–2. On the other hand, Triplopus? youjingensis is similar to North American Triplopus in having the cristid obliqua of m1–3 joining the protolophid in a position slightly lingual to protoconid.
Compared with contemporary Schlosseria from the Arshanto Formation, Triplopus? youjingensis can be distinguished by much larger size, slightly more oblique protolophid, more lingually extended paralophid on m1–2, relatively more anteriorly extended cristid obliqua on m1–3, a reduced hypoconulid on m1–2, and the lack of m3 hypoconulid lobe. Further, the metaconid of p3–m3 in Schlosseria is more or less cuspate with a convex anterior surface, whereas that in Triplopus? youjingensis is merged with the protolophid with a nearly flat anterior surface.
To sum up, this mandible mostly resembles Triplopus in morphology, and its Early Eocene age is earlier than other known species of Triplopus. But the genus Triplopus is also a complex issue to deal with. It contains four species from North America after Leonard Radinsky synonymized Prothyracodon, Eotrigonias, and Ephyrachyus with Triplopus in 1967. However, it is uncertain whether Triplopus is a monophyletic taxon and that all synonymies are reasonable. Thus, Bai et al. have assigned the new species to Triplopus with a query, pending a more comprehensive review of this genus.
The fourth species described is placed in the Family Forstercooperiidae, and given the name Gobioceras wangi, where 'Gobioceras' derives from the root ‘Gobi’ refers to the Gobi area, where the holotype was found; the suffix ‘ceras’ means horn, a common suffix used in Rhinocerotoid names, and 'wangi' honours Jin-Wen Wang, for his contributions to the study of Palaeogene Perissodactyls from China. The species is described from four specimens, IVPP V 26249, a right mandible with m1–m3; IVPP V 26250.1 & IVPP V 26250.2, a right M3, an ectoloph of right M2; V 26251, associated left and right mandibles with talonid of dp3, dp4–m2, and m3 in the alveolus. All are from the Late Early Eocene, basal part of the Arshanto Formation, Nuhetingboerhe.
Gobioceras wangi is a relatively small Forstercooperiid; Differs from Pappaceras by relatively larger and more cuspate M3 parastyle, and the relatively longer and lower anterior branch of the paralophid on m1–3. Differs from Uintaceras by the more lingually appressed M3 metacone, and the more oblique protolophid and hypolophid of m1–3. Differs from Forstercooperia by M3 less triangular in outline with a reduced metacone.
The mandible with m1–3 (IVPP V 26249) was unearthed from the same quarry (east of ‘chalicothere quarry’) where M3 (V 26250) was found; the quarry also bears a new species, possibly of Hyrachyus (V 26253). The juvenile mandibles (V 26251) were unearthed from the ‘chalicothere quarry’.
Gobioceras is distinguishable from Hyrachyus in having a strong cristid obliqua with a high contact with the protolophid on the lower molars, and a reduced, more lingually placed metacone on M3 with a triangular outline. All these features suggest its affinity with Rhinocerotoids. However, the parastyle of M3 still remains relatively large as in Hyrachyus and Tapiroids, but is somewhat more compressed as in Rhinocerotoids. The roughly triangular outline of M3 with reduced, lingually appressed metacone excludes its affinity with Amynodontids. Furthermore, the M3 metacone of Gobioceras is relatively more lingually placed and smaller than those of Triplopus that have rudimentary metacones. The lower molars of Gobioceras are similar to those of Triplopus in having oblique transverse lophids, but different from the latter by having a more Ushaped outline of trigonids with longer paralophids, the cristid obliqua of m1–3 descending slightly rather than sharply from the hypoconid, and joining the protolophid in a relatively higher position based on the slightly worn teeth. The m–3 of Gobioceras is further different from Asian Triplopus? proficiens in having a less lingually extended cristid obliqua which has an angled joint with the hypolophid. The lower molar length of Gobioceras (63.1mm) is considerably larger than in species of Triplopus, although the former from the early Arshantan (roughly equivalent to the early Bridgerian North American Land Mammal Age) is much earlier than Irdin Manhan (or equivalent to the Uintan North American Land Mammal Age) Triplopus. Compared with Triplopus? youjingensis from the same horizon, Gobioceras is larger and has a U-shaped trigonid on the lower molars and a more oblique protolophid and hypolophid. Furthermore, Gobioceras differs from Prohyracodon in having a less reduced metacone, a larger parastyle on M3, and a more oblique protolophid and hypolophid on the lower molars. Thus, Gobioceras is remote from the ancestry of any Hyracodontid Rhinoceroses.
Among Rhinocerotoids, only Pappaceras, which consists of three species, has been reported from the upper part of the Arshanto Formation. Pappaceras was considered to be closely related to Forstercooperia from the overlying Irdin Manha Formation, which gave rise to later Juxia and other Giant Rhinos. It is not surprising to note that Gobioceras from the base of the Arshanto Formation is considerably smaller than Pappaceras from the higher horizon. However, lower molars of Gobioceras show some similarities with those of Pappaceras in having a generally U-shaped trigonid, oblique protolophid and hypolophid that parallel each other, a smoothly curved joint at the hypoconid, and a cristid obliqua contacting the protolophid in a relatively high position. But Pappaceras is more advanced than Gobioceras in having a higher crown, a relatively shorter and higher anterior branch of the paralophid on m1–3, and the buccal branch of the paralophid of m1 slightly more lingually extended. The M3 parastyle of Gobioceras is relatively larger and more cuspate than that of Pappaceras, but both of them are strongly buccally projected relative to the paracone. The M3 metacone of Gobioceras is as lingually placed as those in Pappaceras confluens and Pappaceras minuta, but that of Pappaceras meiomenus is obviously more buccally situated. The M3 metacone of Gobioceras is more distinct than that of Pappaceras confluens, but less prominent than those of Pappaceras minuta and Pappaceras meiomenus, which are even buccally deflected. However, the prominence of metacone on M3 may be a variable character as inferred from Uintaceras and Teletaceras. To sum up, Gobioceras is closely related to Pappaceras and probably represents the ancestral condition for the latter. Forstercooperia from the overlying Irdin Manha Formation (or equivalent Ulan Shireh Formation) is distinguished by a much larger size, and a more triangular outline of M3 without a metacone.
The Uintan Uintaceras radinskyi, which is considered to be a sister group of Rhinocerotidae, also bears a subtriangular M3 with nearly confluent centrocrista and metaloph, a relatively large parastyle, and a reduced metacone as in Gobioceras. But Uintaceras (m1–3 length: 88–93) is considerably larger than Gobioceras. Uintaceras is further different from Gobioceras in having the M3 metacone less lingually placed, and the protolophid and hypolophid of m1–3 more transversely extended.
The fifth species described is placed in the genus Ephyrachyus, and given the specific name woodi, in honour of Horace Elmer Wood, who described the genus and made a thorough revision of Hyrachyids from North America in 1934. The species is described from a single specimen, IVPP V 26252, a right maxilla with P2–M3, right and left p2–3, and fragmentary p4 and lower molar.
The genus Ephyrachyus is diagnosed by upper cheek teeth with the paracone and metacone more merged to form the ectoloph; P3–4 with a high metaconule and a relatively long endoprotocrista. Differs from Hyrachyus and Metahyrachyus by having the paracone and metacone merged with the ectoloph on the upper cheek teeth, the P3–4 metaconule relatively high, and the endoprotocrista relatively long. Further differs from Metahyrachyus by the protocone not joining the metaconule on P2, and the hypocone not budding off from the endoprotocrista on P3–4.
Ephyrachyus woodi differs from both Ephyrachyus implicatus and Ephyrachyus cristalophus in having the endoprotocrista of P3–4 posterobuccally extended from the protocone at a sharp angle, metaconules of P2–4 transversely extended; the metaconule of P4 not fused with the crista; M1–3 parastyle relatively larger. Further differs from Ephyrachyus implicatus by having relatively narrower and longer upper molars with more lingually placed metacones, and by lacking a posterior cingulum on P2 curved up on to the protocone. Further differs from Ephyrachyus cristalophus by having a metaconule on P2, and a relatively shorter M3 metaloph not confluent with the centrocrista. The new specimens clearly show some ‘Hyrachyus’-like characters, including large parastyles closely appressed to the paracones on the upper molars, relatively long postmetacrista on M1–2, M3 metacone reduced, buccally deflected, and perpendicular to the metaloph, and a relatively low cristid obliqua on the lower molars. The length of M1–3 is about 43.2 mm, which is similar to that of Hyrachyus modestus with the mean length ranging from 45 to 50 mm. However, the upper cheek teeth with paracones and metacones merged to form ectolophs, and the relatively high metaconules on P3–4 resemble those of Ephyrachyus erected by Wood in 1934.
The type of Ephyrachyus was based on ‘Hyrachyus’ implicatus (AMNH FM 5078), which was unearthed from the probably late Bridgerian of the Washakie Formation in the Washakie Basin, Wyoming. Wood also erected a new species Ephyrachyus cristalophus from the Bridger C₃ (late Bridgerian) in the Bridger Basin, Wyoming. However, Leonard Radinsky assigned Ephyrachyus implicatus to Triplopus mainly based on its occurrence in the Washakie Formation, from which Hyrachyus is unknown; by contrast, Hyrachyus are much more abundant in the Bridger Formation. Radinsky further considered Eotrigonias petersoni to be a synonym of Triplopus implicatus. Radinsky also considered Ephyrachyus cristalophus to be a synonym of Hyrachyus modestus, representing a small sized species of late Bridgerian Hyrachyus.
The new material, preserving nearly complete P2–M3 from the Erlian Basin, suggests that Ephyrachyus is a valid genus and ‘Eotrigonias petersoni’ is not a synonym of ‘Ephyrachyus’ implicatus. The new material is similar to ‘Ephyrachyus’ implicatus in having a prominent metaconule on P2 separated from the protoloph, paracones and metacones merged with the ectolophs on P2–4, endoprotocristae of P3–4 relatively long, metaconules of P3–4 high and enclosing the medifossette, and p3 with a distinct paraconid and lacking the entoconid. These similarities suggest that the new material and ‘Ephyrachyus’ implicatus should be assigned to the same genus. The new material can be distinguished from ‘Ephyrachyus’ implicatus by the lacking a posterior cingulum on P2 curved up on to the protocone, and in having the endoprotocristae of P3–4 posterobuccally rather than posterolingually extended from the protocone with sharp angles, metaconules of P2–4 transversely rather than posterolingually extended, and the metaconule of P4 not fused with a crista. The lower cheek teeth of the new material are more primitive than those of ‘Ephyrachyus’ implicatus in having the metaconid of p3 placed close to the protoconid, and a relatively lower cristid obliqua. Compared with M1–3 of CM 9384, which was assigned to ‘Triplopus’ implicatus by Leonard Radinsky, those of the new material are different in being relatively narrower and longer, and in having larger parastyles and more lingually placed metacones. Although M3 of the holotype of ‘Ephyrachyus’ implicatus is fragmentary, both the new material and CM 9384 show a reduced metacone of M3 buccally deflected and perpendicular to the metaloph, which are characteristics of Hyrachyids rather than Triplopus. Thus, ‘Ephyrachyus’ implicatus should not be reassigned to Triplopus, and Bai et al. suggest resurrecting Ephyrachyus for those advanced, small ‘Hyrachyids’. The new material represents a new species, Ephyrachyus woodi, first known from Asia.
Compared with Ephyrachyus, the holotype of ‘Eotrigonias’ petersoni (AMNH FM 2341) is distinguishable by smaller parastyles on P4–M3, metacones of M1–2 flatter and more elongated, and metacone of M3 relatively longer and lingually deflected. Thus, ‘Eotrigonias’ petersoni is not a synonym of ‘Ephyrachyus’ implicatus, but probably represents a valid species Triplopus petersoni.
Another species of Ephyrachyus, Ephyrachyus cristalophus, was considered to be a synonym of Hyrachyus modestus. However, Ephyrachyus cristalophus is similar to both Ephyrachyus woodi and Ephyrachyus implicatus in having the paracones and metacones merged to form ectolophs on P2–4, relatively long endoprotocristae and high metaconules on P3–4, and elongated metacones on M1–2. Bai et al. follow Horace Wood in considering Ephyrachyus cristalophus as a valid species of Ephyrachyus. The new material is different from Ephyrachyus cristalophus in having a metaconule on P2, the endoprotocristae of P3–4 sharply rather than smoothly curved from the protocones, metaconules of P3–4 transversely extended and enclosing the medifossette, the metaconule of P4 not fused with the crista, and the metaloph of M3 relatively shorter and not confluent with the centrocrista. The dental morphology of Ephyrachyus woodi is somewhat intermediate between those of Ephyrachyus cristalophus and Ephyrachyus implicatus, but is more similar to the latter. Furthermore, the similarities between Ephyrachyus woodi and North American Ephyrachyus implicatus indicate that the age of the upper part of the Arshanto Formation can be correlated to the late Bridgerian (Br3).
Two species of Hyrachyus have been reported from the Arshanto Formation in the Erlian Basin: Hyrachyus neimongoliensis and Hyrachyus crista. Hyrachyus neimongoliensis is preserved by a fragmentary skull with P3–M3 (IVPP V 5721), and Huang Xue-Shi and Wang Jing-Wen have argued its probable affinity with Amynodontids. Although Qi Tao assigned it to Hyrachyus, he also noticed that its cranial morphology and size resembles those of Pappaceras confluens (Forstercooperia huhebulakensis). Bai et al. consider ‘Hyrachyus neimongoliensis’ likely to be a synonym of Pappaceras minutus or Pappaceras meiomenus. If the latter case is true, the specific name Pappaceras neimongoliensis has priority over Pappaceras meiomenus.
Another species of Hyrachyus, Hyrachyus crista, was reported from the Arshanto Formation at Bayan Ulan. Hyrachyus crista is different from Ephyrachyus woodi in being larger, and in having a more distinct paracone rib on P4, a metaconule of P4 not in contact with the single protocone on the lingual side, parastyles of molars relatively more reduced, the protocone more anteriorly placed related to the level of the paracone on M1–3, the metacone ribs faint or absent on M1–2, the crista more distinct on M1–3, and the metacone of M2 much more elongated.
Leonard Radinsky reported Cf. Hyrachyus (AMNH FM 81801) with P4–M3 from the Arshanto Formation at Huheboerhe in the Erlian Basin. Ephyrachyus woodi is different from Cf. Hyrachyus in having metacone more separated from the paracone on P4, paracone and the metacone of P4 more merged with the ectoloph, the hypocone not separated from the protocone on P4, a distinct metacone rib on M2, and a relatively larger parastyle on M1–3. AMNH FM 81801 probably represent a new species of Hyrachyus as suggested by Huang Xue-Shi and Wang Jing-Wen.
The sixth, and final, new species described is refered to the genus Hyrachyus as Hyrachyus? tumidus, where ‘tumidus’ means swollen, referring to the swollen buccal surface of the P3–4 paracone and metacone. The species is describer from three specimens, IVPP V 26253.1, a right maxilla with broken P3–M2, and IVPP V 26253.2, and IVPP V 26253.3, trigonids fragments of lower molars, all from the basal part of the Arshanto Formation at Nuhetingboerhe.
Hyrachyus? tumidus differs from other species of Hyrachyus by the combination of following characters: P3–4 paracone and metacone rounded and swollen on the buccal surface; P3 with a long endoprotocrista and a metaconule directed to the base of the protocone; M1–2 with a parastyle somewhat separated from the paracone, a prominent metacone rib, and a relatively short postmetacrista.
The new material has the following characters suggestive of Hyrachyus affinity: a prominent metacone rib on M1–2, a relatively long postmetacrista, a weak cingulum on the buccal side of the metacone, a strong, cuspate parastyle on M1–2, and the attachment between the metaconule and the ectoloph higher than the corresponding attachment between the protoloph and ectoloph on P4. Compared with other known species of Hyrachyus (Hyrachyus modestus and Hyrachyus affinis) from early and middle Bridgerian (Br1–2, approximately equal to Bridger A and B) of North America, Hyrachyus? tumidus shows some relatively advanced features, including a protocone posteriorly extended on P3, a high, compressed parastyle on P4, a high and sharp paracone on M1–2 with the parastyle somewhat separated from the paracone. These features are in turn more or less reminiscent of Hyrachyus eximius and ‘Colonoceras agrestis’ from the late Bridgerian (Br3, Bridger C-D), Compared with Hyrachyids from the late Bridgerian, Hyrachyus? tumidus is more advanced than Hyrachyus ‘princeps’ in having more molarized P3, but more primitive than ‘Metahyrachyus’ in lacking the hypocones on P3–4. Furthermore, the upper cheek teeth of Hyrachyus? tumidus is usually larger than those of Hyrachyus from the middle Bridgerian, and approaches the relatively larger size in Hyrachyids from the late Bridgerian. Thus, Hyrachyus? tumidus seems more similar to species of Hyrachyus from the late Bridgerian of North America than those from early and middle Bridgerian. However, the fragmentary material and lack of M3 and most of the lower dentition in the new species make this statement very provisional. Compared with Hyrachyus metalophus from Shandong Province, both have distinct metacone ribs on M1–2, but Hyrachyus? tumidus can be distinguished by larger parastyles and shorter metacones on M1–2.
It is noteworthy that the buccal surfaces of the paracone and metacone on P3–4 are rounded and swollen rather than the riblike as in other species of Hyrachyus. These features are in turn similar to those of Uintaceras radinskyi, which Luke Holbrook and Spencer Lucas considered to be the sister taxon of Rhinocerotidae. In addition, Hyrachyus? tumidus also resembles Uintaceras in having a posteriorly extended protocone on P3 with the metaconule directed toward the base of protocone, and a relatively short postmetacrista on M1–2 with more separated parastyle. These similarities probably indicate that Hyrachyus? tumidus has a close relationship with Uintaceras. However, because of the lack of M3 and complete material, Bai et al. tentatively assign the species to Hyrachyus, pending the new discovery of more complete material in the future.
A cladistic analysis with parsimony criteria results in two equally most parsimonious trees. The tree length of the strict consensus is 2765; the consistency index is 0.234; the retention index is 0.497. The cladogram of the strict consensus tree shows two main clades of Ceratomorpha: Tapiroidea and Rhinocerotoidea; however, the endemic Asian Lophialetidae is a stem group of Ceratomorpha. Regarding the new materials of Rhinocerotoids reported by Bai et al., Yimengia is placed within Rhinocerotoidea, and is a sister group to Triplopus cubitalus, which was considered as an early Hyracodontid by Leonard Radinsky. Minchenoletes forms a sister group to the Yimengia and Triplopus cubitalus clade. Triplopus? youjingensis is most closely related to the ‘True Rhinocerotoids’, which comprises Hyracodontidae, Amynodontidae, ‘Paraceratheriidae’, and Rhinocerotidae. Epihyrachyus is a sister group to Prohyracodon, and both allied with Hyracodontidae. Gobioceras is a sister group to Pappaceras, and they are allied with Forstcooperia. Forstercooperiidae forms a clade as a sister group to the clade comprising Amynodontidae, ‘Paraceratheriidae’, and Rhinocerotidae.
The Bayesian tip-dating analysis generates a majority consensus tree. The relationships within Ceratomorpha are less resolved than in the parsimonious tree, and alternative phylogenetic positions for some taxa or groups are suggested. However, considering the taxa studied in the present paper, their phylogenetic positions generally coincide with those inferred from the parsimony analysis. Yimengia is the sister group to Triplopus cubitalus as suggested by the parsimony analyses. Triplopus? youjingensis is placed in Rhinocerotoidea with a polytomous position (excluding Uintaceras). Gobioceras is allied with Pappaceras and Forstercooperia, but they form a trichotomous clade. Hyrachyus, instead of Prohyracodon, is the sister group of Ephyrachyus, and they form a clade with an unresolved position in Ceratomorpha.
The phylogenetic trees show some interesting results and resolve long-lasting controversies on the phylogeny and biogeography of Ceratomorpha, although some discrepancies are present between the most parsimonious trees and the Bayesian Inference tree. Bai et al. feel it is necessary to mention that the ancestral distributions were reconstructed based on the most parsimony tree. The paraphyletic ‘Isectolophidae’ originated from Asia (excluding India) in the early Eocene, and then dispersed to North America and the Indian-subcontinent. The Karagalax–Gandheralophus clade is most closely related to Ceratomorpha in the most parsimonious trees; however, Meridiolophus and Isectolophus are closer to Ceratomorpha than are other ‘Isectolophids’ in the Bayesian Inference tree. The relatively derived position of Meridiolophus is consistent with its intermediate morphologies between Homogalax-like taxa and Heptodon. The endemic Asian Lophialetidae is excluded from the crown Ceratomorpha and represents a stem group in the most parsimonious trees, and its phylogenetic position is similar to that in the cladogram proposed by Jerry Hooker. Thus, Lophialetidae should neither be placed in Tapiroidea nor in Rhinocerotoidea. The ancestral distribution of Lophialetids is either in the Indian subcontinent or in non-India Asia. But Lophialetidae is placed in an unresolved position within Ceratomorpha in the Bayesian Inference tree. Ampholophus, originally considered as a Lophialetid, is a sister group of Chowliia in both analyses, and the clade is included in a paraphyletic ‘Isectolophidae’.
The crown Ceratomorpha is composed of superfamilies Tapiroidea and Rhinocerotoidea in the most parsimonious trees. Bai et al. consider Tapiroidea to be a monophyletic group, because ‘Isectolophidae’ is excluded from Tapiroidea and may also give rise to Ancylopods. Furthermore, Rhinocerotoids do not originate from Tapiroids, but probably from ‘Isectolophids’ and/or Lophialetids. The crown Ceratomorpha originated in Asia or North America, and the ambiguity is probably attributed to the nearly simultaneous appearances of early Tapiroids and/or Rhinocerotoids during the Early Eocene on both continents.
The superfamily Tapiroidea is supported by several common synapomorphic characters in the most parsimonious trees, such as M1 postmetacrista considerably posterobuccally oriented, cristids obliquae of lower molars highly reduced and directed toward protoconid, and absence of nasolacrimal contact. Heptodon is the sister group to other Tapiroids. The conventional ‘Helaletidae’ is clearly not a monophyletic group, because both Tapiridae and Deperetellidae derived from ‘Helaletids’ in the most parsimonious trees, which is consistent with previous morphologic comparisons. The Asian endemic Deperetellidae is more closely related to Tapiridae than to Lophialetids or Rhodopagids, and Colodon is closer to Tapirus than is Protapirus as suggested by Matthew Colbert. Furthermore, Rhodopagus and Dilophodon form a sister group within ‘Helaletidae’, rather than being allied with Rhinocerotoids. In contrast, both the Rhodopagus–Dilophodon clade and Deperetellidae are placed in Rhinocerotoidea in the Bayesian Inference tree, and Deperetellidae is even the sister group to Rhinocerotoidea. Rhodopagus from the Middle Eocene of Asia was first included in Tapiroidea, but subsequent investigations have suggested that Rhodopagus may be a Hyracodontid or primitive Rhinocerotoid. Similarly, Dilophodon was usually considered to be a small Tapiroid from the Middle Eocene of North America, but Robert Emry suggested its sister relationship with Fouchia and close to Rhinocerotoids. However, a sister group relationship between Deperetellidae and Rhinocerotoidea is somewhat unexpected, because the former has been unequivocally placed in Tapiroidea based on its craniodental characters. But the enamel microstructure found in the molars of Deperetellidae are characterised either by vertical Hunter-Schreger Bands or by compound Hunter-Schreger Bands, which has been seen in unequivocal Rhinocerotoidea, and ‘Hyrachyidae’ and Uintaceras, respectively. In contrast, the enamel microstructure found in the cheek teeth of Tapiroidea have either transversal Hunter-Schreger Bands or curved Hunter-Schreger Bands.
The superfamily Rhinocerotoidea is supported by several common synapomorphic characters in the most parsimonious trees, such as P3–4 postprotocrista absence, M1–2 protolophid somewhat posterolingually oblique, M1–2 metaconid slightly more posteriorly displaced to the protoconid, and m3 hypolophid slightly posterolingually oblique. Beside Yimengia and Deperetellidae, some taxa previously allied with Tapiroidea are replaced in Rhinocerotoidea in both analyses. Those taxa include Minchenoletes from the Early Eocene of Asia, and Selenaletes from the early Eocene of North America. Minchenoletes is either a sister group to the Yimengia and Triplopus cubitalus clade (in the most parsimonious trees) or placed in an unresolved position in Rhinocerotoidea (in the Bayesian Inference tree), instead of being a primitive Lophialetid as originally assigned. Selenaletes was initially considered to be a Helaletid, but it is placed either in a sister group to the ‘True Rhinocerotoidea’ plus Triplopus? youjingensis (in the most parsimonious trees) or forms a sister group to the Rhodopagus–Dilophodon clade (in the Bayesian Inference tree). In the parsimony tree, Indolophus forms a sister group to the Breviodon and Fouchia clade, and together they represent a sister group to other Rhinocerotoidea. Fouchia was originally considered to be in a pivotal position to the origin of Rhinocerotoids, and the statement is supported by the present cladogram. However, Indolophus, Breviodon, and Fouchia are polytomous in Ceratomorpha based on the Bayesian Inference tree. Hyrachyus modestus is a sister group to other Rhinocerotoidea in the most parsimonious trees, but forms a sister group to Ephyrachyus and they are together placed in an unresolved position in Ceratomorpha in the Bayesian Inference tree.
The phylogenetic trees further provide the phylogenetic relationships among four ‘True Rhinocerotoid’ families. In the most parsimonious trees, Hyracodontidae is a sister group to other ‘True Rhinocerotoidea’, and originated from non-India Asia. It is a monophyletic group if the genus Triplopus is excluded from Hyracodontids. Ephyrachyus is the sister group to Prohyracodon, and is remote from Hyrachyus. The Ephyrachyus and Prohyracodon clade forms a sister group to other Hyracodontids. In contrast, Hyracodontidae, which excludes Triplopus cubitalus and Prohyracodon, is more closely related to the Eggysodontidae–Paraceratheriidae–Rhinocerotidae clade in the Bayesian Inference tree. The Asian endemic Paraceratheriidae, usually comprising Forstercooperiinae and Paraceratheriinae, is not a monophyletic group in both analyses. The Forstercooperiidae is a sister group to other ‘True Rhinocerotoids’ except for Hyracodontids in the most parsimonious trees, and its phylogenetic position is somewhat similar to that proposed by Luke Holbrook. However, Forstercooperiidae is placed in a polytomous position in Rhinocerotoidea (excluding Uintaceras) in the Bayesian Inference tree. In the most parsimonious trees, Paraceratheriidae, which is represented by Juxia, Urtinotherium, and Paraceratherium, is most closely related to Rhinocerotidae, as proposed by Kurt Heissig, rather than being closely related either to Hyracodontids or Amynodontids. Current evidence suggests that Rhinocerotidae likely originated from North America. The Rhinocerotidae clade is supported by several synapomorphic characters, including a chisel-like I1 and a tusk-like i2, which were usually considered to be the most conspicuous features of Rhinocerotidae. The lack of metacone on M3 is not restricted in Rhinocerotids, and is also distributed in other Rhinocerotoids except for Amynodontidae, which is characterised by a distinct metacone on M3 with a short postmetacrista. Eggysodon is the sister group to the Paraceratheriidae and Rhinocerotidae clade in the most parsimonious trees. Amynodontidae is a sister group to the Eggysodon–Paraceratheriidae–Rhinocerotidae clade, and originated from non-India Asia. Proeggysodon, previously considered to be a primitive Eggysodontid, forms a sister group to Caenolophus promissus, and both of them represent a sister group to other amynodontids in the most parsimonious trees. Caenolophus was originally considered to be a Hyracodontid, but later became allied with Amynodontids. Proeggysodon was known only from a mandible and the lower dentition, which probably bias its phylogenetic position in the most parsimonious trees. In contrast, Eggysodontidae (Eggysodon and Proeggysodon), Paraceratheriidae, and Rhinocerotidae form a trichotomous clade in the Bayesian Inference tree, and the phylogenetic position of Amynodontidae within Rhinocerotoidea (excluding Uintaceras) is unresolved.
The general topologies are somewhat different between the most parsimonious trees and Bayesian Inference tree. Lophialetidae is a stem group of Ceratomorpha in most parsimonious trees, but placed in an unresolved position in Ceratomorpha in Bayesian Inference tree. However, the phylogenetic positions of some lineages are contradicted between the two methods. The Rhodopagus–Dilophodon clade and Deperetellidae are placed in Tapiroidea in the most parsimonious trees, but both are allied with Rhinocerotoidea in the Bayesian Inference tree. Amynodontidae is closer to the Eggysodontidae–Paraceratheriidae–Rhinocerotidae clade than is Hyracodontidae in the most parsimonious trees; however, the Bayesian Inference tree suggests a closer relationship between the latter two clades. The preference of different topologies generated by the parsimony and Bayesian analysis for morphological data are ongoing debate, and it seems that both have advantages and disadvantages for morphological data. The parsimony method only provides a point estimate (the most parsimonious trees) while Bayesian inference averages over the uncertainties of the topologies by summarising a majority-rule consensus tree. Moreover, the Bayesian tip-dating analysis takes both the morphological characters and geological times into account and models the diversification and sampling processes explicitly, while the parsimony method uses morphological characters solely and absents explicit model assumptions. Nevertheless, the taxa or clade contradictory in both methods indicate that the data might not contain enough information to draw firm conclusions about their relationships. With more fossils and more complete data added in the matrix in combination with improvements of algorithms and parameters two methods probably converge to more compatible results.
The new Rhinocerotoid taxa Yimengia magna, as well as reassigned Minchenoletes, from the Early Eocene Bumbanian is nearly contemporary with Early Eocene Tapiroids, suggesting that the divergence between Rhinocerotoids and Tapiroids occurred no later than the early Eocene (52–56 million years ago). The divergence time between Rhinocerotoidea and Tapiroidea in the Early Early Eocene based on fossil evidence here falls between the roughly 51 million year and roughly 57.5 million year estimates from molecular data. Furthermore, the Forstercooperiid Gobioceras, the Rhinocerotoid Triplopus? youjingensis, and the Rhinocerotid-like Hyrachyus? tumidus from the base of the Arshanto Formation suggest that divergence of these different Rhinocerotoid groups occurred no later than the Late Early Eocene, soon after the split between the Rhinoceroses and the Tapiroids. However, the Bayesian tip-dating estimate suggests that the median value of the divergence time of different Ceratomorph groups (60.1 million years ago) is in the Middle Palaeocene, and that of Rhinocerotoid groups (57.2 million years ago) is in the Late Palaeocene. Both estimates are earlier than current fossil evidence, but the former estimate is close to the divergence time between Rhinocerotoidea and Tapiroidea (57.5 million years ago) based on recent molecular analysis. Similarly, the divergences time of different groups within Lophialetidae, Tapiroidea, and Rhinocerotoidea are in the Early Eocene, and the divergence between Deperetellidae and Rhinocerotoidea occurred 54.6 million years ago. The divergences of the groups within Forstercooperiidae and Amynodontidae occurred in the Late Early Eocene, while those of the groups within Hyracodontidae, Eggysodontidae, Paraceratheriidae, and Rhinocerotidae occurred in the \middle Eocene. The median value of the divergence time of Eggysodontidae, Paraceratheriidae, and Rhinocerotidae is 43.9 million years.
The diverse Rhinocerotoids from the base of the Arshanto Formation are probably correlated with the Early Eocene Climatic Optimum and likely lived in a relatively close, humid environment as inferred from the dental stable carbon isotope analyses of Schlosseria from the same horizon. The habitat of Lophialetidae in the Huheboerhe area is considered to be ‘a relatively open forest environment like a woodland (or a low-density forest)’, and became relatively more arid and/or open over time during the Early–Middle Eocene.
To sum up, the phylogenetic analysis based on both parsimony and Bayesian inference criteria highlights the phylogeny and biogeography of Ceratomorpha, especially for some long-standing controversial groups, such as Lophialetids, Deperetellids, equivocal early Rhinocerotoids, and relationships among Rhinocerotoid groups. Both Tapiroidea and Rhinocerotoidea are independent, monophyletic groups, and derived from ‘Isectolophids’ and/or Lophialetids. Lophialetidae is a stem group of Ceratomorpha in the most parsimonious trees. Some taxa conventionally assigned to Tapiroids are placed to Rhinocerotoidea. However, the phylogenetic positions of Deperetellidae, the Rhodopagus–Dilophodon clade, Hyracodontidae, and Amynodontidae within Ceratomorpha are controversial between the two methods. Furthermore, Bai et al. propose that the divergence between the Rhinocerotoidea and Tapiroidea occurred no later than the Early Early Eocene, or extended to the Middle Palaeocene as suggested by the Bayesian tip-dating estimate. The appearance of various Rhinocerotoids from the base of the Arshanto Formation suggest that the divergence of different Rhinocerotoid groups occurred no later than the Late Early Eocene, or in the Early Early Eocene as inferred from the Bayesian tip-dating estimate. The habitat of diverse Rhinocerotoids from the base of the Arshanto Formation is inferred to have been a relatively close, humid environment. More groups and postcranial characters need to be added into the matrix in future investigations, in order to resolve some controversial issues and illuminate the evolutionary history of the order Perissodactyla.
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