The Case for Cutting Caseasauria Out of the Synapsida

Everyone, from Romer and Price (1940) to Reisz (1986) to Reisz et al. (2011), have reported that the base of the Synapsida was composed of two clades, the Caseasauria and the Eupelycosauria. But expand the taxon list and casesaurs nest elsewhere: with the milleretids and bolosaurs. So what’s the story?

The case for taking Caseasauria out of the Synapsida.

Figure 1. Which of these skulls does NOT belong with the others. New evidence puts caseasaurids with milleretids and bolosaurids.

Cotylorhynchus is famous for being the “fattest” and “slowest” reptile. But was it really a synapsid, as all the experts and past traditions report? Or was it something else?

Cotylorhynchus romeri

Figure 2. Cotylorhynchus romeri, perhaps not a synapsid

Casea was a smaller phylogenetic predecessor, not quite as extreme in size or dimension. Williston (1916) understood that it was not like other synapsids.


Figure 3. Casea, likewise not a synapsid

Oedaleops and Eothryis Are Widely Considered Basal Caseasaurs
Now Oedaleops and Eothyris join Casea to nest with Milleretta, Bolosaurus, BelebeyAcleistorhinus and Eunotosaurus in a previous unnamed and unrecognized clade (Figure 4). Romeria primus and Concordia are outgroup taxa. Concordia appears to have more in common with the eothyrids. This unnamed clade is known chiefly from skulls, but Milleretta, Casea and Eunotosaurus preserve post-crania.

Let’s look at a list of characters from Reisz (1986):
1. Lateral temporal fenestra: also present in cousin taxa Eunotosaurus, Acleistorhinus, Bolosaurus and Belebey. 2. Broad anteriorly tilted occiput: also present in Milleretta, Bolosaurus, Belebey, Acleistorhinus and Eunotosaurus at angles greater than in Oedaleops. 3. Reduced posttemporal fenestra borderd by opisthotic, supraoccipital and tabular: also present in Romeria primus, Milleretta and their descendants. 4. Single postparietal bone – it is not single in eothyrids. 5. Jugal sutured to pterygoid – not in Caseasauria, only in Eupelycosauria. 6. Ectopterygoid present – fairly widespread within the Reptilia. 7.  Septomaxilla comprised of a broad base and a massive dorsal process – as in Romeria primus and its descendants.

Caseasauria, Milleretta and sister taxa

Figure 3. Click to enlarge. Caseasauria, Milleretta and sister taxa

A Tree with 238 Taxa Nests Caseasauria Far from the Synapsida
The diphyletic reptile tree (see below) has recovered many mistakes in the prior nesting of various reptiles simply by expanding the taxon list. Among the past errors is the mistaken nesting of eothyrids and caseids with eupelycosaurs due to a short list of characters, now shown to be shared by convergence (see above). This single tree has more parsimony when Oedaleops, Eothyris and Casea are nested with Millereta, Bolosaurus, Belebey, Acleistorhinus and Eunotosaurus, most of which also had a lateral temporal fenestra and other purported synapsid characters by convergence.

Speaking of Purported Synapsids That No Longer Nest with Synapsids
Tetraceratops now nests with Tseajaia and Limnoscelis. Placing Tseajaia in the matrix of Amson and Laurin (2011) nested it with Tetraceratops as a basal synapsid. Removing Tetraceratops left Tseajaia at the same leaf and node above Haptodus, not as an outgroup. This experiment demonstrated the error of adding Tetraceratops to the Synapsida without first employing a larger umbrella study of reptile relationships.

The Reptile Tree

Figure 5. Click to enlarge. The new diphyletic reptile tree. Note the position of Eothyris, Oedaleops and Casea far from Dimetrodon, Varanops and other synapsids.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Amson E and Laurin M 2011. On the affinities of Tetraceratops insignis, an Early Permian synapsid. Acta Palaeontologica Polonica 56(2):301-312. online pdf
Reisz RR 1986.
Pelycosauria. Handbuch der Paläoherpetologie / Encyclopedia of Paleoherpetology 17A:1-102.
Reisz RR, Maddin HC, Fröbisch J, and Falconet J 2011. A new large caseid (Synapsida, Caseasauria) from the Permian of Rodez (France), including a reappraisal of “Casea” rutena Sigogneau-Russell & Russell, 1974. Geodiversitas, 33(2): 227-246. doi: 10.5252/g2011n2a2.
Romer, AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Williston SW 1912. Primitive Reptiles – A Review. Journal of Morphology 23(4):637-666.

tolweb: Synapsid Classification


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