Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals.

Ictidosaurian genera are allocated to two families, Tritheledontidae and Therioherpetidae. This paper provides a diagnosis for Ictidosauria. The previously named family Brasilodontidae is shown to be a junior synonym of a family, Therioherpetidae. It is concluded that Ictidosauria originated from Late Permian procynosuchid non-mammalian cynodonts rather than from Middle Triassic probainognathid non-mammalian cynodonts. The structure of the skull and jaws of a derived traversodontid Ischignathus sudamericanus from the early Late Triassic of Argentina supports an earlier view that tritylodontids are more closely related to traversodontid than probainognathid non-mammalian cynodonts. Tritylodontids should not be included in Ictidosauria, nor should they considered to be a sister group to mammaliaforms. [ABSTRACT FROM AUTHOR]