Re: Platyhystrix and dinosaur humps/sails

[AM Yates <Adam.Yates@bristol.ac.uk>]

On Fri, 10 Mar 2000, [iso-8859-1] Bill Adlam wrote:

> 
> I'm afraid I don't know anything about _Platyhystrix_ beyond what's
> been mentioned here.  In what sense is it an amphibian: crown group
> amphibian, stem amphibian or stem tetrapod?  And how big is it?
> 

This is an uresolved question! There is simply no consensus as to which
fossil group(s) are the most closely related to the modern amphibians.
In the 80's and early 90's consensus view, as a dissorophoid
temnospondyl, Platyhistrix would be a stem amphibian close to the origin
of Lissamphibians. However the most comprehensive cladistic analyses to
date are saying that the temnospondyls are an extinct clade not a grade
of proto-lissamphibians and that they are quite remote from the tetrapod
crown group.  


> Modern amphibians breathe through their skin, so a sail could increase
> oxygen supply.  I can't remember any particular evidence for cutaneous
> respiration in early terapods, but it would seem likely.  Perhaps 
> someone more informed than me could comment...
> 

I don't know if there is any evidence for cutaneous respiration in
sarcopterygians, living or extinct, so the ancestral condition for the
tetrapoda may have been gill and lung based respiration. The development
of cutaneous respiration could well be a derived feature of
lissamphibians. It seems their naked, unscaly skin is a derived feature
of the group. A number of early tetrapods such as ichthyostega and
Temnospondyli retain  scales. Maybe cutaneous respiration is coupled
with the small body size of lissamphibians. 

Adam Yates

 > Bill Adlam
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