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Re: Quo vadis, T. rex?

On Thu, 1 Feb 1996 Dinogeorge@aol.com wrote:

> (1) The phalangeal formula of _C._ AS PRESERVED is 2,2,0,0,0 (only two
> digits: not 2,3,3,0,0), 

Sorry.  I spaced out there for a sec.  That's what I meant to say.  Really.

> (2) Didactyly is not at all easily developed in parallel. Practically all
> theropods had three or four manual digits, even those with absurdly reduced
> forelimbs, such as _Carnotaurus_. _Only_ tyrannosaurians had didactyl
> forelimbs (and I include _Compsognathus_ as the earliest, most primitive
> known member of that suborder). You should decouple the ideas of "forelimb
> reduction" and "digit-count lowering" from each other; they do not go "hand
> in hand" (arghh--no pun intended).

I didn't mean to suggest that they did.  I just don't believe it takes 
that much to lose a finger.

> The progressive reduction in manual digit count seen in theropods is the
> result of some pretty painstaking evolution of the wing (BCF theory here;
> ignore at your peril). Digits do not just drop away willy-nilly without a
> struggle.

Actually, I'll agree that it is quite possible that tyrannosaurs evolved 
from advanced birds (enantiornithians of some sort, probably) that had 
lost most of digit III as modern birds have (and that the arctomet foot 
evolved separately in tyrannosaurs and bullatosaurs from the ancestral 
avian [true birds, that is, not theropods in general] foot design, in 
which metatarsals II and IV contact each other over the face of met III, 
but met III is not "pinched").

What I do not agree with is the linking of tyrannosaurs with the much 
more primitive (ornitholestid-grade) _Compsognathus_ purely on the 
occurrence of the didactyl condition in these two taxa (or that the 
arctomet foot evolved in tyrannosaurs from scratch, i.e. from a 
completely uncompressed foot like that of _Compsognathus_).

Actually, I'm not being quite fair to little Compy.  If it could be shown 
that mets II and IV touch each other in this animal, it could make a 
suitable tyrannosaur ancestor.  The original specimen of _Compsognathus_ 
is not preserved well enough to convince me entirely that the phalangeal 
count is correct.  Is the French specimen any better?

Another help would be if someone could figure out how nerve V1 exits 
the braincase in _Compsognathus_.  If it exits out the front, then Compy 
is a bird and a possibly a suitable tyrannosaur ancestor.  If not, then 
it is neither.

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447

"If you can't convince them, confuse them." -- Harry S. Truman