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Re: BADD BCF



In a message dated 95-12-07 04:21:57 EST, ornstn@inforamp.net (Ronald
Orenstein) writes:

>GO writes:
>
>>>This, however logical it may be, is not evidence.  If I understand you you
>>>are saying that non-flying maniraptors share a suite of characters in the
>>>forelimb that, in volant birds, serve as part of the flight mechanism, and
>>>suggest that this involves too many coincidences to dismiss as anything
>>>other than a secondarily flightless condition in those forms.
>>
>>In the forelimb (various modifications for a wing), in the foot
(retroverted
>>hallux), in the skeleton (pneumatization), and probably in the soft anatomy
>>as well. Not to mention a number of less important but noteworthy cranial
>>modifications, and flight feathers.
>
>The retroverted hallux, as we have discussed here, is NOT a flight
>adaptation.  It may be an arboreal adaptation (or not) - but that is not the
>same thing.  Pneumatization (I think?) is also known in other dino lines
>(sauropods?) and may serve other functions (air-sac type respiration,
>lightening) that could either be flight adaptations or "exadapted" for this
>purpose in birds.  "Probably" ain't evidence; the cranial characters may
>indicate relationship, but what  does that have to do with flight?  And, yet
>again, there is NO DIRECT EVIDENCE that ANY non-bird maniraptor had flight
>feathers or any other kind of feathers.  A character state cannot be coded
>because you think it ought to be there.

Oh, but the retroverted hallux is definitely a flight adaptation. To
construct a crude analogy (the only kind I seem to be capable of these days),
the theropod foot with the retroverted hallux is to an ordinary archosaur
foot what the retractable landing gear on an airliner is to ordinary wheels.
It's not part of the wing apparatus, but it does help the animal to fly
better. Before the advent of the retro-hallux, arboreal archosaurs had to use
all four limbs for clambering about in trees; this limited the development of
the forelimb into a wing, because it also had to serve in the more mundane
grade of locomotion. But the advent of the retro-hallux allowed the foot to
assume more of the climbing and grasping burden and freed the forelimb to
become more winglike. This was a key stage in the evolution of avian
flight--the decoupling of the forelimb from portal locomotion--and it was
made possible (in this instance) by the retro-hallux. Of course, the
proximate cause behind the retro-hallux was simply to improve the grasping
ability of the hind foot, an arboreal adaptation.

What possible use could a retro-hallux have for a strictly cursorial biped?
None. That's why it vestigialized (into a dew claw) in each different
theropod lineage.

Pneumatization in non-avian dinosaur lineages may in all instances be traced
back to the arboreal ancestral forms. At least, this accords with the
phylogeny of dinosaurian pneumatization recently presented by Brooks Britt.
Indeed, the _loss_ of pneumatization occurred only within Prosauropoda and
Ornithischia, and I have been considering this as a synapomorphy uniting
those two clades within Phytodinosauria.

>>When those characters _all_ involve modifications that significantly
improve
>>an animal's capacity for flight, I begin to suspect that they arose in that
>>context.
>
>This improvement (if present at all) could arise prior to the development of
>flight - in fact some of these characteristics must have been present in a
>non-fllying ancestor or flight would not have evolved!
>
>>I'm dealing in likelihoods here--and in the unified picture of
>theropod/avian
>>evolution that BCF presents. 
>
>Likelihood isn't evidence.  Neither is  a "unified picture" (and I don't see
>why BADD isn't also a unified picture).

BADD is too ad hoc; it's less a unified picture than a collage. In the
absence of hard evidence to the contrary, I stick with the simpler, more
unified picture. The fact that numerous paleontologists apparently accept the
BADD ground-up theory of the origin of avian flight is not a good reason to
accept it. The history of paleontology is littered with the debris of
falsified ad hoc theories and hypotheses that were once accepted by
mainstream paleontologists.

>>If theropods started out as cursorial animals, I
>>would expect to see evolutionary modifications that improved their
>>cursoriality, or their ability to use their forelimbs better for grasping
>and
>>holding prey, and so forth.
>
>It seems to me that an animal like Velociraptor is beautifully designed for
>both, in many ways:  the balancing tail, the long grappling arms, the whole
>build of the critter.    Again, I'm no palaeontologist - is there anyone out
>there NOT committed to BCF who thinks otherwise?

The question is, which of the splendid evolutionary modifications in
_Velociraptor_ occurred to overcome the limitations inherent in its arboreal
origin.

> It is true that evolution sometimes hands us the
>>unexpected, and to that extent I'm willing to entertain the idea that
small,
>>bipedal cursorial theropods acquired feathers (somehow), lost two or three
>>manual digits (who needs 'em?), enlarged their grasping hands into grasping
>>wings (for some reason), retroverted the hallux (well, why not?), etc.,
etc.
>
>As I noted (much) earlier, even if these adaptations were evolved in an
>arboreal ancestor that later became terrestrial this does not conflict with
>BADD.

And to that extent, BCF is in full agreement with BADD!

>>>These are cautions, not fatal objections.  But the fact remains that the
>>>only way to settle the matter is by discovery of further transitional
>forms.
>>>For example, the absolute clincher for showing that, say, Velociraptor was
>>>secondarily flightless would be an analysis showing that it lay WITHIN a
>>>clade of known winged forms.  We do not have the taxa to show this.
>>
>>Actually, it does--Archaeopterygiformes. _Archaeopteryx_, _Deinonychus_,
>>_Velociraptor_, and other dromaeosaurids share enough apomorphies that even
>>cladists will soon admit that they belong to a single clade. The big
>problem,
>>of course, is that only one (maybe two or three, if _Archaeopteryx_
>comprises
>>more than one species) of the known archaeopterygiform taxa was volant. But
>>soon the Korean archaeopterygiform will be described, and there will
>>undoubtedly be others.
>
>This is not what I meant.  You must show that Deinonychus, Velociraptor et
>al PLUS a winged taxon form a clade with ANOTHER winged taxon as the sister
>group to that clade.  We are hardly in a position to do that.

Boy, I'll say.

>My point was that the stem taxon
>>>could have included quite closely related species diverging widely in size
>-
>>>and, if so, could have given rise to both large and small forms.
>>
>>It probably did. So--?
>
>Then you can't argue that size points away from BADD.  If the ancestral
>stock included both large and small forms, otherwise very similar, then the
>small form could have given rise to birds without size reduction, the large
>form to larger maniraptors, and the whole group would still form a single
>clade.

Again, if this is how you interpret BADD, then you are also in substantial
agreement with BCF. The difference lies mainly in the presumed lifestyle of
that common ancestor: in BADD, it would have been a bipedal cursor; in BCF,
an arboreal climber or glider.

>>>But, as I said above, there is no reason to assume that BADD requires that
>>>Archaeopteryx was derived from a LARGE theropod - only that both
>>>Archaeopteryx and the large theropods like it evolved from a common
>ancestor
>>>whose size is unknown.  Thus, unless you can produce a known ancestral
>taxon
>>>acccepted as such by BADD theorists that is considerably larger than
>>>Archaeopteryx, I think this particular argument is a straw man.
>>
>>This will never happen, because the BADD cladists weasel out of presenting
>>_any_ taxa as ancestral.
>
>I'd like to hear one of them on that.  So far the only taxon BCF has come up
>with is Longisquama, which seems far too derived (and perhaps too distant)
>to qualify (I mean, Eusthenopteron may be an ancestor too, but what does
>that tell us?).  The point is that we do not have an ancestral
>maniraptorian, and that is not dependent on whose theory you buy.

BCF didn't "come up" with it, the fossil record did. Presently, I maintain
that _Longisquama_ should at least be scrutinized as a possible bird
ancestor. Most workers, however, dismiss Sharov's identification of it as a
"pseudosuchian" (don't ask, don't ask--just say "archosaur"), but accept his
interpretation of it as a sprawling, lizard-like climber, even though we know
only the forequarters of the animal. This, I think, may be exactly backward,
but we desperately need a better description of the specimen before we can be
certain.