[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Forelimb function: an alternative scenario

>The semilunate carpal is a winglike characteristic of dromaeosaurid
>forelimbs. Remember that the wings of _Archaeopteryx_ retained a good
>grasping function; the digits were all separate and movable (although there
>may have been ligamentous locking mechanisms that kept them extended straight
>when the wings were being flapped). So a flightless descendant of
>_Archaeopteryx_ or a similar dino-bird would likely have retained the
>grasping function, too.
>The main problem I've always had with the BADD/ground-up scenario is getting
>a 20-50 kg theropod with relatively small forelimbs to evolve into a 2-5 kg
>dino-bird with grasping wings. It _could_ happen, but in my opinion it is
>_far_ less likely than having a 2-5 kg dino-bird with grasping wings to
>evolve into a 20-50 kg theropod.

An alternative scenario, based on recent theropod phylogenies:

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).

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.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661