Re: Archosaur Origins...was:MESENOSAURUS ERRATA.

[Dinogeorge@aol.com]

In a message dated 8/24/01 6:43:26 AM EST, larryf@capital.net writes:

<< Perhaps I should have been more careful in definining my "terms". I 
suppose most people think of birds as the modern form. I always assume they 
include the stem group stretching back (I believe) into the Triassic, and 
would be defined as a true flying form possessing flight feathers (of some 
kind).>>

We're trying to get away from typological definitions of taxa, so it's not a 
good idea to define birds as "true flying forms possessing flight feathers." 
After all, there are many birds that don't fly and do not possess flight 
feathers, and who knows what they might have done or what kinds of feathers 
they might have possessed in the Middle Jurassic. Rather, we're trying to 
define taxa in terms of their phyletic relationships. So when I describe 
birds as "all animals more closely related to modern birds than to modern 
crocodiles," this is actually a very precise (and highly inclusive) 
definition. It's also the most natural group in which to include the extinct 
as well as extant birds (and among extinct birds are all the dinosaurs).
 
<< At any rate, I was curious as to how close to pterosaurs you think these 
Triassic forms might have been? Assuming that  both evolved from 
prolacertiforms, and within a fairly narrow time frame of the late Triassic 
(if you count Protoavis as an example), I would think that they might have 
had a common prolacertiform ancestor. Do you agree?>>

Perhaps earlier than Triassic: Permian is not outside the realm of 
possibility, and indeed, the Permian glaciation and generally cool climate 
may have fostered the evolution of endothermy and an insulatory integument in 
some of the earliest prolacertiforms.

Dave Peters has suggested taking a look specifically at Prolacerta as a 
possible prolacertiform archosaur ancestor. I haven't had a chance to do so, 
but the idea is interesting. Prolacertiforms were outside my paleo field of 
interest for many years and I don't yet know very much about them.
 
<< And for that matter, if "birds" did get their start from the 
prolacertiform group, and theropod dinos were their flightless descendants, 
dosen`t this imply that the other dino groups (Sauropod and Ornithischian) 
also had prolacertiform ancestry??? >>

Yes, exactly. The common ancestor of Dinosauria (which is a crown subclade of 
Aves, or birds, according to the definition of birds above) would have had 
some kind of prolacertiform ancestor somewhere along the lineage leading from 
prolacertiforms to birds--assuming that prolacertiforms were indeed the 
ancestors of the archosaurs (which would make Archosauria a subclade of 
Prolacertiformes), of course.

I cannot now exclude the possibility that the croc stem clade (let's call it 
Suchia rather than Crocodylia, and reserve the latter for the more familiar 
"true" crocs) descended from a different kind of prolacertiform from the bird 
clade. After all, there were lots of aquatic prolacertiforms as well as 
arboreal ones, and the proterosuchians, erythrosuchians, and other 
such-and-suchians seem to me more comfortably ensconced among the aquatic 
prolacerts. I really must read up on those prolacerts. There may be many 
connections there.