ANGIELCZYK, K.D.; University of California, Berkeley: Unexpected Homoplasy in the Anomodont (Synapsida; Therapsida) Feeding System

Anomodont therapsids were the most abundant and diverse group of terrestrial vertebrate herbivores during the Late Permian and much of the Triassic. The success of anomodonts has been hypothesized to be related to their complex mastication system, which was based on a propalinal sliding of the lower jaw. Recent phylogenetic work has provided new insight into the relationships of the most basal anomodonts as well as the evolution of their distinctive feeding system. Specializations related to propaliny occur in two clades, the endemic Russian Venyukovioidea and the unnamed clade including Galeops and the Dicynodontia. These specializations have been interpreted as being present in the common ancestor of both clades, implying a single origin of propaliny in Anomodontia. However, optimization of recoded characters related to propaliny on recent cladograms shows that a dual origin is equally parsimonious in several cases. Furthermore, despite sharing characters related to propaliny, the two clades show divergent specializations of the feeding system. Mastication in Venyukovioids involved extensive occlusion of robust teeth, whereas the Galeops + Dicynodontia clade reduced or lost the teeth completely and replaced them with a keratinous beak. Given that the power stroke of mastication in basal synapsids involved the application of a posterodorsally-directed force on the lower jaw, propaliny may not have been difficult to evolve and other non-mammalian synapsid clades (e.g., Edaphosauria, Dinocephalia) also include members in which some sliding of the jaw was possible. Finally, the notably different longevity and diversity of the two anomodont clades suggests that propaliny alone was not enough to guarantee success in Late Permian and Triassic ecosystems.