Re: synapsids are reptiles

["Steve Brusatte" <dinoland@lycos.com>]

On Wed, 3 Apr 2002 15:45:17   
 David Marjanovic wrote:
>
>  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. 

Those are good points.  However, is the possession of four color receptors a 
plesiomorphy for amniotes?  Everything I've read has stated exactly what Dr. 
Holtz posted back in 1995: that Dimetrodon (and other "early" synapsids) likely 
had poor color vision (along with glandular skin, etc.)

>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).

I *think* that the scales in caecilians are more fish-like, but I could be 
mistaken.  Hmmm...a lissamphibian apomorphy of scalelessness?  Interesting 
idea.  Much of what I've read says that early tetrapods and amniotes had 
glandular skin without the beta keratin of reptiles.  So, I don't know if 
scalelessness would be an apomorphy, but it is an interesting suggestion.

>  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).

That (diadectomorphs as the sister group fo Amniota) certainly seems to be the 
consensus, although some researchers agree with Ken and place diadectomorphs 
within Amniota.  There is also the possibility that _Diadectes_ and kin were 
stem amniotes.

>  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? :-)

Yup, the precise problem in resorting to the synapsid-diapsid-anapsid 
classification.  I don't like it for the reason David mentioned.

>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.. :-)

Certainly...yes!

Ken Kinman wrote:

>I would agree with most of what Alberto said.  I would only add that 
   since turtles are probably diapsids which reversed back into an anapsid 
   condition, defining Reptilia as a clade was a very bad idea in the first 
   place (and for other reasons as well, as has been often pointed out).

How many published analyses support the turtle-diapsid linkage?  I know of 
Rieppel's Nature paper published a few years back, but, IIRC, the hypothesis of 
turtles as diapsids is far from certain.

>And given the probable amniote nature of diadectomorphs, I proposed 
   here a month ago (DML, March 4th) that Order Diadectiformes be placed inside 
   of Reptilia (rather than as the sister group immediately outside of 
   reptiles).  The cladistic topology remains the same.

I really don't care either way.  I don't think that the placement of 
diadectomorphs is very important.  What is important, IMHO, is studying how 
diadectomorphs obtained herbivory and how that herbivory led to a plethora of 
changes that helped lead to the success of the amniote stock.  Also important 
is attempting to understand how early tetrapods that employed buccal pumping 
evolved into amniotes that used the more effective costal ventilation system of 
breathing.  This was the subject of a paper recently published in Acta 
Palaeontologica Polonica:

Janis, C.M. & Keller, J.C. 2001. Modes of ventilation in early tetrapods: 
Costal aspiration as a
key feature of amniotes. ? Acta Palaeontologica Polonica 46, 2, 137?170.

Janis and Keller find that the evolution and development of costal breathing, 
not the amniotic egg per se, was the key development that led to the 
proliferation of the amniote lineage.  In order to ascertain when this 
transition was made, the authors examined the skull, cervical vertebrae, and 
ribs of living and extinct amniotes.  They find that _Diadectes_ is more 
similar in morphology (longer necks, more mobile ribs, narrower, deeper skulls, 
etc.) to amniotes.  They propose _Diadectes_ as a stem amniote, and hypothesize 
that change from buccal pumping to costal ventilation changed in a stepwise 
fashion among stem amniotes, including _Diadectes_.  They also trace this 
change to the acquisition of herbivory, and rise of the amniote stock.  

This, to me, is much more important than trying to pinpoint exactly where 
diadectomorphs fall in the family tree of anamniotes and amniotes.  However, it 
would be nice if we could find a few more fossils that could give some insights 
into a better phylogeny...:-)

>Whether one wants to put diadectomorphs in Class Amphibia or Class Reptilia 
>makes no 
big difference to me since they were making the transition to basal amniotes 
>(i.e. basal reptiles, in the traditional sense).  

Yes!

>An Order Diadectiformes as the 
   basal clade of Class Reptilia seems the best current option and predict that 
   it will remain in that position (thus promoting stability).

Hmmm...I don't know how stable this would be.

Steve

P.S.: Congrats to Dr. Holtz for Maryland's victory Monday evening.  As a 
diehard Illinois fan, I was very pleased to see Indiana lose!

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