Is Glaucosaurus an Edaphosaurid?

Yes.
The  single, small, round, big-eyed, small-toothed skull of Glaucosaurus megalops (Fig. 1) was first described by Williston (1915) as most resembling the much larger Edaphosaurus. Broom (1932) linked it to Mycterosaurus from the same locality in Texas. Romer and Price (1940) followed Williston (1915). Reisz (1986) considered Glaucosaurus a synapsid of uncertain affinities, noting a lack of discernible sutures. Most recently, Modesto (1994) nested Glaucosaurus with Edaphosaurus. He noted “…many of the elements of the skull and roof are either absent or damaged…” and, despite the juvenile proportions, he wrote,“Glaucosaurus possesses a suite of autapomorphies which indicates that this form cannot be recognized as a juvenile of any other synapsid taxon.”

Ianthasaurus is a small pelycosaur close to Edaphosaurus, but is known from only post-cranial material, which is frustrating as Glaucosaurus is closely related, but the two taxa preserve different portions of their skeleton.

Glaucosaurus megalops

Figure 1. Left: Glaucosaurus megalops (FMNH UC 691) from Modesto (1994) alongside several  images of Edaphosaurus, Australothyris and Romeria primus for comparison. Despite resemblances to other taxa, Glaucosaurus nests with Edaphosaurus in the large reptile tree. What does the variety in the skull of Edaphosaurus tell us?

Glaucosaurus shares with Edaphosaurus a prefrontal ventral process transversely expanded,  similarly-sized teeth, a lack of canines and a lack of a pterygoid transverse process. In the large reptile tree Glaucosaurus nests with Edaphosaurus, but its worthwhile to also check it against Haptodus.

So, yes, Glaucosaurus does nest with Edaphosaurus in the large reptile tree. And it is a late survivor of an earlier radiation. Interestingly, the addition of this taxon shifted Milleropsis to the base of the Heleosaurus branch, but yesterday the additions of the two Varanosaurus specimens moved it back. It’s only a step away. That’s not unexpected as certain data from this clade is largely crushed and based on various low-rez drawings, rather than hi-rez tracings of the original materials.

In any case, it’s exciting learning about the various paths evolution took and the many, many dead ends that had to happen to bring us today’s living synapsids and diapsids.

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.

References
Modesto SP 1994. The Lower Permian Synapsid Glaucosaurus from Texas. Palaeontology 37:51-60/

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