Diadectes is not an Amphibian. And Procolophon is a diadectid.

Tradition and Wikipedia reports that “Diadectes is an extinct genus of large, very reptile-like amphibians.” This is an outdated hypothesis that has to go. Wiki further reports, “Diadectes combines a reptile-like skeleton with a more primitive,seymouriamorph-like skull.”

Earlier we looked at Diadectes, noting that it nests deep inside the plant-eating side of the Reptilia, the new Lepidosauromorpha.

Let’s take a look at that skull again. 
Wiki reports, “Among its primitive features, Diadectes has a large otic notch (a feature found in all labyrinthodonts, but not in reptiles) with an ossified tympanum.” Other reptiles with a large otic notch include several close relatives of Diadectes, including a sister taxon, Procolophon (Fig. 1). The resemblance is not just superficial, yet Wiki reports, “Procolophon was a genus of lizard-like procolophonid reptiles.” Why was Procolophon considered a reptile and Diadectes an amphibian? It can’t be the notch. It’s the same on both. Procolophon was simply smaller diadectid that lived later in time. Take a look a the various Diadectes skulls  on the Wiki page and you’ll see that the otic notch is bigger on some, smaller on others. It’s not homologous with the similar structure in amphibians. Seymouria retains an intertemporal bone and has palatal fangs. Diadectes does not.

Figure 1. In the large reptile tree Procolophon nests with Diadectes, and both share a large otic notch, a trait Wiki says makes Diadectes an amphibian.

The Otic Notch
The otic notch simply redeveloped by convergence in diadectids and procolphonids, yet one got labeled a reptile and one an amphibian. I don’t know why. Both Procolophon (Owen 1876) and Diadectes (Cope 1878a, b) were first described long ago. Perhaps this is some sort of tradition from a time when we didn’t know very much about prehistory. If anyone has original literature, I’d like to see it.

Ancestors in the Large Reptile Tree
Orobates has a small otic notch and nests primitive to the diadectids and procolophonids. Tseajaia, Solenodonsaurus and the chroniosuchids all have an otic notch and all are considered by Wiki to be amphibians, but here nest in the Reptilia. All these taxa and the diadectids + procolphonids have Concordia in their pedigree. It has virtually no otic notch, but you can see how it could have begun here. Funny thing is, Wiki reports, Concordia is close to the origin of the captorhinid reptiles, and it is too in the large reptile tree. These sorts of problems emphasize the importance of adding lots of taxa to basal reptile studies. The more you add, the more Diadectes nests with Procolophon deep inside the Reptilia.

Herbivorous
Wiki reports, “Diadectes was one of the very first herbivorous tetrapods.” One look at the chronological reptile tree indicates that two other herbivores, Cephalerpeton and Concordia preceded Diadectes chronologically. So do Orobates and Stephanospondylus. Captorhinids are likewise herbivores, but they are found in younger rocks despite their more primitive nesting.

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
Cope ED 1878a. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proceedings of the American Philosophical Society 17:505-530.
Cope ED 1878b. A new Diadectes. The American Naturalist 12:565.
Owen R 1876. Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum. London, British Museum (Natural History).