FROM MEAT TO BEET: PHYLOGENY AND EVOLUTION OF DIADECTOMORPHA (TETRAPODA: COTYLOSAURIA)

Based on dental, cranial, and postcranial anatomy, members of the Permo-Carboniferous clade Diadectidae are generally regarded as the earliest tetrapods capable of processing high-fiber plant material. Phylogenetic analyses support the monophyly of Diadectidae within Diadectomorpha, the sister-group to Amniota, with Limnoscelis paludis as the sister-taxon to Tseajaia campi + Diadectidae. Analysis of diadectid interrelationships of all known taxa for which adequate specimens and data are known—presented here—positions Ambedus pusillus as the sister-taxon to all other forms, with Diadectes sanmiguelensis, Orobates pabsti, Desmatodon hesperis, Diadectes absitus, and (Diadectes sideropelicus + Diadectes tenuitectes + Diasparactus zenos) representing progressively more derived taxa in a series of nested clades. The presence of heterodont dentition and molariform cheek teeth exhibiting substantial wear suggests that even the most primitive diadectids were herbivorous, and the phylogenetic hypothesis presented here details an evolutionary history leading to more efficient oral processing within the lineage, with successive nodes characterized by features increasingly indicative of a high-fiber diet. Within Diadectomorpha, diadectids constitute the majority of the species, suggesting that the advent of herbivory resulted in a relatively rapid radiation of species within the group, producing a clade that is markedly more species-rich than other, non-herbivorous diadectomorph taxa.