Re: Thecodontia defined and saurischian Marasuchus

[Tim Williams <twilliams_alpha@hotmail.com>]

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