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Archosaur origins (was reply to Euparkeria)



> At 12:34 PM 5/4/97 -0600, LN Jeff wrote:
> 
> >    How controversial is Paul Sereno's placement of _Euparkeria_ at the
> >base of Archosauria, its dissasociation from the ornithosuchids, and the
> >placement of the ornithosuchids with (Gauthier's) pseudosuchians to form
> >Cruotarsi?  These last two seem pretty radical based on the little
> >research I've done, and I was curious what other people's take on it was.
> >Sereno 1991 is the most up to date thing on it I've read on "thecodonts"
> >(even _I_ don't mind using quotation marks for THAT paraphyletic group).
> 
> Euparkeria is pretty convincingly basal within Archosauriformes, and I don't
> think anyone would be terribly surprised with it as a basal ornithosuchian,
> a basal pseudosuchian, or an archosauriform outside of Archosauria proper.
> It's association with Ornithosuchidae was never based on particularly strong
> positive evidence.

Perhaps this is the right moment to put another question
about the same topic 

When I read the older literature about the origins of Archosauria proper, two 
different theories/points of view seemed to emerge.
First, there is the theory of Proterosuchia (is this still a valid 
monophyletic taxon?) came first: first archosaurs were semi-aquatic 
piscivores and near-shore lurkers; their powerful hind limbs gave a 
strong "back wheel driven" force to launch an attack in shallow 
water, their descendants invaded the real terrestrial biota and the 
same strong hindlimbs were very useful for bipedal gait (exaptation); 
the tendency to bipedalism in Archosauria stems from an aquatic 
environment.
Second, first archosaurs were agile terrestrial small carnivores such 
as the euparkeriids, which descended from tiny "eusuchians" (yes a 
ragbag name I know) as Robert Carroll's Heleosuchus and Heleosaurus, which 
could already atteign some degree of bipedalism (such as the extant Basilisc 
lizard); the large semi-aquatic Proterosuchia were an offshoot that secondarily
invaded the water.
Has the recent taxonomic work on archosaur origins clarified this 
issue?

Pieter Depuydt, M.D.
Department of Respiratory Diseases
University of Ghent
Belgium