The AMNH specimen
of Cotylorhynchus is a spectacular sight to see (Fig. 1). It’s huge! And complete! It’s bigger than a Galápagos tortoise with a skull just as small.
Figure 1. Cotylorhynchus AMNH specimen. Note the angle of the ribs. Is that a whip-lash tail? Compare to Datheosaurus (Fig. 4). Just look at those massive elbows!
Romer and Price 1940
pictured Cotylorhynchus with vertical dorsal ribs (Fig. 2 lateral view).
Figure 2. Cotylorhynchus romeri
The Sam Noble Museum Oklahoma’s Museum of Natural History
in Norman, Oklahoma, USA, has a mount of Cotylorhynchus (Fig. 3) that follows the Romer and Price illustration with vertical ribs. Here, in this 2-frame GIF animation, I have angled them back to match the in situ specimen and most other quadrupedal tetrapods.
Figure 3. Cotylorhynchus mount in the Sam Noble Museum of Natural History with vertical ribs modified here to have diagonal ribs more typical of tetrapods and reflective of the in situ fossil.
Cotylorhynhcus romeri (Stovall 1937) Kungurian, Middle Permian, ~265 mya, ~6 m in length, was the largest sister to Casea and Ennatosaurus. It was the largest land animal of its time.
Figure 4. Milleretta, caseasaurs and kin. The LRT nests these taxa together apart from the Synapsida, with which they share a lateral temporal fenestra. Note the angle of the ribs in the Milleretta reconstruction, similar to the suggestion for Cotylorhynchus. Casea and Ennatosaurus continue to have invalid vertical ribs in the above figure due to my laziness.
All prior and other current reports
nest Cotylorhynchus with the synapsid pelycosaurs, but here in the large reptile tree (LRT. 1315 taxa) the caseid clade nests more parsimoniously with Milleretta, Feeserpeton and Australothyris and other plant-eaters, many of which share a lateral temporal fenestra in the new Lepidosauromorpha, opposite to the coeval pelycosaurs nesting in the new Archosauromorpha.
We looked at this traditional mistake
based on taxon exclusion here back in 2011. Even so, synapsid workers continue to follow this outdated tradition without testing validated alternatives proposed here.
References
Romer AS and Price LI 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Stovall JW 1937. Cotylorhynchus romeri, a new genus and species of pelycosaurian reptile from Oklahoma. Arnerican Journal of Science (5) 34: 308-313.
Stovall JW, Price LI and Romer AS 1966. The Postcranial Skeleton of the Giant Permian Pelycosaur Cotylorhynchus romeri. Bulletin of the Museum of Comparative Zoology 135 (1): 1-30. online pdf
The Pterosaur Heresies 