Avesuchia and Avemetatarsalia: Avoid these taxa!

Earlier we looked at another widely used and accepted taxon, the Ornithodira (dinosaurs + pterosaurs and sometimes Scleromochlus), that turns out to be diphyletic according to the large reptile tree, or at least redundant with Amniota (= Reptilia) and therefore should be avoided. Today we’ll look at two taxa proposed by Benton (1999) in his study of Scleromochlus, in which he and I tilt the unseen quadrate different ways.

Benton (1999) defined Avemetatarsalia as all “avesuchians/crown-group archosaurs” closer to Dinosauria than to Crocodylia. After review in the large reptile tree that definition is now redundant with Dinosauria since no taxon is closer to Dinosauria than Crocodylomorpha. Avemetatarsalia was meant to include Scleromochlus + pterosaurs + dinosauromorphs, but here that clade is paraphyletic (or redundant with Reptilia) because pterosaurs are on the new lepidosauromorph branch while dinosaurs and Scleromochlus are on the new archosauromorph branch These branches divided during the earliest Carboniferous at its very base. If pterosaurs were removed from this definition, the resulting inclusion set would be redundant with Archosauria (crocs + dinos).

Benton (1999) defined “Avesuchia/crown-group Archosauria” as the taxon comprising “Avemetatarsalia” and “Crurotarsi” (and sister taxa of “Crurotarsi” that are closer to Crocodylia than to Aves), and all their descendants. Because the definition included parasuchians, pterosaurs and Lagerpetonhere this creates a paraphyletic clade redundant with Reptilia for the same reasons listed above. If pterosaurs were removed from this definition, the resulting inclusion set would be redundant with Archosauriformes.

I realize this is heretical thinking, but it reflects recovered results after testing the hypothesis, which is what Science is all about. Wish others would test this hypothesis with equal rigor.

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.

Benton MJ 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London B 354: 1423-1446.