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Re: Giant birds



In a message dated 11/9/99 4:48:47 PM EST, rjmeyer@ix.netcom.com writes:

<< Wait a minute.  Do you include all dinosaur groups, including 
ornithischians, under BCF (or are we really talking about BCF here)?  I 
thought that BCF referred to theropod evolution solely.  Or are we talking 
about something different.  Now, I understand that the first ornithischians 
are bipedal, and that a case could be made for a tree dwelling ancestor, but 
I'm not sure that makes them birds.>>

BCF explains the evolution of primitive, sprawling, quadrupedal archosaurs 
into birds. Sauropods and ornithischians belong somewhere on this 
archosaur-bird lineage. Since sauropods are very likely the most primitive 
dinosaurs (because only they retain all five functional hind foot digits), 
they probably branched off at the base of the dinosaur clade. All 
ornithischians lack a fifth hind-foot digit, which they could either have 
lost independently (if they branched off even before the sauropods) or 
inherited from an ancestral theropod or prosauropod. I don't see very many 
theropod features in early ornithischians, but I do see some prosauropod 
features in them, so I think it is likely that ornithischians descended from 
a prosauropodlike saurischian, one something like Thecodontosaurus or 
Azendohsaurus (the latter having originally been classified as an 
ornithischian--which is an interesting and significant fact).
 
<< I have thought that the radical difference in hip designs implied *two 
separate* instances of bipedal evolution (I envision several protodinosaur 
groups existing, where only the ornithischians and saurischians survived).  
Personally, I can't see a saurischian evolving an ornithischian hip, even 
with a truckload of generations.  There doesn't seem to be any advantage to 
the "new" hip design. >>

Saurischians did evolve ornithischian hips, when theropods evolved into 
birds; so it certainly could have happened earlier, when the dinosaur hips 
were much less specialized than theropod hips (fewer sacrals, shorter ilia, 
apronlike pubes, etc.). I tend to agree that there were several protodinosaur 
groups with only two surviving, but evidence for such groups is pretty much 
zero right now. This is probably because those other groups were small 
arboreal archosaurs that left no large, ground-dwelling descendants, and no 
fossil record because of preservational bias.