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Re: More Bird origins: ignore if you wish...



In a message dated 95-12-07 10:06:27 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes:

>>I'm dealing in likelihoods here--and in the unified picture of
>theropod/avian
>>evolution that BCF presents. 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.
>
>Quite so.  You know, every time this particular theropod worker looks at his
>favorite fossils, he sees abundant evidence of enhanced cursorial ability
>(relative to other archosaurs) and derived grasping functions in the jaws
>and/or forelimb.  I don't understand how others do not see these clearly
>manifested...
>
>

To the business about "enhanced cursorial ability" and "derived grasping
functions" I should add something about the existence of such adaptations
_outside the context of flight adaptations_. It is indeed correct that there
are many, many cursorial adaptations among theropods. But these seem to arise
_after_ the lineages diverge from their arboreal ancestors. For example,
"bipedality" itself is a cursorial adaptation--but it is a cursorial
adaptation for lineages in which the forelimbs have become too modified to
serve as portal locomotor organs. They are adaptations that in some cases at
least arose to compensate for arboreal adaptations that hindered
cursoriality. For another example, the dew claw on the foot, which is a
cursorial adaptation compensating for the retro-hallux of arboreality.
Reduced forelimb size is also just such an adaptation, compensating for
forelimbs that would have been much too big and too much in the way had they
maintained their arboreal relative size in the cursorial forms (the forelimbs
were reduced even in _Deinocheirus_; either that, or it was one odd-looking
dino).

The trouble is, it is very difficult to separate cursorial adaptations of the
kind described above from cursorial adaptations that would have arisen
without an intervening arboreal stage. Had there not been an arboreal stage
in dinosaur evolution, I suspect a lot more of them would have been
quadrupedal--like their earlier, pre-dino-bird thecodontian relatives--sleek,
slender-limbed (like sphenosuchians), gracile runners. Something like that.
They might also have retained an enlarged calcaneum, and there would have
been no retro-hallux.

But this isn't even speculation anymore; it's speculation squared.