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Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



Date: Sat, 03 Oct 1998 20:05:33 PDT
From: "Matthew Troutman" <m_troutman@hotmail.com>
To: dinosaur@usc.edu, larryf@capital.net
Cc: m_troutman@hotmail.com
Subject: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE
Message-ID: <19981004030534.15328.qmail@hotmail.com>


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).

I still say, out of all three cases, the structure of birds and pterosaurs
are much, much, MUCH more closely allied morphologically! (did I say much
too much?)

<<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).>>

Not if prolacertiforms and Dave Peter's phylogeny are used.

I DO think that prolacertiforms are the best candidates for pterosaur
ancestry, (although I personally would choose one that had clavicles
intact), but it still is not visible in them the extended fourth wing digit
that one would expect in a pre- pterosaur form, and therein lies the " gap".
There might have developed a more "gracile form", with the extended
membrane, retaining clavicles (furcula), that became adjusted to living  in
the narrow branched world of gymnosperms, and developing angiosperm trees.

<<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 guess that you can then argue that tritylodontids are
multituberculates, bats are related to birds and pterosaurs, etc.

I don`t think that the similarities I am pointing out are all that distant,
ie. The specialized redirection of the supracoracoideus tendon to act as
wing elevator,  hollow bones,(whose ONLY function that I can think of would
be as a flight adaptation), and there`s another feature I left out, that
actually makes for a pretty strong argument in my favor (I think), that is
.....the Advanced Mesotarsal ankle joint present only in Dinosaurs, Birds
and (whadda ya know!) Pterosaurs....

So,...we`re agreed that pterosaurs are derived from prolacertiforms that had
no AM ankle joint. My theory would propose that this condition first
developed in the pterosaur line, and it would explain the passing on of this
primary diagnostic condition to the group aves, and hence to their
descendants the dinosaurs. Padian (I think) tries to show sisterhood of the
pterosaurs to the Dinosauria by claiming them to be descendants of
Lagosuchus (or similar), again implying a (to me) weak ground upwards
evolution of the flight mechanism. Of course, if you agree with  Dave
Peter`s phylogeny (or Wild`s ?), I guess then you`re calling the AM ankle in
Pterosaurs just another example of Convergence???

<<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
hypothesis.

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
waist.
7) Replacement pit in the tooth root closed ventrally until late
developement.
8) Root cementum.
9) Implantation of teeth in a groove without development of interdental
plates.
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.


My larger view of  Diapsid evolution is that the whole line essentially
developed  within an arboreal enviornment, with a constant periodic sending
of descendants back to ground level to lead a terrestrial, (or in some cases
aquatic) existance. So, in my view, all of the Thecodonts, came from such
prior arboreal ancestors, some no doubt from the direct line leading towards
the avian group, therefor, they would be expected to have many
similarities,...and NOT by convergence.

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.

Convergence seems (to me) to be an overused, if not abused concept. One
thing I learned in reading Feduccia`s book, is that convergence (or not) is
sometimes very hard to detect. Still, I cannot see how he fails to see the
dino-bird relationship to be what it is, that is very close, actually one
and the same  with theropods. So, how much better are many of you who see
this close theropod-bird relation, and yet dismiss the other "birdlike"
characters of the remaining sauropods ,and the ornithischians even, as
being..."Convergence"??? What enviornmental force is acting to make these
dinos appear "Bird-like", if in fact they weren`t directly descended from
the bird line at some point in time?? Someone PLEASE tell me why they
developed hollow bones...many of them in seeming anticipation of flight (as
this flies in the face of evolutionary theory as I know it!). (Oh boy, I
didn`t want to start up the flames of BCF vs. BADD, but here it goes!).

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.

Yeah, but they had long enough to " specialize",(or not),  after my proposed
pterosaur-bird split in the early Triassic...no?

<<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.

Again, I was thinking more along the lines of the BCF-BADD split of opinion.

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.

Matt Troutman

Well, I guess that last statement is a matter of opinion,...and certainly
not mine!

Larry Febo