The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil

JOSE BONAPARTE; AGUSTIN MARTINELLI; CESAR SCHULTZ; ROGERIO RUBERT

Simposio; 51st Symposium on Vertebrate Palaeontology & Comparative Anatomy; 2003

Two new derived cynodonts from the Late Triassic of southern Brazil demostrate the unexpected variety and distribution of very small, presumably insectivorous probainognathians, from which the morganucodontid mammals eventually originated. Mov. gen., nov. sp. A is characterized by rectangular upper and lower postcanines with a summetrical distribution of cusps and little morphological variation along the tooth row. Nov. gen. nov. sp. B has rectangular upper postcanines as in A but asymmetrical lower postcanines, some of them similar to those in Megazostrodon. The different proportions of the preorbital and orbitotemporal region and the different structure of the lower postcanines recorded in sp. A and B support the distinction of both genera. Several derived characteristics of the skull in each of these genera and a morganucodontid-like lower dentition in nov. sp. B, suggest closer relationships to the morganucodontid mammals than for either tritheledontids or tritylodontids.