Re: Stego/Ankylo limbs

[Dinogeorge@aol.com]

In a message dated 96-01-30 13:49:10 EST, Robert.J.Meyerson@uwrf.edu (Rob
Meyerson) writes:

>I'm afraid I can't agree.  Recent work on _Triceratops_ has lead to the=
> conclusion that sprawling forelimbs actually do coincide with trackway=
> data.  I recently had the pleasure of talking with Bruce Erickson, from
the=
> Science Museum of Minnesota (who's collection has one of the few=
> non-plaster mounts of _Triceratops_, placed in a sprawling posture).  By=
> measuring the width between the feet, he was able to put together a=
> composite of what trackway his T-tops would make.  Suprise, Suprise, the=
> composite track coincides with the dimensions of a real trackway.
>
>Therefore, and back to my original question, since split posture types are=
> possible, could this also be accurate for other quadreped dinosaurs?
>
>

Since ceratopians were secondarily quadrupedal (the ancestral ceratopians
were small bipedal marginocephalians and psittacosaurs), there is no reason
to expect their graviportal forelimbs to have had a pose as fully erect as
their hind limbs, which had been primitively erect (a dinosaurian apomorphy)
for about 150 million years. Indeed, because the large, graviportal
ceratopians had evolved quite rapidly during essentially the second half of
the Late Cretaceous (about 20 million years altogether), one would not expect
much in the way of a fully erect pose--the result of lengthy evolution--from
the formerly grasping forelimbs. The forelimbs became suborned in a "fast and
dirty" way to holding up the animals' forequarters, letting the "evolutionary
chips" fall where they may. Perhaps, had there been no terminal Cretaceous
extinction event, the forelimbs of ceratopians would eventually have trended
toward a more fully erect pose, but now we'll never know.