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Re: Paradoxically temporal



At 02:48 PM 7/22/98 -0400, Jeff Poling wrote:

>   Why does the fact that _Eoraptor_ appears later or contemporaneously
>with _Plateosaurus_ in-and-of-itself disqualify it from being
>_Plateosaurus_'s ancestor?

At present, all known specimens of _Eoraptor lunensis_ occured too late in
time to be **directly ancestral** to prosauropods.  In this particular sort
of situation, Feduccia and Martin have made an excellent point: an ancestor
MUST be older than its descendants.  (Of course, where they make their
mistake is thinking that dromaeosaurids are advocated as bird ancestors,
rather than bird sister taxa).  We have, at present, no fossils of
_Eoraptor_ which are old enough to be _Plateosaurus_' ancestor.

This is not to say that an older dinosaur, closely resembling _Eoraptor_,
might not be the ancestor of _Plateosaurus_, nor do the ages of these
fossils in and of themselves dismiss the possibility of a sister group
relationship.  However, _Eoraptor_ itself is known only from fossils too
young to be that ancestor.

Evidence for ancestor-descendant relationships require stricter rules of
evidence than mere sister taxon relationships.

(In fact, there ARE anatomical reasons to suspect that _Eoraptor_ is not
directly ancestral to any prosauropod, such as the marked reduction of
digits IV and V in the manus, more so than in prosauropods (or ceratosaurian
theropods, in the case of digit IV)).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661