Search

The “condylarth” genus Hyopsodus is diverse and abundantly represented in Eocene mammalian faunas of North America. In contrast, fossil specimens of Hyopsodus are rather sparse in Eurasia. Only four species of Hyopsodus are known from Asia and two from Europe, as compared to the 18 species of Hyopsodus described from North America. Here, we report a new species of Hyopsodus, Hyopsodus arshantensis sp. nov., from the middle part of the Arshanto Formation in the Erlian Basin, Inner Mongolia, China. The holotype and only specimen of the new species, a right mandible with m1–m2, exhibits a unique combination of characters on m1–m2 not present in other species of Hyopsodus, including a moderately lophodont crown, a long trigonid without a paraconid, an obliquely aligned protolophid, an angle between the cristid obliqua and the posthypocristid slightly greater than 90°, a midline position of the hypoconulid, and a relatively large entoconid. The m1–m2 morphology of H. arshantensis is intermediate between specimens of Hyopsodus from the Wasatchian and Uintan North American Land Mammal Ages (NALMA), and is comparable to that of Hyopsodus from the Bridgerian NALMA. Moreover, its relatively large size is near the size range present among the late Bridgerian species of Hyopsodus. Based on those similarities, in combination with a few fossil mammals from overlying layers, the middle part of the Arshanto Formation could be correlated in part to the late Bridgerian, and the upper part of the Arshanto Formation may bracket the time interval equivalent to the Bridgerian/Uintan boundary. That proposed correlation and somewhat different faunas recognized within the Arshanto Formation suggest that it may be necessary to subdivide the Arshantan Asian Land Mammal Age (ALMA) and/or redefine the Arshantan/Irdinmanhan ALMA boundary in future comprehensive studies.

The paraceratheriid Pappaceras is the earliest unequivocal rhinocerotoid genus to date, for which the osteological morphology is relatively unique compared to other perissodactyls. Due to the poor preservation condition, paleobiological aspects of Pappaceras (or forstercooperiines), such as chewing and feeding behavior, still remain unknown. Under the Extant Phylogenetic Bracket, the cranial musculature of the newly erected Pappaceras meiomenus has been reconstructed using two-dimensional illustrations, drawings and interpretations of the position and general morphology of cranial muscles for which origins and insertions on the skull are visible. In this study, eight muscles are reconstructed, described and compared to the corresponding muscles known or inferred in other perissodactyls, including the m. levator nasolabialis, the m. levator labii superior, the m. caninus, the m. zygomaticus, the m. masseter, the m. temporalis, the m. buccinator and the m. pterygoid. The reconstruction of the masticatory muscles suggests that Pappaceras meiomenus is strictly herbivorous, probably folivorous, with a primary component of vertical biting. The relatively well-developed m. pterygoid (particularly the m. pterygoideus medialis) indicates that Pappaceras meiomenus is similar to hyracodontids, having more advantages in rotary chewing than other non-hyracodontid rhinocerotoids. The configuration of basicranial features shows differentiation between non-hyracodontids and hyracodontids, demonstrating that the well-developed, specialized postglenoid process and the wide glenoid fossa, along with the postcotyloid process of the mandible, serve as a strong fulcrum during the power stroke in non-hyracodontids. Based on its rostral morphology, we suggest that Pappaceras meiomenus was a general browser. The morphology of its incisors and canines further indicate the ability to feed on hard plants, using the postulated puncture-crushing and grinding function.