New material of Aprotodon lanzhouensis (Perissodactyla, Rhinocerotidae) from the Early Miocene in Northwest China

Aprotodon is an extinct genus of the family Rhinocerotidae with a peculiar morphology that is mainly embodied in the wide mandibular symphysis and a pair of prominent second low incisors. The genus once lived on the vast grasslands of Asia, from Pakistan, and Kazakhstan to China, during the Late Eocene to the Early Miocene. The scarcity and incompleteness of the record, however, make the morphological characteristics of Aprotodon rather perplexing. Here, we report on a better mandible fossil with an intact second right incisor from the Xianshuihe Formation from the Early Miocene age (approximately 21–20?Ma) in the Lanzhou Basin, Gansu Province, China. Several important features, notably the elongated strongly curved tusk-like incisors and mandibular symphysis, which widens sharply in the anterior part of the mandible, support attribution to A. lanzhouensis. Compared with previous specimens, the material described here is distinct in the lack of p1, horizontal ramus, and the ventral morphology of the mandible symphysis. Based on the new fossil specimen, we provide new material for the understanding of this extinct peculiar rhinoceros.