The well-preserved skull is unquestionably referable to Forstercooperiinae mainly for its cranial morphology (e.g. shallow nasal incision and distinct preorbital fossa) and unique anterior dentition (e.g. conical, sub-equal upper incisors and relatively stubby canines). We assign the new specimen to a new species of Pappaceras based on the stratigraphical evidence (same with Pappaceras rather than Forstercooperia in Arshantan age) and its characteristic dental morphology (see diagnosis). The comparisons of craniodental features between Pappaceras meiomenus and the earliest representatives of main groups of early rhinocerotoids, which were considered to be highly comparable as proposed by various authors7,8,11,18,20, are essential for the study of rhinocerotoid evolution.

P. meiomenus is evidently distinguishable from rhinocerotids by lacking characteristics of rhinocerotids, such as the tusk-like I1/i2, which occurs as an incipient condition later in the earliest unquestionable rhinocerotid Teletaceras28, Compared with Juxia, an ancestor to the later giant paraceratheriids, P. meiomenus differs by its smaller body size, shallow nasoincisor notch terminated above upper canines, nasal-premaxilla contact, relatively stubby upper canines, reduced upper premolars and distinct metacone of M3. Moreover, P. meiomenus is similar to Juxia, but distinguishable from other early rhinocerotoids (except for Rostriamynodon as discussed below) in following features: (1) relatively long basicranium29; (2) the posterior end of zygomatic plate-like and dorsoventrally compressed; (3) a posterior end of zygomatic arch forming lobe-shaped blade; (4) a shallow preorbital fossa; (5) a wide frontal between the orbital and followed by a flange overhanging the postorbital cavity; (6) incisors arranged in straight lines and weakly converging anteriorly; (7) ridge-like nuchal crest. The posttympanic process and paroccipital process of Juxia are more developed, wider, and thicker than those of P. meiomenus, however, and the latter began to show the rudimentary coalescence and enlargement of the former. Similarly, despite lacking the characteristics of amynodontid cheek teeth that possess straight ectolophs on upper molars and short premolar series, P. meiomenus shares the character (5), (6), (7) as listed above with the most primitive amynodontid Rostriamynodon30. Furthermore, it is interesting to note that the some features of P. meiomenus are reminiscent of those of amynodontids: a relatively large canine, distinctly reduced P1, a short postcanine diastema, P4 transversely wide with high and strong protoloph, weak and short metaloph (metaconules), and M3 quadrate in outline. The derived amynodontid Sharamynodon shows a similar condition to P. meiomenus and Juxia with coalesced and anteroposteriorly thick paroccipital and posttympanic processes, although the paroccipital process is more posteriorly extended. Generally, the younger group of eggysodontids are similar to forstercooperiines with short diastema and lingually extended protolophids of lower cheek teeth18,31. In spite that P. meiomenus is close to the Late Eocene eggysodontids Proeggysodon and Guangnanodon in size (based on length of lower molar series)18,31, eggysodontids differ from the latter by having complete upper premolars, large and nearly vertically oriented canines, primitively speculate incisors that vary in number (2 or 3) across different genera, and more molarized premolars32,33, and the known craniodental features of eggysodontids34 are more comparable to those of rhinocerotids rather than forstercooperiines. The enigmatic genus Uintaceras was considered as the sister group of rhinocerotids for its distinctive features of anterior dentition (buccolingually compressed upper incisors with a triangular profile) and characteristics of cheek teeth and postcranial elements8,35. Although Holbrook and Lucas35 pointed out some cranial differences between forstercooperiines and Uintaceras, it should be noted that the reconstruction of skull of Uintaceras was mainly based on laterally compressed and distorted materials of UCMP 69722 and UW 2410. Some characters used for distinguishing Uintaceras from fostercooperines show similarities in mentioned taxa, when compared to those of other early rhinocerotoids. For instance, both of them possess shallow nasal notches, high maxillae, prominent sagittal crests, the occipitals not strongly posteriorly inclined, and probably shallow maxilla fossae. In terms of dentitions, they are similar in having non-molarized premolars, presence of M3 metastyle and short diastema. It is interesting to mention that some similarities between Uintaceras and amynodontids in both dental and postcranial characters were mentioned, but considered as a result of scaling35. The most conspicuous feature that distinguishes Uintaceras from fostercooperines lays in its buccolingually compressed upper incisors with triangular profile. This specular feature, however, is even different from anterior teeth of all rhinocerotoids. It seems that Uintaceras is still more closely related to forstercooperiines and amynodontids than to rhinocerotids. In addition, the late Bridgerian Hyrachyus princepts (AMNH 12364)36 shows some similarities with either fostercooperines or Uintaceras in craniodental characters, but different from other species of Hyrachyus in having a flange posterior to the orbital, somewhat coalesced and enlarged posttympanic and paroccipital processes (with forstercooperiines), and especially relatively chisel-blade I1-2 (with Uintaceras). These similarities seem not unexpectable, since Hyrachyus princepts, the second largest species of Hyrachyus beside H. grandis, which was included in Uintaceras, was considered to give rise to the Uintan H. grandis by Wood36. However, the large parastyles of upper molars and relatively reduced cristid obliqua of lower molars displaced the species in the side of tapiroids rather than rhinocerotoids.

Phylogenetic analysis yielded one most-parsimonious tree (MPT) that is displayed in Fig. 4. Our phylogenetic analyses placed Pappaceras meiomenus as the most basal taxon of the monophyletic forstercooperiine clade (node F). The separate placement of P. meiomenus from the group of P. confluens and P. minuta implies that P. meiomenus represents an alternative evolutionary direction by the reduction of upper premolars in this genus. The monophyletic hyracodontids (node H), amynodontids (node A), paraceratheriids (node P), and rhinocerotids (node R) are also supported in the result of phylogenetic analyses. Despite the fact that only craniodental features are known for unequivocal forstercooperiines, the close affinity between forstercooperiines (node F) and paraceratheriines (node U) was supported in our results with solid evidence that consists of several synapomorphies (Supplementary Information, Table 1).

Figure 4 Correlation of geographical distributions and phylogenetic relationship of early rhinocerotoids based on the single most-parsimonious tree (tree length = 430, consistency index = 0.540, retention index = 0.704). For convenience, node A for Amynodontidae, E for Eggysodontidae, F for Forstercooperiinae, H for Hyracodontidae, P for Paraceratheriidae, R for Rhinocerotidae, U for Paraceratheriinae. Numbers by the nodes denote the Bremer values. Full size image

Our phylogenetic analysis reveals novel interrelationships of early rhinocerotoids (Fig. 4). Based on the node J, the clade comprising amynodontids, paraceratheriids and Uintaceras is monophyletic. The genus Uintaceras, which was considered as “North American forstercooperiines” and subsequently regarded as the sister group of rhinocerotids35, is closely related to the Asian paraceratheriid clade, suggesting that Uintaceras should be North American relatives of paraceratheriids. The similarities of incisors and postcranial elements between Uintaceras and rhinocerotids as previous proposed to favor the rhinocerotid affinity of Uintaceras, shows some parallelism in the evolution of early rhinocerotoids according to the result. Meanwhile, Hyrachyus princeps, possibly ancestral to Uintaceras, is positioned at the basal part of the present parsimonious tree, despite resembling Uintaceras in having uniquely buccolingually compressed I1 and distinctive flanges overhanging postorbital cavity. Interestingly, it is the amynodontid clade that is closely placed to paraceratheriids and North American Uintaceras, contrasting with the previous view that regarded amynodontids as a unique family divergent from other rhinocerotoids at early stage of rhinocerotoid evolution7,10,37. Particularly, the reduction of P1 makes P. meiomenus linked with amynodontids, for this feature is present in P. meiomenus as the earliest occurrence across rhinocerotoids, and characteristic of all amynodontids as well. The eggysodontid clade (node E) is revealed as the sister group of the rhinocerotid clade (node R), in contrast to the previous views of either paraceratheriines or forstercooperiines affinity11,18. The hyracodontid clade, is redefined as the most basal group of rhinocerotoids in our results, and comprises the primitive Triplopus, Prohyracodon, several derived types from North America (e.g. Hyracodon and Triplopides) and Asia (e.g. Ardynia) with small to medium body size and distinctively fast-running adaption of the skeletons21,29.

A parallel evolutionary trend is evident in the evolution of dental characters of early rhinocerotoids, particularly the anterior dentition and M3, which were generally regarded as the diagnostic features of the four main families of rhinocerotoids5,6,21. Large lower canines are present in forstercooperiines, amynodontids, and eggysodontids, of which the incisors are uniquely conical for the former and primitively spatulate for the latter two groups. On the other hand, the specialization of incisors is represented by tusk-like form for paraceratheriines and rhinocerotids with enlargement of I1/i1 and I1/i2 respectively, whereas it turns to be conical for forstercooperiines and North American hyracodontids. The lingually deflected metacone of M3, that was regarded as characteristic of hyracodontids7,21, is plesiomorphic for all clades except for eggysodontids, of which M3 specimen from Eocene is still unknown. Meanwhile, the homologies of cranial and postcranial characters, which were not extensively explored under the frame of phylogeny but equally significant as dental evolution, are revealed in the present results, for instance the hyracodontid clade is supported by 7 postcranial features, while 7 synapomorphies supporting the monophyly of amynodontid-paraceratheriid-Uintaceras clade are related to cranium.

Given the phylogenetic relationship of early rhinocerotoids, it reveals the primary trend of independent origination and evolution in different continents in relevant groups of paraceratheriids, amynodontids and hyracodontids. The origination and evolution of the paraceratheriid clade is definitely restricted into Asia, whereas its sister group Uintaceras appeared slightly later (Uintan) in North America than Pappaceras (Arshantan) in Asia, and like Hyrachyus princeps, evolved only in the North American continent. Likewise, the earliest records of amynodontids are represented by Rostriamynodon and its contemporary Amynodon occurring in Asia (Irdinmanhan) and North America (Uintan), respectively. Triplopus, the most primitive hyracodontid, first appeared in the Irdinmanhan and Uintan, and its descendants evolved independently to obtain the incisors with two different forms (spatulate and conical) in Asia and North America (the anterior dentition of Prohyracodon unknown). The distinctively cursorial adaptation of hyracodontids is relatively conservative from primitive Eocene Triplopus to derived Oligocene Hyracodon. By contrast, the eggysodontids and rhinocerotids seem to have arose independently in Asia and North America, respectively. The Asian origination of European eggysodontids is supported by the earliest occurrence in the Late Eocene of Asia, suggesting that this group dispersed into Europe after the “Grande Coupure”18. Given the presence of the earliest representative of rhinocerotids in North America28, as well as the poor record of Asian Eocene rhinocerotids and the systematically problematic status of those assigned specimens29,38,39, the North American continent should be central for rhinocerotids at early stage of the evolution. Another evident evolutionary trend is the maximum size of amynodontids and paraceratheriids, which is generally congruent with other Cenozoic large land mammals40. This pattern is greatly contributed to the niche expansion, in which Juxia evolved to the largest terrestrial land mammal Paraceratherium in paraceratheriids (Fig. 4), whereas Rostriamynodon turned to be the assumedly uniquely semi-aquatic Metamynodon in amynodontids41. On the other hand, the paraceratheriids, amynodontids and hyracodontids gradually faded away after the Oligocene epoch, but the rhinocerotid clade just began to diversify since the Late Eocene.