An early Late Cretaceous mammal from Japan, with reconsideration of the evolution of tribosphenic molars

Abstract

The morphology of a mandibular fragment with a left lower molar discovered in the "Upper Formation" (upper Cenomanian-lower Turonian) of the Mifune Group in central Kyushu, southwestern Japan, suggests that this fossil should be assigned to a new species of Late Cretaceous mammal, Sorlestes mifunensis sp. nov. (Infraclass Eutheria; Order Proteutheria; Family Zhelestidae). S. Mifunensis is the oldest zhelestid yet recorded. Some workers suggest that the Zhelestidae have a close affinity with ungulates. A detailed comparison between the lower molar of the new species and those of ungulates supports this suggestion. The comparison also suggests that the Zhelestidae have a closer affinity with ungulates than the Zalambdalestidae and other contemporary mammals, and that S. mifunensis has a relatively primitive character within the Zhelestidae. This comparison leads us to revise the diagnoses of the family Zhelestidae and of the genus Sorlestes. The unique character of the entoconid-hypoconulid twinning seen in the Zhelestidae was probably caused by the movement of the hypoconid (the presumed first single talonid cusp seen in the first therian Kuehneotherium) to the buccal side, far away from the other talonid cusps. This twinning pattern is distinct from the twinning pattern seen in marsupials.