How close are Caseasaurs and Diadectids?

Pretty darn close it appears.

And these two clades have NEVER been closely associated before. This is the sort of wonderful nesting you get when you just let it happen with a large gamut of reptiles.

Of course this goes against the grain of traditional paleontology that puts caseasaurs like Eothyris, Oedaleops, Casea, Cotylorhynchus and Ennatosaurus improbably and awkwardly alongside synapsids, like Varanodon and Ophiacodon.

Tradition also puts diadectids like Orobates and Diadectes outside the Reptilia alongside other amphibians. The large reptile tree solves all such problems. The two clades, Diadectidae and Caseasauria, are close kin as it turns out. And the skull images bear this out, if you just ignore the lateral temporal fenestra, which is a trait that comes and goes with the Millerettidae, of which the caseasaurs are members.

Figure 1. The diadectid, Orobates, alonside the casesaurs, Oedaleops, Ennatosaurus and Casea.

Figure 1. The diadectid, Orobates, alonside the casesaurs, Oedaleops, Ennatosaurus and Casea. Boy they sure do look alike, overall and in several details save the lateral temporal fenestra. That used to be a big deal that falsely segregated taxa. Now it’s just another character trait. Part of the mix.

I hate it when a blind eye is turned to toward such relationships. Tradition trumps testing in most cases. That’s why I’m here… to encourage young free thinkers to test everything in the Reptilia to see if it matches tradition or the large reptile tree.

Both clades find a common ancestor close to Romeria primus and Concordia, two taxa known chiefly from skulls close to the the base of the new Lepidosauromorpha, Cephalerpeton and Captorhinidae.

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.

3 thoughts on “How close are Caseasaurs and Diadectids?

  1. Suspicions are good. Try to find a competing analysis that includes the same taxa. It’s not just the teeth, by the way. It’s every character from nose to tail. According to the large reptile tree, that keeps growing without changing topology, these two clades had a common ancestor.

  2. Looking at the literature a bit, this isn’t that heterodox. While Benson (2012) found varanopids+ophiacodontids to be the most basal synapsids, it only took two more steps to make caseasaurs the most basal.

    Berman et al. (1992) and Berman (2000) both suggested diadectomorphs were the sister group of synapsids as opposed to the sister group of amniotes. Kemp (2003) analysed their evidence, concluding-
    – A posterolateral corner of the skull table formed entirely or nearly entirely by the supratemporal is only found is Tseajaia, which I note would make it ambiguous synapomorphy is diadectomorphs and synapsids were sister taxa.
    – A long posterior expansion of postorbital that contacts supratemporal to exclude the parietal lappet from contacting the squamosal is primitively present in cotylosaurs, including basal sauropsids.
    – Presence of an otic trough of the opisthotic is shared, but not by varanopids, which I note isn’t a problem if caseasaurs are basal.
    – A deep, nonsculptured component of the tabular which contacts the distal end of a ventrally displaced, laterally directed paroccipital process, enclosing laterally a small, ventrally displaced, posttemporal fenestra is absent in Limnoscelis and Desmatodon, so is more likely convergent in synapsids and Diadectes.

    Laurin and Reisz (1995) list several characters of Amniota. Diadectomorphs lack-
    – Frontal contacting orbit.
    – Occipital condyle almost as high as it is broad.
    – Labyrinthodont infolding of enamel absent (Kemp, 2003).
    – Axial centrum tilted anterodorsally.
    – Cleithrum restricted to anterior edge of scapulocoracoid.
    – Presence of three scapulocoracoid ossifications.
    More problematic are-
    – Occipital flange of squamosal gently convex. Even if I understood it, they say diadectomorphs’ condition may be an autapomorphy.
    – Transverse flange bearing a row of large teeth on its posterior edge. Also in Limnoscelis, so could work with synapsid diadectomorphs.
    – Presence of astragalus. They followed Rieppel’s (1993) model, but O’Keefe et al. (2006) showed amniotes have the same astragalar homology as Diadectes.

    Then there are the 13 synapsid synapomorphies in Benson (2012), which I won’t list as me doing so for ophiacodonts didn’t get a response from you past “I haven’t looked at most of these characters, and it’s someone else’s job to add them to my matrix to test things”. Not to mention diadectomorphs aren’t even sister to caseasaurs in your tree. You have derived diadectids by procolophonids, basal diadectomorphs strewn with Solenodonsaurus, Tetraceratops and chroniosuchians, and caseasaurs sister to various taxa usually placed in Parareptilia, and these three clades are successively less closely related to lepidosaurs.

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