Re: Not dinosaurs but chondrichthyans

["Tim Williams" <twilliams_alpha@hotmail.com>]

Colin McHenry wrote:

> , but anything that gets people to question the old chestnut about sharks 
> being some relict primitive group is good in my book. I used to emphasise 
> to students that white sharks are late model, highly evolved, warm blooded, 
> live young bearing, mammal eating machines). The standard doco line about 
> sharks being some relict group of monsters is absurd.

I agree entirely, Colin.  The modern shark clade (= crown group Neoselachii) 
does not appear in the fossil record until the Mesozoic.  Alas, the word 
"shark" also tends to be applied to many basal chondrichthyan or basal 
elasmobranch taxa.  But modern sharks are more closely related to rays than 
to fossil 'sharks' such as _Cladoselache_ and _Hybodus_.

White sharks, and a few other lamniform sharks, are warm-blooded in the 
sense that they display something called "regional endothermy", the 
anatomical mechanism for which I've never fully understood.  It does allow 
these sharks to raise their body temperature above that of the surrounding 
water.  Some rays can do it too.



Tim

_________________________________________________________________
Get rid of annoying pop-up ads with the new MSN Toolbar ? FREE! 
http://toolbar.msn.com/go/onm00200414ave/direct/01/