Symmetrodonta's a basal group of Mesozoic mammals characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. The traditional group of symmetrodonts ranges in age from the latest Triassic to the Cretaceous. One species, Spalacotheridium noblei,'s notable for its small size. It's one of the smallest known mammals. Each individual molar's little more than 1/100th of an inch across. Symmetrodonta are generally rare and poorly represented… (More on Symmetrodonta)