A new species of the 'condylarth' Hyopsodus from the middle Eocene of the Erlian Basin, Inner Mongolia, China, and its biostratigraphic implications

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.