Re: Dimetrodon

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

On Mon, 6 May 1996, Mark Sumner wrote:

> Call me a lumper, but I'm not ready to generate new orders to
> hold transitional forms.  

Neither am I (see below).

> My feeling is still "reptile" best
> describes the form and lifestyle of these creatures.  

Indeed.  Just remember that this has no formal taxonomic bearing.

> 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.

Ick!  Both of those sound paraphyletic, and I would not agree with the 
establishment of either.

How about this:

Theropsida
   caseids
   sphenacodonts
   blah, blah, blah...
   Therapsida
      dicynodonts
      gorgonopsians
      cynodonts
      etc., etc., ...
      Mammalia

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

Certainly not!

> 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.

Which, bear in mind, is the default and the norm for amniotes!  It's the 
mammals and birds that are unusual.  We tend to look at things from the 
wrong end.  Go back to the Permian.  Disregard anything you know about 
mammals, birds, or anything that evolved later.  Look at the anmiotes.  
They all look rather like lizards.  Classify them into different groups.  
This one has a hole behind its eye socket.  That one has two holes behind 
its eye socket and another hole in front.  Now run time again, and as new 
species evolve from the old ones, expand your groups.  Name new groups 
when it seems appropriate, but remember that your new groups are still 
members of the old groups.  You will end up with a phylogeny like what I 
suggest.

> 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.

UGH!  My condolences.

> 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.  

If you're advocating a typological taxonomy rather than a phylogenetic 
one (let's see...  I think this caecilian, this snake, this 
amphisbaenid, and this glass lizard look and act similar enough...), then 
the above is relevant.  But if your goal is accurate representation of 
evolutionary events, then where you personally would draw the line is 
irrelevant.

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

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