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Synapsids are NOT reptiles!



>>synapsids, which I gather aren't even called reptiles any more though they
>>are still referred to as reptiles in many books.
>
>Actually, synapsid is still a valid taxon within reptilia.  The two=
> important ones, the pelecosaurs and therapsids, are considered true=
> reptiles (warm-bloodedness appearing in the advanced therapsids).

No, no, no, no, no!!  For about ten years now, the movement has been on to
use cladistic definitions of amniote (& other!) taxa.  Under the new
definitions, Reptilia is defined as the most recent ancestor of turtles,
lepidosaurs, and archosaurs, and all of that ancestor's descendants.  This
EXCLUDES the Synapsida from Reptilia, and raises some important points.  For
example, uricoteley, enhanced color acuity, "reptilian" scales, and so forth
were probably never present in Dimetrodon and it's ilk.

Synapsida is mammals and all taxa closer to mammals than to reptiles.
Sauropsida is reptiles and all taxa closer to reptiles than to mammals.

>Things start getting dicey when we try to draw the line between therapsids=
> and mammals.  We have so many good transitional forms, that the line has to=
> be placed in an arbitrary way.  For now, it has been placed somewhere=
> between the two (I'm not sure where).

Tim Rowe and colleagues define Mammalia as the most recent common ancestor
of Monotremata, Metatheria, and Eutheria, and all of that ancestor's
descendants.  Hopson and others place it further down, to include
Adelobasileus (sp?) and Sinoconodon.

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