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