Re: Bird evolution (long)

[skeptix@cyberwarped.com (Leon Retief)]

I wrote about bird evolution :

>>Martin feels that instead of being descendants of dinosaurs, birds
>>should rather be regarded as cousins, descending directly from
>>dinosaur ancestors called thecodonts.
>>In fact, Martin convincingly points out a number of apparent
>>fallacies in the arguments advanced by the proponents of the
>>dinosaur-bird connection, such as a seeming misunderstanding of
>>some anatomical aspects given much weight by Padian and Chiappe.


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..."

"Another dufficulty 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...
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. Even more implausably, dinosaur evolution would have stood still
for
the subsequent 145 million years of its known fossil record."

"I began to grow disenchanted with the brid-dinosaur link when I
compared
the eighty-five or so anatomical features seriously proposed as being
shared
by birds and dinosaurs. To my shock,virtually none of of the
comparisons
held up. For example, the characteristic upward-projecting bone on the
inner
ankle in dinosaurs lies on the outer ankle in birds. In some cases I
even
discovered that the supposedly shared features occured on entirely
different
bones. This is a bit like saying that you and i are related because my
nose
resembles your big toe."

"The confusion over anatomy stems at least in part from spotty
ornithological literature. Although many ornithologists study the
songs,
brilliant plumage and behaviour of birds, few choose to study the
smelly
bones and muscles. By the same token, dinosaurs specialists who
advocate a
bird-dinosaur link have been largely content to leave avian anatomy to
the
ornithologists."

Martin then goes on a bit about how such a situation led to the some
paleontologists discovering that everything is not the way it was
thought to
be, how band-aid (paleontologically speaking) has had to be applied
and
dmage control then being a way out : the inconsistencies may be
ignored or
the intrepretation of the anatomy may be changed to suit the
hypothesis.
Cladism comes in for a certain amount of (not very ehavy, IMO)
criticism,
since it may lead researchers astray by noting too many features.

"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."

""...Ostrom says Deinonychus (which he regards as a bird ancestor -
LW) has
a wrist 'almost identical' to Archaeopteryx. But that statement cannot
be
true. Deinonychus has only two bones in the wrist, 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. One
row of bones is connected to the arm, the other to the hand."

"Ostrom saw only half as many bones in the wrist of Deinonychus. He
reported
that both of them, including the half-moon bone, lay in the row
connected to
the arm. (The row connected to the hand, he believed, did not exist.)
Hence
by Ostrom's own logic the half-moon bone in Deinonychus occurs in a
different part of the wrist from where it occurs in birds. Therefore,
although the half-moon bones in the wrists of birds and dinosaurs look
alike, they develop from completely different wrist bones during the
animal's growth. Birds and dinosaurs must have developed the trait
seperately. The half-moon bone should have been dismissed immediately
from
consideration as a homologous bone, a shared characteristic. instead,
it has
continued to be cited for decades."

Hope this is sufficient.

Regards

Leon Retief
Bellville
South Africa