Re: synapsids are reptiles

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

On Mon, 8 Apr 2002 22:17:05   
 David Marjanovic wrote:
>----- Original Message -----
>From: "T. Michael Keesey" <mightyodinn@yahoo.com>
>Sent: Monday, April 08, 2002 8:00 PM
>
>
>> `--+--+--Gymnophiona (dermal scales)
>>    |  `--+--Urodela (smooth skin)
>>    |     `--Salientia (smooth skin)
>
>Might be more complicated... in both the MPT and its bootstrap test in the
>recent Can. J. Earth Sci. paper on *Solenodonsaurus* (can't find it at the
>moment) Urodela comes out as paraphyletic with respect to Gymnophiona. In
>some SVP meeting abstracts there's also the idea that frogs are
>temnospondyls while the other lissamphibians are not. Let's joyfully wait
>for the paper... :^)

Is there a paper in press on this issue?  If so, I'll have to look for it.  
However, there are a few papers (already published!) that mention this very 
issue, although none go into much detail (hopefully, if a new study is in 
preparation, it will answer many of the questions I have on this issue).

There is a decent paper in _The Age of Dinosaurs in Russia and Mongolia_ (M.A. 
Shishkin et. al, 2001, Permian and Triassic Temnospondyls from Russia...) that 
discusses this very issue.  Basically, Shishkin et al. believe that the notion 
of a temnospondyl-derived Clade Lissamphibia is incorrect, and that there may 
be two or more major clades of what we commonly refer to as "amphibians."  They 
reconstruct anurans as derived from temnospondyls based on two(!) 
synapomorphies: the presence of a sound-conducting tympanic system with the 
dorsal stapes disconnected from the hyoid, and a unique pattern of cranial 
arteries, as deduced and restored from impressions on various skulls.  

Characters that support Lissamphibia, the authors say, reflect paedomorphic 
trends seen in recent amphibians or non-skeletal soft parts that cannot be seen 
in fossils.  I don't have much knowledge about either of these two trends, but 
from my studies of heterochrony it has become obvious that there are some 
paedomorphic trends that do characterize modern amphibians.  It seems logical, 
then, that such trends may be convergences or something homoplastic.

While Shishkin et al. have no problem with the temnospondyl-anuran link, they 
believe that caudates and gymnophionians may have been derived from 
lepospondyls, possibly microsaurs.

Very interesting ideas.

Steve

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