Abstract

A tegotheriid docodontan Sibirotherium rossicum from the Early Cretaceous of Western Siberia, Russia, was considered to have six lower premolars, as in the tegotheriids Agilodocodon and Microdocodon from the Middle Jurassic of China. The micro-computed tomography of two dentary specimens with a supposed submolariform ultimate deciduous premolar (dp6) revealed absence of a replacing tooth germ in this locus. Also, the morphology of the roots of this tooth is more consistent with that of molariform teeth. Based on the new data, we interpret Sibirotherium to possess five lower premolars rather than six and that the supposed dp6 might in fact be better interpreted as the first molariform (m1). The results of our phylogenetic analysis suggest that this is a plesiomorphic condition for Docodonta and, under slow character optimization, for Tegotheriidae, with the number of premolars reduced to three to four in Docodontidae (Haldanodon, Docodon, and Docofossor).