Re: Eoraptor question

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 08:41 PM 11/18/98 -0800, Larry Dunn wrote:
>Why is _Eoraptor_ supposedly closer to the basal saurischian than,
>say, _Herrerasaurus_?

Because, under the Sereno model, _Herrerasaurus_ shares several derived
features with neotheropods that _Eoraptor_ lacks.  In other words,
_Eoraptor_ is less divergent in form from the ancestral condition.

>Is that in fact the most common opinion of the
>placement of E.?
>

I do not know of anyone who considers _Herrerasaurus_ less derived
anatomically from an ancestral position than _Eoraptor_.

However, here are the two extremes of the phylogenetic position of _E._ and
_H._:
Sereno et al.:
Dinosauria
  Ornithischia
  Saurischia
    Sauropodomorpha
    Theropoda
      _Eoraptor_
      unnamed
        _Herrerasaurus_
        Neotheropoda

So, in this case, _Eoraptor_ diverged from other theropods first, and
_Herrerasaurus_ second.

In the most extremly different of several trees from Holtz & Padian (1995)
(incidentally, the updated analyses do not support this as a most
parsimonius tree):
_Eoraptor_
unnamed
  _Herrerasaurus_
  Dinosauria
    Ornithischia
    Saurischia
      Sauropodomorpha
      Theropoda

In this case, _Herrerasaurus_ is closer to the basal saurischian condition,
because it is closer to Dinosauria than is _Eoraptor_.

There hasn't been much work on _Eoraptor_'s phylogenetic position yet,
beyond the studies of Sereno, Novas, and colleagues (mine and Padian's
remains unpublished, but I'm working on it!!!).

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