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Reptilia (was RE: Sundry responses of Bois)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> KiernanCR@aol.com
>
> >>I don;t have a problem with "2"; however, Reptilia is very much
> monophyletic
> as long as you put Aves in Reptilia (as is done by many workers today).<<
>
> But what about Mammalia? If we exclude them from Reptilia, doesn't that
> render the class paraphyletic, since mammals did evolve from reptilian
> ancestors? I agree that Reptilia is monophyletic, if Mammalia and
> Aves are
> included (and that's what I should have said in the first place,
> instead of
> stating that Reptilia, in the older sense of the order, was
> paraphyletic --
> which is why I originally qualified "reptiles" with quotation marks).

After Gauthier et. at. (1988), Reptilia = all descendants of the most recent
common ancestor of turtles, lepidosaurs, and archosaurs.  As such, birds are
part of Reptilia, but mammals are not: synapsids (mammals and their
ancestors) diverged from the lineage leading to reptiles prior to the split
among the various reptile lines.

Thus, the so-called "mammal-like reptiles" are not reptiles at all.  Mammals
did NOT evolve from reptilian ancestors.  _Ophiacodon_, _Dimetrodon_,
_Placerias_, _Cygnognathus_, etc. are no longer considered reptiles: they
are synapsids.

It is now recognized that a monophyletic Reptilia does have a suite of
derived skeletal, soft-tissue, and physiological features not found in
lungfish, amphibians, and mammals (nor presumably in the ancestors of the
same).  The old-style "reptilian" traits (cold-bloodedness, shelled egg,
etc.) are simply common to all amniotes.  So mammals are amniotes, but they
aren't descendants of reptiles.

Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843