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Re: Quadraped Sailbacks-All non-dinos?



>>Okay - Dimetrodon, Edaphosaurus, and its relatives are not dinosaurs, nor
>>reptiles (under cladistic taxonomy), but are primitive representatives of
>>the synapsids (mammals and their ancestors).
>
>Weren't the Synapsids reptiles (hence the name "mammel-like reptiles")?
>
>This leads me to the question, where do we draw the line between reptiles
>and mammels? What features of modern mammels were known to be present in
>synapsids?
>
>I hope this isn't getting too far away from dinosaurs to be of interest to
>this group.

Under the older, "gradisitic" taxonomy, it was fair to refer to reptiles as
"all amniotes which aren't birds or mammals", so Dimetrodon would then be a
reptile.

Under a cladisitic taxonomy, all named groups must be an ancestor and ALL
of that ancestor's descendants.  One idea is to reserve 'Reptilia' for the
group composed of the most recent common ancestor of turtles, lepidosaurs,
and archosaurs.  Under this definition, dinosaurs and birds are reptiles,
but the so-called "mammal-like reptiles" are not.  Instead, they are
regarded as synapsids.  In fact, there is evidence that the primitive
synapsids were not particularly "reptile-like" with regards to scales,
eyes, excretory system, etc.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.