RE: PTEROSAURS: AVIAN ANCESTORS?

["Larry Febo" <larryf@capital.net>]

Date: Sun, 27 Sep 1998 16:25:37 PDT
From: "Matthew Troutman" <m_troutman@hotmail.com>
To: dinosaur@usc.edu, larryf@capital.net
Cc: m_troutman@hotmail.com
Subject: RE: PTEROSAURS: AVIAN ANCESTORS?
Message-ID: <19980927232539.12418.qmail@hotmail.com>


<<Yes, I can see in the anatomy of bats a good example of convergence.
Pterosaurs and birds however appear, to me, to be more than just
parallel evolution.>>

Why?

Pterosaur shoulder girdles are not similiar to bird shoulder girdles in
many details. For one, the pterosaur scapula is shorted and is fused to
the coracoid. The sterni of birds and pterosaurs are very different
too. Pterosaurs have a seperate element (the cristospine) immediately
anterior to the sternum that functions like the carina of the bird
sternum. However, it is very different in placement, shape, and nearly
everything else. Pterosaurs also have sternal ribs that are more
numerous than bird sternal ribs and lack the joints of bird sternal
ribs.

Reply:

I`m not sure if these details aren`t related to the more powerful
musculature needed by these pterosaurs to move their enormous wings without
collapsing the ribcage. These might be  the features expected in a more
advanced, and hence specialized form, not the more primitive, generalized
form that birds might have evolved directly from. Even Wellnhofer states how
even the earliest pterosaurs recorded seem already specialized to their
particular mode of flight , and a great gap existing in the fossil record as
to what exactly led up to the first pterosaurs,(as we know them). I guess
what I`m looking for  as a common link between aves, and pterosaurs is as
yet a hypothetical form. Still, I would think that what can be observed of
the similarities between the two forms, even in their advanced, specialized
states might suggest the existance of such a hypothetical ancestor. I know
that "hypothetical" arguments tend to be weak, and subject to flaw, but ,
again, in these areas of scant fossil evidence, what recourse is there? I
guess this will remain as another of my "hunches" until some solid fossil
evidence shows up to prove, or disprove my hypothesis. Thanks for the
"debate", it has given me much more to think over. And, I don`t want to get
into the "maniraptoran ancestry of birds," or vice versa, as I`m sure that`s
already been debated to the hilt!

By the way, you mentioned:


"Pterosaur wrists seem to been rather specilized. The elements do not
seem to be analogs to the cuneiform, scapholunar, and semilunates of
bird wrists. _Archaeopteryx_ has four carpal elements. Pterosaurs have
two or three. "

According to Carroll (pg. 333), ..."There are five or six carpals in
primitive pterosaurs, but these are fused into one proximal and one distal
element in later genera that form a simple hinge joint between the ulna and
radius at one end and the remainder of the limb at the other."

 So, I guess I can consider my original question as yet unanswered.