Article: Dental morphology of the Jurassic holotherian mammal Amphitherium, with a discussion of the evolution of mammalian post-canine dental formulae

Four fragmentary mandibles from the Stonesfield Slate facies, Taynton Limestone Formation, Middle Bathonian (Middle Jurassic), England, represent two species of AmphitheriumA. prevostiiA. rixoni sp. nov. Both species had five lower premolariform teeth. The composite formula for the lower dentition of Amphitherium appears to have been I/4, C/1, P/5, M/6-7. The seventh molar is not present in the presumably oldest individual, and its presence is regarded as probably an individual variation. The mosaic evolution of patterns of differentiation of the postcanine dentition from nonmammalian cynodonts to modern therian mammals is reviewed. It is concluded that Amphitherium probably had reached the grade of modern therians in the division between diphyodont premolars and monophyodont molars. The common ancestor of Amphitherium and zatheres probably had lost the primitive pattern of posterior shift of the postcanine dentition, which appears to have consisted of five premolars and four or possibly five molars. In Amphitherium the number of molars probably was secondarily increased.