Re: Dimetrodon

["Nicholas J. Pharris" <pharrinj@PLU.edu>]

On Sun, 5 May 1996, Mark Sumner wrote:

> Steve Jackson wrote:
> 
> > The sail-backed <I>Dimetrodon<$> is one of the best-known prehistoric
> > animals, and is often found in sets of dinosaur models, but it was not a
> > dinosaur.
> > It was a <I>sphenacodont,<$> and was more closely related to mammals than
> > to reptiles. It actually lived in the Permian Age, long before the
> 
> Hmm.  Perhaps you should insert the word "modern" in here, as in
> "more closely related to modern mammals than to modern reptiles."
> 
> As a "mammal-like reptile," sphenacodonts are still reptiles,
> and so about as closely related to reptiles as you can get. <g>

Sorry, all, for yet another post about cladistic definitions.

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.

> Mark

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8206
PharriNJ@PLU.edu