Yesterday we talked about the Parareptilia, a so-called amniote clade invalidated by novel nestings recovered by the larger numbers of taxa within the large reptile tree. Today we’ll look at its opposite “branch,” the so-called Eureptilia, which is also invalidated (shown to be diphyletic) when tested against the large reptile tree.
Muller and Reisz (2006) included the following taxa in their classification of the Eureptilia: Coelostegus, Thuringothyris and the Captorhinindae, Broffia, Paleothyris, Hylonomus, Protorothyrididae and the Diapsida.
The large reptile tree tested these relationships using many additional taxa and
did not recover the same tree topology. Most of the taxa appear together in the new Archosauromorpha (which is a good thing), but Thuringothyris and the Captorhinidae do not (which makes the clade diphyletic, which invalidates the Eureptilia).
According to Wiki, the Eureptilia is defined by the skull having greatly reduced supraoccipital, tabular, and supratemporal bones that are no longer in contact with the
postorbital. Unfortunately these traits turn out to be primitive for reptiles as they occur in
Cephalerpeton, the most basal reptile. The expansion of these bones in diadectids and other reptiles converges with various pre-reptile amphibians, hence the confusion. There is still widespread belief (not based on a comprehensive analysis) that diadectids are not reptiles. This belief has to go away with further testing, following the lead of the large reptile tree.
There is no doubt that the taxa identified by Muller and Reisz (2006) are primitive,
but the captorhinids nest on a separate branch, the new Lepidosauromorpha, while
the others nest with and around the Synapsida, some preceding that clade. Others, like the Diapsida, are actually derived from basal members of the Synapsida.
Muller and Reisz (2006) would have found the same tree topology had they
expanded their taxon list to include most of the genera used by the large
reptile tree, including the use of the reptile-like amphibian, Gephyrostegus, as
an outgroup. Here’s hoping someone will pick up this scientific challenge.
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.
Muller J and Reisz RR 2006. The phylogeny of early eureptiles: Comparing
parsimony and Bayesian approaches in the investigation of a basal fossil clade.
Systematic Biology, 55(3):503-511.