Earlier here and here we looked at Mycterosaurus (Fig. 1). This cat-sized taxon nests as a basalmost protodiapsid (along with Archaeovenator), not far from the basalmost synapsids, Aerosaurus, Varanops etc. and their last common ancestor, Protorothyris, a basal taxon which does not have a lateral temporal fenestra. Other basal protodiapsids include Milleropsis and Erpetonyx and these ultimately give rise to diapsids like Spinoaequalis and Petrolacosaurus in the large reptile tree. This new reconstruction (Fig. 1) is based on more precise data from Berman and Reisz (1982) than originally available from Williston (1915).
This subset of the large reptile tree (Fig. 2) shows relationships at the base of two large clades of reptiles, Synapsida and Diapsida (sans lepidosauriformes, which have convergenently developed a similar temporal morphology).
Mycterosaurus longiceps (Middle Permian, Williston 1915, Berman and Reisz 1982) nested with Heleosaurus as a basal protodiapsid. Botha-Brink and Modesto (2009) also correctly nested it with Mesenosaurus and Heleosaurus but considered those taxa varanopid synapsids unrelated to diapsids.
The tip of the snout is unknown in Mycterosaurus, but probably straight as in sister Heleosaurus. Berman and Reisz 1982 considered the AMNH 7002 specimen (above) another Mycterosaurus, but it has recurved canines more like those of Mesenosaurus, the maxilla is lower and the jugal had a different shape.
No DNA studies link mammals (synapsids) to birds and crocs (diapsids) yet embryological studies show that both develop of jugal with a quadratojugal process, something lizard and turtle embryos do not produce. I continue to be perplexed about DNA vs. morph studies. But I also continue to urge cladogram builders to include key taxa, like Mycterosaurus in studies on basal diapsids and vice versa.
Berman DS and Reisz RR 1982. Restudy of Mycterosaurus longiceps (Reptilia, Pelycosauria) from the Lower Permian of Texas. Annals of Carnegie Museum 51, 423–453.
Botha-Brink J and Modesto SP 2009. Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29(2):389-400.
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Willistion SW 1915. A New Genus and Species of American Theromorpha: Mycterosaurus longiceps. The Journal of Geology 23(6):554-559.