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Re: Forelimb function: an alternative scenario



In a message dated 95-12-06 11:40:31 EST, Thomas_R_HOLTZ@umail.umd.edu (th81)
writes:

>The long limbs of most coelurosaurs probably derives from a combination of
>predatory and/or scansorial function.  Dong & Russell likened it to the arms
>of a praying mantis, which I agree with with one caveat: whereas mantids
>grasp
>on the ventral surface of the limbs, dromaeosaurids and their kin grabbed on
>their palmar surface of their hands.
>
>This condition (present in troodontids, dromaeosaurids, oviraptorosaurs, and
>probably the ancestors of tyrannosaurids and ornithomimosaurs) was exapted
>into the flight mechanism of fully volant birds, as discussed by Padian and
>Gauthier in the mid-1980s (the German Archaeopteryx conference).  Since the
>forelimb was already elongated for predatory and/or scansorial function,
>birds were not derived from theropods with "short forelimbs" (at least not
>the immediate ancestors).

Actually, this condition, present as a flight adaptation in the more volant
ancestors of troodontids, dromaeosaurids, oviraptorosaurs, and probably of
tyrannosaurid and ornithomimosaurs, was exapted into the grasping and
catching maniraptoran forelimbs. It did not appear so the theropods could
snatch better; it appeared so their ancestral forms could _fly_ better.

>And, reduction in size is a common feature of evolution.  For example, the
>majority of the Permian and Early-Middle Triassic ancestors of mammals were
>large, and became much smaller during the Late Triassic.

Few if any of the large therapsids of the Permian left any known small
descendants in the Triassic. Small Triassic mammals were almost certainly
derived from small Permian therapsids; the large forms just became extinct
without significant issue. Reduction in size may be not nearly as common in
evolution as has been thought.