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Re: Thecodontia defined and saurischian Marasuchus



David Marjanovic wrote:

Thecodontia (Protorosaurus + Thecodontosaurus)

IMHO the idea of defining Thecodontia is at least as wrongheaded as that of defining Reptilia. Thecodontia is _meant_ to be paraphyletic! Even more so than Reptilia (which is older than 1859)!

Like David, I have grave reservations about resurrecting Thecodontia as a clade. A great deal of effort has been expended (by Benton, Parrish, and others) to demonstrate that the Thecodontia of traditional usage is meaningless, and therefore the term should be removed from phylogenetic usage.


When Owen conceived the term he did so within a typological framework. Since then, the group Thecodontia was used as a receptacle for the "primitive" grade of archosaur, including the putative ancestors of dinosaurs (multiple origins), birds (origin separate from dinosaurs), crocodiles and pterosaurs. But the concept of a paraphyletic Thecodontia really only came about with the rise of cladistics as the generally accepted framework for classification.

Rhynchosaurs were (of all things...) rhynchocephalians, protorosaurs were first lizards and then "eosuchians", and I guess trilophosaurs were "eosuchians", too...

Eosuchia has also been redefined, such that it is now a clade. Laurin (1991) defined Eosuchia as all descendants of the most recent common ancestor of _Coelurosauravus_, _Apsisaurus_, Younginiformes, and Sauria (sensu Gauthier et al., 1988; includes archosaurs and lepidosaurs). So technically, dinosaurs and crocodiles are eosuchians as well as (now) thecodonts.




Tim