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Re: Thecodonts?



 From: jpoling@magnus.acs.ohio-state.edu (Jeff Poling)
 > 
 >    Are you sure the crocs branched off from the thecodonts?  My dino class
 > professor mentioned that the term "thecodont" was falling into disfavor,
 > with "protodinosaur" taking its place. 

The term thecodont refers to a paraphyletic group, which strict
cladists refuse to accept as valid taxa.  This is why it is
falling out of favor.  There really is no adequate replacement
term.  One either uses it or one talks about "stem archosaurs".
[I would take "protodinosaur" to refer to stem archosaurs that
are closer to dinosaurs than to other derived archosaur groups,
that is stem ornithodires].

So, in a sense it *is* true that crocodilians branched off from
thecodonts.  That is, dinosaurs and crocs had entirely seperate
origins within the stem archosaurs.  In fact they came from
two quite divergent lineages.  In particular crocs are related
to the stem archosaur group called the pseudosuchians, while
dinosaurs are more closely related to the lagosuchian group.

swf@elsegundoca.attgis.com              sarima@netcom.com

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