Re: Dimetrodon

[Mark Sumner <range@inlink.com>]

Nicholas J. Pharris wrote:

> Whether you call _Dimetrodon_ and its ilk "reptiles" depends on your
> definition of a reptile.  Reptiles have been defined cladistically as
> "anything closer to a modern lizard [turtle, bird, croc, snake, tuatara]
> than to a modern mammal.  Sphenacodonts are quite clearly NOT reptiles
> under this definition.
> 
> Or you may prefer the typological lay definition of a "reptile" as "a
> (usually) terrestrial tetrapod which lays shelled eggs, is ectothermic,
> and possesses scales."  _Dimetrodon_ was undoubtedly an ectothermic,
> terrestrial tetrapod which laid shelled eggs, but there is no particular
> reason to think it possessed scales, at least of the type seen in modern
> "reptiles."
> 
> As I have mentioned before, I am in favor of abandoning the terms
> "reptile" and "Reptilia" in scientific writing and leaving them with only
> their typological definitions.  They're just too confusing and, in many
> cases, misleading.

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.  

To create a "Synapsoid" group, or some other mishmash, as
has been suggested now for better than twenty years, strikes
me as no more sensible than creating a "dinobird" order 
to hold transitional bird forms.

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

The fine points of cladistics are not lost on me.  I spent large
parts of my career trying to pigeonhole Pennsylvanian fishes, 
which presents enough headaches to knock a few more holes in any 
skull.

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.  

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.

Mark "let's hear it for lay terms" Sumner
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