Earliest known unequivocal rhinocerotoid sheds new light on the origin of Giant Rhinos and phylogeny of early rhinocerotoids

Abstract

Forstercooperiines are a group of primitive rhinocerotoids with a relatively large body size in the Eocene, and normally considered to be closely related to Giant Rhinos. Here we report a new forstercooperiine, Pappaceras meiomenus sp. nov., from the late Early Eocene Arshanto Formation, Erlian Basin, Nei Mongol, China. Pappaceras is the earliest known unequivocal rhinocerotoid, and the holotype of the new species, represented by the most complete cranium of forstercooperiines known to date, shows the earliest evidence of reduction of the first upper premolar in rhinocerotoids, and resembles paraceratheriine Juxia in basicranial features, supporting the interpretation that the forstercooperiine clade is ancestral to paraceratheriines. The new species also displays some similarities with amynodontids in craniodental structures. Phylogenetic analysis identifies P. meiomenus as a basal taxon of the monophyletic forstercooperiines. It also reveals novel phylogenetic relationships of early rhinocerotoids that indicates Uintaceras is the sister group of paraceratheriids, to which amynodontids are more closely related than to any other group of rhinocerotoids. Furthermore, the eggysodontid clade is excluded from hyracodontids and placed as the sister group of rhinocerotids. Hyracodontidae, excluding paraceratheriids and eggysodontids, is placed as the most basal group of the rhinocerotoids.

New data on Amynodontidae (Mammalia, Perissodactyla) from Eastern Europe: Phylogenetic and palaeobiogeographic implications around the Eocene-Oligocene transition

“…Suprageneric relationships within Rhinocerotoidea are divergent from other recent phylogenies, especially concerning the Hyracodontidae, which is clearly paraphyletic (following the “classical” definition of the family), as also recovered by recent study [ 23 ]. The results presented here are disputable for several reasons.…”

“…The phylogenetic position of Amynodontidae within perissodactyls is quite unstable [ 19 ]. The potential relationships of this family, outside the rhinocerotoids, have only been phylogenetically tested by including two amynodontid taxa ( Rostriamynodon grangeri , and the type genus of the family: Amynodon ) [ 20 – 23 ]. However, the monophyly of Amynodontidae and the synapomorphies of this clade within Rhinocerotoidea are well-established [ 2 ]: quadratic M3, loss of upper and lower P1, enlarged canines, elongated talonids and presence of a preorbital fossa.…”

The First Tertiary Fossils of Mammals, Turtles, and Fish from Canada's Yukon

“…However, given that the cheek teeth of the largest North American amynodont Metamynodon planifrons are noticeably taller or higher crowned, yet have thinner enamel, than YG 514.12, it seems unlikely. The Peraceratheriidae (= Indricotheriidae), an Asian clade that contains the largest terrestrial mammals that have ever lived on Earth (Radinsky, 1967;Wang et al, 2016), is known to have vertical HSB (specifically in Forstercooperia and Juxia\ Koenigswald et al, 2011), although the enamel of many members of this clade has not yet been investigated. North American Uintaceras, considered the sister taxon to the Asian peraceratheres (Wang et al, 2016), has compound enamel in which horizontal (or transverse) HSB occur in an inner zone, and vertical HSB in an outer zone (Koenigswald et al, 2011).…”

Eocene Pachynolophinae (Perissodactyla, Palaeotheriidae) from China, and their palaeobiogeographical implications

“…; Wood ; Qiu & Wang ; Wang et al . ), as well as the rhinocerotoid Hyrachyus douglassi (Wood ). Thus, the combination of a molariform P2 and a non‐molariform P3–4 is known in different groups of perissodactyls as a result of a parallel evolution.…”