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Re: polarity of bipedality in dinosaurs
At 09:57 AM 9/23/96 -0500, Dinogeorge wrote:
>[Jeff. Martz wrote...]
>> How much Triassic "dino-bird" material is known? A lack
>> transitional fossils for what was probably a pretty small group at
>> the time is hardly surprising given the sparsity of remains of even
>> the relatively common animals from the Triassic. This absence of
>> material, and even if it does not work against your theory,
>> certainly doesn't support it unless you have a great deal more
>> material from your hypothetical transitional form.
>I'm not looking for support for my theory here; I'm saying that evidence
>previously used to support pre-dinosaurian, thecodontian, bipedality doesn't
That's going a long way. You may be prepared to simply discard all
of the evidence concerning the proliferation of bipedality amongst
archosaurian taxa, but I would certainly look very skeptically on any theory
which requires such a seemingly casual rejection of so much work. While it
is true that the fossil record is not complete in this area, a theory which
requires one to ignore a significant body of scholarly work cannot be as
favorable as one which addresses (even in an abbreviated manner) that work.
Should we take this to mean that you don't believe in bipedal
pterodactyls either? For certainly they arose from stock closely realted to
the dinosaurs. Did they develop from quadrapedal animals while "dinobirds"
were going bipedal? If so, when did they find the time to specialize their
forelimbs? Funny, they're beginning to sound like "dinobirds". I suppose
it could happen...
>Besides, there >are< fossil archosaurs that fit my criteria of what some
>dino-birds might have looked like. These include _Longisquama_, various
>megalancosaurids (or drepanosaurids),
If I am correct, these are poorly understood forms. Except for that
one with the long scales on it's back, which looks about as likely to learn
to fly as a goat.
>and whatever small, birdlike thing has
>gotten mixed into the type specimen of _Protoavis texensis_.
So certain are you that Protoavis is a chimera?
>If hard evidence for BCF, in the form of real fossil dino-birds (for
>example), were abundant, it would have become the standard theory
I'm afraid that this doesn't always hold. There was abundant
evidence, even some papers published, arguing for active dinosaurs, and it
was ignored. That is beside the point....
>The major point I keep trying to make is that BCF is a single,
>unified theory that >explains< the existence in dinosaurs,
>particularly theropods, of a
The one thing I have seen consitantly throughout the best, longest
lasting, most robust theories is that they don't explain everything at once.
Your theory requires that a group of animals stay around for around
200my, not changing very mush, spitting out radiation after radiation of
critters. It is not more parsimonious than assuming that these critters
developed from each other.
>number of characteristics that standard BADD (birds are dinosaur descendants)
>theory just logs _ad hoc_ without adequate explanation. This makes BCF
>>better< than BADD. Among these characteristics are:
>Loss of outer digits of the manus in theropods
If they were bipedal, why would they need a wide hand? Two or three
solid meathooks sounds like it would work fine. Still, interesting question.
>Bipedality and forelimb reduction in theropods
See Tom Holtz's recent post on "forelimb" reduction. Personally, I
can't see why a bird would want *smaller* forelimbs, anyway. As for
bipedality, how many flying tetrapod forms are bipedal? 1+1/2 (I count
pterosaurs as 1/2). That's half of the total, not counting the numerous
quadrapedal gliders. How many bipedal tetrapods are flying? 1+1/2. If my
calculations are close, and I don't count the aussie frilled-lizard, that's
about 1/3rd of the total (kangaroo-forms and primates and a half a
pterodatcyl are the others). You haven't yet proved this point.
>Large size of theropods versus small size of birds
Cope's Rule is a _generalization_, it is *not* a hard-and-fast rule!
one of the valid routes of selection is DWARFISM. This may be difficult to
see in the fossil record, because homoplasy, paedomorphy, and allometric
considerations can mask the relationships, but there is certainly no
evidence that small animals *must* come from smaller animals. Otherwise,
after a certain point, we might run out of small animals, and then we'd be
out of new animals. God may play dice with the universe, but you don't have
to be omincient to avoid them odds!
>Hollow-boned and pneumaticized skeletons of theropods and primitive dinosaurs
Could be simply related to bipedality.
>Stiffened tail in theropods
Balance, manoeverability [sic?], general predacious habits.
>Oversize manual and pedal unguals in theropods
Huh? You have got to be kidding? That's like saying we *won't* win
a nuclear war because we have *more* missiles than anyone else! Big
claws=big kills. Period.
>Maniraptoran forelimbs of certain theropods
When someone explains what this means to me, I'll be happier. I'll
throw this one at you: Why is it that those "certain theropods" seem to be
the ones that are the most birdlike. If these dinobirds were constantly
spitting out new radiations, I would expect to see the maniraptoran wrist in
more than one group, and perhaps in other early forms. Shouldn't
Lesothosaurus have one?
>Retroverted hallux of most theropods
Most? I was not aware that it was in most. I was aware that this
occurred in dromaeosaurids and not in troodontids, indet. in ornithomimids,
?in Oviraporoids, not in therizinosauroids, I haven't heard of it outside of
the coelurosaurs, but you're welcome to enlighten me.
>Keeled sternum in theropods at and above the phyletic level of _Allosaurus_
I presume you mean fully ossified keeled sternum, and the taxon you
mean is Coelurosauria. So birds developed earlier, but they didn't develop
the keeled sternum until later?. As several folks have pointed out in the
Mononykus debate, a keeled sternum doesn't necessarily mean flight. I'd
like one, myself. In any case, all you've then proved is that coelurosaurs
are secondailyy flightless.
>Presence of a furcula in certain theropods
I am sick of this argument, myself. Some thecodonts may have had
>Scarcity of pre-_Archaeopteryx_ "birds,"
It's more parsimonious to presume there weren't any.
> and existence of all so-called "bird precursor" dinosaurs in strata
> >younger< than that of _Archaeopteryx_
So the fact that there are echidnas today proves something about the
way mammals developed? Let's be real, the raptors _were_ "The Nastiest
Dinosaurs." If _I_ met one, I couldn't make it extinct! :)
>Microfossil theropod teeth in Jurassic strata
Uh... So there couldn't have just been small theropods like the
fifteen or so that have been found in the past decade?
>Presence of feathers in birds versus apparent lack of feathers in
>most if not all theropods
_Apparent_ lack of evidence, indeed. Since there haven't been any
published accounts of skin impressions on small dinosaurs other than
Archaeopteryx and birds yet, I think you are, in fact, quite wrong in your
assertion that there is something in need of explanation here. We have two
cases of feathered small theropods (Archy and Confusci), one ?pitted?
dromaeosaur skin, and whatevertheheck Pelicanomimus has, versus no scaley
And don't forget:
THAT WHICH MAKETH THE DINOSAUR IS A DINOSAUR
If your "dinobirds" exist, after a certain point, they _are_
dinosaurs. I think it's a pretty simple calculation to see that modern
birds probably branched off of a BCF tree after some other dinosaurs.
>BADD and BCF are in substantial agreement that birds and theropods are
>intimately related, and agree in most of the major features of dinosaur-bird
And BCF is an interesting hypothesis. Whenever I get new data, I
test it against BCF as a control. BCF has some stuff going for it, at least
as far as theropods are concerned. I really wouldn't be surprised if
someday someone is able to make a convincing case (not that Greg Paul isn't
already trying) that some theropods were secondarily flightless.
| Jonathan R. Wagner "You can clade if you want to, |
| Department of Geosciences You can leave your friends behind |
| Texas Tech University Because your friends don't clade |
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