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PTEROSAURS, PROTOBIRDS, AND CONVERGENCE
REGARDING: Differences between bird and pterosaur shoulders
<<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.>>
All flying vertebrates have to have some sort of bracing to resist the
compressive forces generated by the flight muscles. All go through
different ways. How they do it probably depends on what kind of animal
they are and their ancestor (Hill and Smith 1984; Flight Chapter).
<<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
Not if prolacertiforms and Dave Peter's phylogeny are used.
<<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
I guess that you can then argue that tritylodontids are
multituberculates, bats are related to birds and pterosaurs, etc.
<<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?>>
Look at hypotheses with abundant fossil evidence? :-)
Lets do just that... I think that maniraptorans ARE the ancestors of
birds, but just for argument lets look at the main, most viable
alternative to the maniraptoran/bird condition; the crocodylomorph
Morphological evidence in crocodilians for avian relationships:
1) Fenestra pseudorotunda.
2) Superior tympanic diverticulum with corresponding recess.
3) Posterior tympanic diverticulum " " "
4) Anterior tympanic recess crossed by carotid.
5) Siphonium between pneumatic quadrate and articular.
6) Teeth with the crown seperated from an expanded root by a distinct
7) Replacement pit in the tooth root closed ventrally until late
8) Root cementum.
9) Implantation of teeth in a groove without development of interdental
10) Quadrate cotylus near the base of the paraocciput.
11) Bipartite quadrate articulation.
12) Anterior-medial origin of temporal musculature.
13) Similiar development of the sternum.
Most of these are valid characters, though most all are found in
theropods. But look at how strong this origin of birds is compared to
the pterosaurian ancestry of birds. Now, if the main alternative to the
maniraptoran/bird connection are this strong, you most have an
alternative hypothesis that is at LEAST as strong as this one.
Oh, and also. If theropods are the ancestors of birds (and even if they
are not), most of the characters above are CONVERGENCES between
crocodylomorphs and maniraptoriform theropods.
Now, pterosaurs are much too specialized to be avian ancestors with
their procoelus vertebrae, elongate digit IV, uropatagium,
brachioptagium, and specialized wrists. They are also very primitive in
antorbital and periotic pneumatization.
<<Now, 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!>>
It hasn't really been debated much on the list lately. Everybody thinks
that birds are dinosaurs :-) Now, if we had any combination of Sam
Tarsitano, Larry Martin, Max Hecht, Kenneth Whetstone, or Alan Feduccia,
it will be debated more.
Another weakness of your hypothesis is its interpretation of secondary
flightlessness in maniraptoriforms. A far better interpretation is that
they are avian ancestors not avian descendents.
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