A probable pseudo-tribosphenic upper molar from the late Jurassic of China and the early radiation of the Holotheria

A three-cusped, mammalian upper molar was recovered from the type locality of Shuotherium dongi Chow and Rich, 1982 in the Late Jurassic, Upper Shaximiao Formation Sichuan, China. This tooth is too large to be referable to S. dongi, which is known only from a fragment of a left mandible with teeth. Shared, derived (possibly autapomorphic), functionally related characters of the reconstructed dentition and occurrence at the same locality suggest that the new specimen probably is an upper molar of a closely related species, here named Shuotherium shilongi, sp. nov. Crown morphology and development of wear facets on the new specimen accord with the reconstruction of occlusal relationships of pseudo-tribosphenic molars proposed by Chow and Rich (1982). An alternative, less probable, interpretation is that the new specimen is an upper molar of a tribosphenic mammal and indicates a greater antiquity and diversity of the Tribosphenida than previously thought. This uncertainty highlights the problems in identification of isolated, primitive tribosphenic-like teeth, particularly isolated upper molars. Phylogenetic analysis indicates that Yinotheria Chow and Rich, 1982, including Shuotherium dongi and, probably, S. shilongi, sp. nov., most likely is not a member of the symmetrodont clade but is a sister group to Cladotheria McKenna, 1975 (and see McKenna and Bell, 1997), consisting of the Dryolestidae, Paurodontidae, Amphitheriidae, Peramuridae, and Tribosphenida.