A new study of Solenodonsaurus janenschi, and a reconsideration of amnioteorigins and stegocephalian evolution

A restudy of the holotype and other known specimens of Solenodonsaurus, a L ate Carboniferous relative of amniotes, suggests that this taxon did not ha ve a tympanum. Its teeth lack labyrinthine infolding, and the frontal did n ot reach the orbit. A new phylogenetic analysis of stegocephalians suggests that Solenodonsaurus is an anthracosaur, and that it is the sister group o f Cotylosauria. It also shows that the problematic taxon Westlothiana may n ot be an anthracosaur (not a stem-amniote). A recent suggestion that diadec tomorph cotylosaurs laid amniotic eggs is shown to be based on tenuous evid ence, although this remains a possibility. The evolution of the otic region is studied using parsimony to determine whether the emargination present i n Solenodonsaurus and in some diadectomorphs is a primitive or a secondary character, but this procedure yields equivocal results.