A new tritylodontid from the Middle Jurassic Shaximiao Formation of western Hubei, China.

The tritylodontids are a group of highly specialised, herbivorous non-mammalian synapsids, which have been thought to be closely related to mammals. By using CT scanning technique and 3D reconstruction, a new tritylodontid, Bienotheroides xingshanensis sp. nov., is described herein from the Middle Jurassic Shaximiao Formation of western Hubei, China. The only and holotype specimen shows a part of skull, with well-preserved right dentition bearing two incisor and six cheek teeth. It is assigned to Bienotheroides based on its short and broad snout, obtusely rounded corners of upper cheek teeth and the cusp formula of 2-3-3. It differs from the known species of Bienotheroides in having short and wide upper cheek tooth outline, as well as two much more reduced anterior cusps of the median and lingual rows. Based on a new compilation of the known record of tritylodontids, the evolution of spatio-temporal distribution of this group is addressed. It is supposed that the apparent extinction of the Early Jurassic tritylodontids might be related with the Toarcian global warming and that the northward migration of tritylodontids during the Middle to Late Jurassic might have been influenced by aridification trend in East Asia. [ABSTRACT FROM AUTHOR]