Brachyscirtetes tomidai, a new Late Miocene dipodid (Rodentia, Mammalia) from Siziwang Qi, central Nei Mongol, China

摘要

Based on the dipodid materials collected from Siziwang Qi, Nei Mongol in 2009, a new species Brachyscirtetes tomidaisp. nov. is erected. It is characterised by its smaller size, more bunodont paracone on M2, more separate mesoloph and posteroloph on M2 and separate mesolophid on m2, more longitudinally symmetrical protoconid-metaconid complex diverging at an obtuse angle and elongated anterior part of ectolophid on m1 than existing species. It represents the most primitive known species of this genus. Judging from the associated mammals, the age of Brachyscirtetes tomidai should be late Late Miocene. Brachyscirtetes is possibly derived from a form of early Late Miocene Paralactaga. Its dental trends include increasing size and crown height, and increasing lophodonty of occlusal morphology, which is reflected as gradual fusion of mesoloph and posteroloph with paracone, metacone on M2 and of mesolophid with entoconid on m2, respectively; the protoconid-metaconid complex on m1 shifts from a longitudinally symmetrical position to an anterolabial orientation, and the anterior part of ectolophid on m1 is gradually shortened, shifting from a longitudinal to an oblique orientation. The appearance of Brachyscirtetes and abundant herbivores indicates an arid and steppe environment in late Late Miocene Siziwang Qi area.