Re: Re: Dimetrodon

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

>Call me a lumper, but I'm not ready to generate new orders to
>hold transitional forms.  My feeling is still "reptile" best
>describes the form and lifestyle of these creatures.  

"Synapsida" is hardly a new term... (and, as stated below, is no longer
accepted as a name for a transitional group).

>To create a "Synapsoid" group, or some other mishmash, as
>has been suggested now for better than twenty years, strikes

Care to try for "nearly one hundred years now"?

Also, "Synapsida" is no longer a term for a transitional stage, but is the
entire clade (i.e., it includes Mammalia).

>me as no more sensible than creating a "dinobird" order
>to hold transitional bird forms.

I would certainly not hold with naming formal paraphyletic groups!

>I've seen no evidence that would make me classify Dimetrodon and
>its kin as a mammal.

And, except for Van Valen, no one has.  Dimetrodon is closer to mammals than
it is to anything else living, but is still MUCH less advanced than even the
gorgonopsians, dinocephalians, dicynodonts, and the rest of the host of
nonmammalian therapsids.

>On the road (or at least an off ramp thereof) that leads to mammals?
>Yes.  But though it has reached an intermediate structure of skull
>and teeth, this animal continues to operate as a reptile.

Operate as a basal amniote?  Yes.  Operate as a reptile, with the uricotelic
excretion, phi keratin scales, four-to-five color receptors in the eyes,
etc.?  No evidence for it, and phylogenetic evidence against it.

>But if I was drawing the reptile / mammal line in the sand, I 
>would definitely draw it "uphill" from Dimetrodon or, for that
>matter, any of its close relatives.  

Why draw that line at all?  Under almost all modern phylogenies (all that I
have seen, but there may ve others out there), Mammalia did not evolve from
Reptilia, nor are they even sister taxa per se.  Amniotes are divided into
Synapsida (of which Mammalia is the only surviving clade) and Sauropsida (of
which Reptilia is the only surviving clade).

>This argument has been bubbling since I was in school (you know,
>back when Dimetrodons were common pets), so I don't suspect it
>is likely to be decided soon.  My position was (and is) that
>the debasement of perfectly fine terms such as "mammal-like 
>reptile," only serves the fine technician at the expense of the
>larger picture.

Perfectly fine, as long as your goal is to confuse the whole situation that
is... :-(

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

"There are some who call me...  Tim."