Re: Bird evolution (long)

[Stanley Friesen <sarima@ix.netcom.com>]

At 07:55 PM 4/3/98 GMT, Leon Retief wrote:
>To which Stanley replied ;
>
>>Hmm, could you elaborate on this?  What does he say?
>
>OK, here goes (in part) :
>
>"They (i.e. dinosaurs that most closely resembling birds) come 
>mainly from the very late Cretaceous period. They are much younger 
>fossils that those of Archaeopteryx. Because related animals 
>usually become less alike with the passage of time, one would 
>expect that the most birdlike dinosaurs should be even older than 
>Protoavis. But they are not. In fact, the resemblance between 
>dinosaurs and birds _increases_ with the passage of time. Such a 
>historical pattern is a hallmark of convergence..."
>
Well, this might be so if the fossil record of bird-like forms were
complete enough, and if it were certain that _Protoavis_ really was bird.

But, as it is, there is really no way to tell how "bird-like" the small
woodland theropods of the mid-Jurassic really were, as we have no fossils
of them one way or the other.

[And this leaves out the as-yet undescribed maniraptorans from the Morrison].

>"Another difficulty for Chatterjee's account is that it casts doubt 
>on a cladistic analysis that has been the chief prop for the
>dinosaurs-to-birds theory. That analysis was done in 1982 by the 
>paleontologist Kevin Padian... and was expanded in 1986 by Padian's 
>former student Jacques A. Gauthier...

Hmm, I haven't seen this so far.  But I have not yet finished reading
Chatterjee's book, so it might be in the upcoming chapter.

But, even if the original cladistic analysis is rejected, Chaterjee *does*
use cladistic results in constructing his model.  And, as in all areas of
science, cladistic results are always open to re-analysis.  Only if the new
analysis placed birds outside of the theropods would it actually weaken the
conclusion that birds are derived from dinosaurs.

>Padian and Gauthier's studies describe birds as the ultimate
>innovation of
>the dinosaurs. But if their view is correct, and if Protoavis turns
>out to
>be a bird, then every branch of the dinosaurian evolutionary tree
>would
>already have appeared by the late Triassic period, some 210 million
>years
>ago.

Yep, it would mean that.  But that is not the same thing as casting doubt
on the cladistic analysis!!!

>"A classic example of a cladistic trap is the trait that convinced
>Ostrom of his theory  : the half-moon bone in the wrists of 
>dinosaurs and birds.  A half-moon shape is a sign that the animal 
>rotates its hand extensively in a single plane. In birds the bone
>enables the flight feathers to be tucked against the body when the 
>wing is folded. In theropod dinosaurs the bone enables the hand
>to be tucked against the body while running. Both birds and 
>dinosaurs have fewer wrist bones than than do their more primitive
>relatives. Some of the wrist bones of the birds and dinosaurs, 
>along with part of the hand and two fingers, were lost during 
>evolutionary development.
>Thus it is crucial in the bird-dinosaur debate to make certain that
>the bones being compared are homologous, and that the observed
>similarities actually came from a common ancestor."

Yep.  And the bones in question almost certainly *are* homologous.
>
>""...Ostrom says Deinonychus (which he regards as a bird ancestor -
>LW) has a wrist 'almost identical' to Archaeopteryx.

Yep, and Ostrom is probably right.

> But that statement cannot be true. Deinonychus has only two bones 
>in the wrist,

Correction: only two wrist bones have been *found* in _D._.  This in no way
means they did not exist.  During fossilization skeletons often get
scattered or crushed, and the small, loose bones of the wrists do so the
easiest.  Very few dinosaurs (or any other fossil tetrapods) are ever found
with a complete wrist (or ankle).

The articular surfaces of the various bones that *were* found strongly
suggest the prior presence of other, now lost, bones.

> though its half-moon bone may be the result of a fusion with 
>another bone. Archaeopteryx resembles all other known birds in 
>having a total of four bones arranged in two rows.

AND, it is represented by some of the most complete fossils known for any
tetrapod.  I would put the better _A._ specimens in the top 10% in terms of
completeness.

>Hope this is sufficient.

Depends on what you mean:

Sufficient example? Yes

Sufficient refutation? No.

--------------
May the peace of God be with you.         sarima@ix.netcom.com
                                          sfriesen@netlock.com