Adding a few
and distinct Diadectes specimens (no two appear to be conspecific) opens the door to new insights into that corner of the cladogram. Some of the data are from 3D skull images with sutures delineated. Others are from firsthand observation. Some data are from drawings. Berman et al. 1992 made an interesting observation that prior authors illustrated the skull roof of Diadectes in a variety of ways (Fig. 1). The caption does not indicate that all were drawn from the same specimen. I suspect they were not.
The Berman et al. phylogenetic analysis
included seven taxa, including two suprageneric taxa, Pelycosauria and Captorhinomorpha. They included only nine characters. The anamniote, Seymouria, was the outgroup. The first clade included Pelycosauria + (Limnoscelis +(Tseajaia and Diadectes). The second clade included Captorhinomorpha + Petrolacosaurus. The large reptile tree includes hundreds more taxa and characters. The pertinent subset is shown here (Fig. 2). It is also clear from the Berman et al. taxon set that they thought they were dealing with a small set of basal reptiles and pre-reptiles. In 2015 it is clear that they did not include the pertinent taxa they should have as some of these taxa are not related to any of the others except distantly.
Now, with current data
it is becoming increasingly clear that both bolosaurids and procolophonids nest within a fully reptilian Diadectes clade. It is also clear that the genus Diadectes needs to be further split, as Kissel (2010) started to do by renaming Silvadectes and Oradectes from former Diadectes species.
the placement of Stephanospondylus as a proximal sister taxon to the diadectids nesting at the base of the pareiasaurs (including turtles). Turtles are sisters to pareiasaurs and they ARE pareiasaurs because they are derived from pareiasaurs, just as birds are derived from theropod dinosaurs.
Like everyone who studies prehistoric reptiles
there is a day when you don’t know anything about a taxon and later there is a day when you are making contributions to Science. Those days keep on coming.
Berman DS, Sumida SS and Lombard E 1992. Reinterpretation of the Temporal and Occipital Regions in Diadectes and the Relationships of Diadectomorphs. Journal of Vertebrate Paleontology 66(3):481-499.
Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Thesis (Graduate Department of Ecology & Evolutionary Biology University of Toronto).