Phylogenetic evidence for and implications of a dual origin of propaliny in anomodont therapsids (Synapsida)

Anomodont therapsids were the most diverse and abundant group of terrestrial vertebrate herbivores during the Late Permian and much of the Triassic. Their success has been widely attributed to a complex mastication system that was based on a propalinal sliding of the lower jaw. Traditionally, anomodont phylogeny has been viewed as an essentially linear sequence of taxa, each with more specializations for propaliny, culminating in the radiation of the dicynodont anomodonts. However, recent phylogenetic work has shown that similar specializations for propaliny can be found in members of two distinct anomodont clades, the endemic Russian Venyukovioidea and the unnamed clade consisting of Galeops + Dicynodontia. These specializations have been inferred to be present in the common ancestor of the venyukovioids and dicynodonts, implying a single origin of propaliny in anomodonts. Here a reevaluation of the anomodont feeding system is presented, and the possibility that propaliny evolved twice within th...