Re: Platyhystrix and dinosaur humps/sails

[Jason Anderson <jander3@po-box.mcgill.ca>]

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?

Its an armoured dissorophid temnospondyl, like the more familiar
_Cacops_.  There is another closely related form, _Aspidosaurus_, with
very high neural spines that are expanded laterally rather than
compressed.  _Platyhystrix_ was a respectable size (sail spans approx.
25 cm).

Whether they are crown or stem amphibians depends on your views of
amphibian evolution, and whether you follow the crown-based definition
of Amphibia.  If lissamphibians are monophyletic and arise within
temnospondyls, they are stem amphibians, following the crown-based
definition.  If they are monophyletic and arise within the lepospondyls,
then they are stem tetrapods. If lissamphibians are diphyletic (with
caecilians deriving from microsaurs), then they would be crown
amphibians.  Clear? ;-)

> 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...

The large size some of these critters attained, coupled with the dermal
ossicles of this family, suggests to me at least that cutaneous
respiration would not be sufficient.  Modern amphibians also breathe
using a buccal pump (I'm watching an aquatic caecilian _Typhlonectes_ as
we speak), reserving dependence on cutaneous respiration for periods of torpor.

Jason
-- 
******************************************************
Jason Anderson
Redpath Museum, McGill University

http://www.mcgill.ca/redpath/jsahome2.htm 
jander3@po-box.mcgill.ca