Re: synapsids are reptiles

Thomas R.Holtz, 13.Dec.1995 in "Synapsids are not reptiles" wrote:

Reptilia is defined as the most recent ancestor of

turtles, lepidosaurs and archosaurs. Uricotely, enhanced color acuity, reptilian scales and so forth were probably never present in Dimetrodon.

If I'm correct in interpreting "enhanced color acuity" as having 4 color receptors instead of 2 like all mammals except those few primates that have 3 (or, sometimes, another 4), then there's no reason to assume Dimetrodon didn't have that -- having 4 color receptors is the plesiomorphy for vertebrates in general, and it is assumed that mammals lost 2 as a result of a nocturnal lifestyle. About scales... at least some caecilians have scales (though I don't know whether they are more fish- or reptile-like), so the fact that most living amphibians are scaleless may be their apomorphy (evolved several times).

A big fully terrestrial amniote like Dimetrodon needs skin protection against trauma, considering also the sprawling position of the limbs that maintains the body so near the ground.

Good argument IMHO.

Thomas R. Holtz, 13. Dec. 1995 wrote:

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

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

There is a strong possibility that diadectomorphs are amniotes (Lee & Spencer), in that case where we have to place them in sauropsida or synapsida?

There has been the suggestion that diadectomorphs are synapsids (would offer a comfortable opportunity to separate the meanings of Synapsida and Theropsida...). The consensus is that they are the sister group to Amniota (even though they may well have laid amniotic eggs... unknown).

(Can't find the ref for the most recent phylogenetic study... Michel Laurin & Robert R. Reisz, Can. J. Earth Sci., has Solenodonsaurus janenschii in the title.)

The situation in amniote phylogeny is so fluid that perhaps is better, at least for the moment, to maintain in use the traditional distinction: synapsida-diapsida-anapsida, all inside the Reptilia.

You do know that by your suggestion you are a reptile, and Reptilia becomes the same as Amniota? :-)

The situation in amniote phylogeny is so solid that perhaps it is better to abandon the term Reptilia altogether. We have beautiful, precise terms like Amniota and Sauropsida, let's use them instead. If only because they don't carry connotations like "low vertebrate", "cold-blooded", "sluggish" etc. etc.. :-)