New primitive therian from the early Cretaceous of Mongolia

Abstract

THERIAN mammals with tribosphenic molars were probably in existence at the beginning of the Cretaceous¹. This conclusion is based on a single heavily worn lower molar from the Lower Wealden (Neocomian) of England, named Aegialodon dawsoni. The molars of the Jurassic therians, (Pantotheria and Symmetrodonta) were capable only of puncturing and shearing. Tribosphenic molars have an additional cusp on the upper molars (protocone) which fits into a basin (talonid) on the matching lower molar. Unfortunately, the single lower molar of Aegialodon is extensively worn and slightly damaged. Consequently, it is difficult to discuss with confidence the structure of the earliest known tribosphenic molars, or the inferred structure of the matching upper molars, or the occlusal relationships. In this paper a brief description is given of an early Cretaceous mammalian lower molar from the Aptian of Asia. It is almost identical to that of Aegialodon. Because it is exceptionally well preserved and practically unworn, it provides critical information on the structure and function of the very early tribosphenic molars.