Abstract

Stereospondyls were a very successful radiation of early tetrapods. The key to their success is not resolved but could lie in their diversity of feeding strategies, reducing interspecies competition. The toothrow and subtemporal vacuities of this group, representing functional morphology of feeding, were assessed for allometry and morphospace occupation. Toothrow length affects prey holding ability, and the subtemporal vacuity is the attachment site for the muscle responsible for jaw closure, which influences ability to close the jaw and to keep it closed. Relationships between skull length, as a measure of body size, and either toothrow length or subtemporal vacuity area were investigated using phylogenetically controlled regression and principal component analysis of size-controlled semilandmarks. Taxa studied represented a range of families and temporal epochs. Linear models were tested for Brownian evolution. Semilandmarks around the outline of the subtemporal vacuity assessed shape differences and morphospace occupation. There was a strong, significant isometric relationship between toothrow length and skull length but there was no evidence of Brownian evolution. From this and the power function, we can accurately predict the toothrow length of stereospondyls, which is 68% of skull length. There was positive allometry in subtemporal vacuity area and skull length, with Brownian evolution, and larger species had proportionally larger vacuities. There was a lot of variation in morphospace occupation, differing over geological time but not by geographic location. The Permian stereospondyls had significantly narrower and longer subtemporal vacuities than the Triassic. Late Triassic taxa possessed wider and posteriorly extended vacuities compared to early Triassic species. We conclude that toothrow length was conserved in stereospondyls, but that jaw-closing action and strength varied considerably.