Reinterpretation of the temporal and occipital regions in Diadectes and the relationships of diadectomorphs

Abstract

New materials from the Permo-Pennsylvanian of north-central New Mexico permit a new description of the temporal and occipital regions of the diadectomorph Diadectes. The important issue of the fate of the intertemporal bone is resolved by demonstrating its absence and apparent incorporation into the parietal as a lateral lappet. Four cranial autapomorphies of Diadectes are recognized: 1) loss of contact between postparietal and tabular; 2) supratemporal greatly enlarged with well-developed occipital process; 3) tabular no longer exposed on skull roof, but greatly reduced and incorporated into occipital plate, with a coarse, posteromedially facing surface; and 4) skull roofing bones thick and porous, with a consistent network of U-shaped grooves. The temporal–occipital region of Diadectes is compared with those of holotypic and recently collected specimens of Limnoscelis and Tseajaia, the type genera of the other two recognized diadectomorph families, Limnoscelidae and Tseajaiidae. On the basis of the literature the comparisons are extended to include certain late Paleozoic amniotes: synapsid Pelycosauria, Captorhinomorpha, and the primitive diapsid Petrolacosaurus. The results are subjected to a cladistic analysis, which supports the following hypotheses of relationships: 1) Diadectidae, Tseajaiidae, and Limnoscelidae form a natural group, the Diadectomorpha; 2) Diadectes and Tseajaia share a more recent common ancestor than either does with Limnoscelis; 3) Diadectomorpha, Pelycosauria, and their descendants form an unnamed, primitive sister clade to that consisting of Captorhinomorpha, Petrolacosaurus, and their descendants; and 4) the taxon Cotylosauria (sensu Heaton, 1980), consisting of Diadectomorpha and Seymouriamorpha, is paraphyletic and invalid. The third hypothesis dictates the assignment of Diadectomorpha to Amniota.