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RE: PTEROSAURS: AVIAN ANCESTORS
Problems:
1) The pectoral girdle of pterosaurs was not similiar to the pectoral
girdle of birds. Yes, it is similiar to the flight apparatus of birds
with a slender scapula, elongate, strut-like coracoid, and an outwardly
facing glenoid. The problem with this corellation is that all of the
features stated above are NECESSARY features for flight. Consider bats
(I'll refer to them collectively even though I think that Chiroptera is
diphyletic)... Bats have a clavicle that functions much like an avian
coracoid; it stands nearly vertical, it is strut-like, and it
articulates similiarly with the sternal elements. The acute angle
between the scapula and coracoid in birds and pterosaurs is also
approached in bats, just substitute clavicle for coracoid.
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.
Further complicating the issue is the fact that most maniraptoriforms
have a similiar arrangement in the pectoral girdle as birds.
My view that these maniraptoriforms are descended from arboreal avian forms
would account for a similar, non-convergent arrangement in these pectoral
girdles as well.
Early pterosaurs also have a unique coracoidal articulation to the
sternum; the coracoids are articluated one behind the other.
Wellnhofer states that this overlapping arrangement of the coracoids is
typical of the Ramphphorynchcoidea, but that the Pterodactyloidea sport a
symmetrical juxtaposition on the sternum. (It`s interesting that the
pterodactylodia are also the more "birdlike" in many other features as
well).
2) Theropods have hollow bones.
So are many pterosaur bones, including the skulls.
3) Pterosaur brains are not very similiar to bird brains (no pun
intended). They are rather similiar to croc brains in many details.
Theropod brains are more similiar to bird brains.
I`m no neurologist. Here I defer to an excerpt from Wellnhofers book
"Prehistoric Flying Reptiles"(pg 38)..."Tilly Edinger`s research showed
that, even in the Upper Jurassic, pterosaurs had developed brains that were
more like those of birds than the brain of their contemporary, the'
primordial bird' Archaeopteryx. Thus the pterosaur brain was by no means
reptile-like and small, as in modern crocodiles and lizards, but closer to
that of a bird in shape and size."
4) There is really no "time" problem in the theropodian=>bird
hypothesis. Early maniraptorans are springing up all over.
Yes, as descendants of pre-existing arboreal forms.
5) Not all advocates of the theropod=>bird hypothesis believe that
flight evolved from the ground-up.
But even those that don`t , have the problem of resolving feathers as being
both adapted to flight and insulation at the same time. (It`s the one thing
Feduccia seems to make sense on....my opinion).
6) There is lots of evidence (as outlined by Dave Peters) that suggests
that pterosaurs are not related to dinosaurs, or are even in the
Archaosauria. Peters (following Wild) suggest that pterosaurs are
descended from prolacertiforms, a group that might be close to either
Archosauria or Lepidosauria
Yet even Padian argues pterosaurs being more closely related to dinosaurs.
(Although , as I mentioned previously, he`s little inclined toward my
theory on bird origins).
<<As for the missing furcula, or even clavicles in pterosaurs, I
attribute this to most pterosaurs being "top-heavy" in form, i.e. large
crania relative to body size. Such top-heaviness would have made for
fairly hard landings upon their forelegs, involving stresses that would
have broken any clavicles (if in existance).>>
This could well be true, but prolacertiforms seem to lack clavicles.
I know, I know...and Wild sees them as the true pterosaur ancestors. To fit
my theory, I`d have to pick another candidate. (Can`t agree with everyone!).
<<Therefore, evolution would have eliminated them in the true pterosaur
form. But if there existed a smaller, more primative form, with much
smaller cranium (insectivior), more adapted to making perched landings
on its hind legs, it would have been able to retain the furcula to aid
its powered flight. Such a smaller form would be able to land on, and
negotiate the more intricate branches of the gymnosperms, and newly
evolving angiosperms, where larger pterosaurs couldn`t follow. These
newer trees could invade colder enviorns, where insects were sure to
follow, and in turn, so would these smaller, insectivorous pterosaurs.
In order to adapt, these proto-birds had to develope feathers, at first
just for insulation. Then, hairy proto-feathers evolving into contour
feathers (to keep the body streamlined), finally into true flight
feathers, to aid both in lift and maneuverability. There would have been
definite selectionary pressure against a long wing digit in this colder
enviorn,(Bergman`s Rule), and hence its eventual loss. It could have
been a gradual exchange, a trade-off, loss of length of wing digit for
increased length of flight feathers until finally...a bird!>>
1) The pterosaur form was just fine the way it was.
Sure, until the avian form developed and outcompeted it.
2) This scenario assumes that bird ancestors had a large amount of
aerodynamic control because hind leg first landings are hard to
accomplish.
Yes.
3) It seems to speculate that pterosaurs did not have insulation or
pelage, which they did. Now, feathers are not better insulators as hair
so there is no reason to select for them, even for aerodynamics since
gliding membranes are still present.
Yes, but was it really "hair" that the pterosaurs had, or hair-like
protofeathers??
4) There is no evidence that early birds were endothermic.
I think that anything capable of true flight must certainly have been
endothermic. (my view anyway).
5) Pterosaurs were already stream-lined with their "hair".
O.K.
6) Pterosaur hindlegs seen to have been bound in a uropatagium,
something that birds lack.
All of them? Some of them?? Isn`t this still a debated issue?
7) Pterosaur legs seem to have been bowed to a certain extent. Some
workers think that early bird legs were bowed to an extent, but not to
the extent seen in pterosaurs.
P.S. don`t forget, most of the pterosaur fossils we have are already quite
advanced and specialized, too much so to be exactly analogous to birds in
all features.
8) The pterosaur acetabulum is not fully perforate.
Not as exactly a bipedal posture as birds,...(maybe having to make perched
landings had something to do with the evolution of bipedal stance in both
birds and their descendants?
9) If pterosaurs did go through a process such as this, there is little
reason why the elongated digit IV should be lost.
Bergman`s Rule in a colder enviorn would dictate this, ...I would think.
10) There is no evidence embryologically that birds had an arrangement
in their manus like the pterosaur wing.
No indication of even a fourth metacarpal? Then what about Protoavis??
<<Anyway, in looking for possible negative evidence that might test this
hypothesis, I was once told by my old professor of evolution, (Max
Hect), that..."it`s all in the wrist!" Even Dr. Kevin Padian, who was
kind enough to review my hypothesis, told me..."it`s a long way from a
pterosaur wrist to a bird wrist." I`m not sure exactly what this means,
(I`ve seen evidence of the vertebrate structure doing some pretty
spectacular "morphing").>>
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.
Still, Fushion can be confusun. I`ll have to research this one further.
So, in conclusion, it seems that pterosaurs and birds share no real
similiarities other than classic flight features.
I don`t know. Too much seems to be "lumped" as a product of Convergence
(another topic that bothers me).
Matt Troutman
Thanks again for listening....Larry
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