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Re: Pre-archosaur...was : What came before Eoraptor?



Larry Febo  <larryf@capital.net> wrote about two cladograms at
<http://dinosauricon.com/taxa/archosauromorpha.html>:

> I`m not sure I like either of them. In Dave Peter`s, Prolacertiforms are
> grouped under Ornithodirans, but I thought to qualify as Ornithodiran, it
> must have an "advanced mesotarsal" ankle, and I thought that
prolacertilians
> had not yet achieved that feature.


Cladograms aren't meant to be liked or disliked, just tested.  The reason
that the David Peters puts the Prolacertiformes in the Ornithodira is that
Paul Sereno defined Ornithodira as all descendents of the common ancestor of
Pterosauria and Dinosauria in his early 1990's Memoir of the JVP (I'm sure I
have not given the exact wording or citation, but the essence is correct).
In my 1996(?)  paper on the phylogenetic position of the Pterosauria, I
argued that the phylogenetic position of the Pterosauria depends on the
interpretation of a suite of hindlimb characters.  If they are interpreted
as synapomorphies with the Dinosauria, then the Pterosauria is the major
sister group of the Dinosauria.  However, if the suite of characters are not
shared with the Dinosauria, then the Pterosauria are much further down the
cladogram.  In my analysis (assuming those characters were homoplasic rather
that homologous) the Pterosauria came out as the sister group of
Erythrosuchia + everything else up the cladogram.  I noted in that paper
that if that cladogram was correct, then the Ornithodira as defined by
Sereno would be much more inclusive than he intended.  David Peters
presumably either noted my comment or came to the same conclusion
independently.

As for David Peters' cladograms, I cannot remember whether he lists the
characters he used to support them on his web page, but a couple years ago
he sent me copies of his analyses and characters, and in my opinion he was
playing fast and loose with his characters.  Again, let's not like or
dislike cladograms, let's test them.

Chris


S. Christopher Bennett, Ph.D.
Asst. Prof. of Basic Sciences
College of Chiropractic
University of Bridgeport
Bridgeport, CT  06601-2449
cbennett@bridgeport.edu