At the synapsid/diapsid divide

All nesting data for this post comes from the large reptile tree which primarily separates taxa closer to lepidosaurs from taxa closer to archosaurs all the way back to about the time that amniotes first arose. So diapsids, as traditionally thought, are not monophyletic. We’re not going to talk about the lepidosaur branch today. Only diapsids like Petrolacosaurus that shortly thereafter gave rise to enaliosaurs and protorosaurs, including archosaurs. Such diapsids had only to develop an upper temporal fenestra as proto-diapsids already had a lateral temporal fenestra inherited from their very basal synapsid kin.

Earlier we looked at the taxa at this node. Paleontologists preceding or unfamiliar with the large reptile tree consider these proto-diapsids as the varanopid branch of the Synapsida because they have excluded pertinent taxa. Today a short note on the precursor, Protorothyris known from the Early Permian.

Figure 1. Three specimens attributed to Protorothyris and other more derived taxa among the basal synapsids and basal proto-diapsids.

Figure 1. Three specimens attributed to Protorothyris and other more derived taxa among the basal synapsids and basal proto-diapsids. Note the great resemblance between the skulls of Orovenator and Archaeothyris.

We know of at least three types, traditionally considered smaller form, laterally crushed, P. morani (CM 8617) and the larger types P. archeri (Fig. 1) one with a flatter skull, some of that via dorsal crushing (MCZ 1532) and one with a taller skull, some of that due to lateral crushing (MCZ 2149).

Clark and Carroll 1973 provided the base illustrations for the above presentation, modified only slightly to remove crushing. They also provided a composite sketch of all three specimens to show what this taxon looked like. Nesting as it does, so close to the synapsid/diapsid split, I had to see if the split went any deeper, into these three specimens. It did not. Echinerpeton still nests closer to the split.

Even with the various distortions, there are some interesting variations here. The jugal has a distinct shape in each one. So does the squamosal. The ventral maxilla has a deeper convex curve on the MCZ 2149 specimen and it has two canines. Or is that a replacement tooth coming in?

References
Clark J and Carroll RL 1973. Romeriid Reptiles from the Lower Permian. Bulletin of the Museum of Comparative Zoology 144 (5): 353-408.\
Price LI 1937. Two new Cotylosaurs from the Permian of Texas: Proceedings of the New England Zoological Club, v. 16, p. 97-102.

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