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Re: New refs (short)

Quoting Tim Williams <twilliams_alpha@hotmail.com>:

> There is evidence that the agile, upright ancestors of crocodylians were 
> endotherms.  If endothermic sphenosuchians could give rise to ectothermic 
> crocs, why couldn't there be metabolic changeovers within (say) the 
> ornithopod or theropod lineages?  Your argument is "Because placentals all 
> have the same RMR."  My reply is "So what?"

If that is indeed his argument, then I agree it is a weak one.  However, on a 
slightly different interpretation, I think there is value in it.  Crocs 
invaded (and still inhabit) a niche (aquatic ambush predation) in which, it 
has been argued, it is beneficial to be ectothermic.  Placental mammals have 
occupied many of the same niches occupied by dinosaurs (large herbivore, small 
flyer), and yet none of these have become ectothermic.

I think the argument would be, starting (by hypothesis) from endothermic 
ancestors, dinosaurs occupied various niches for which placental mammals offer 
no evidence that ectothermy is advantageous.  They did not, to our knowledge, 
occupy the few niches (aquatic ambush predator, tropical communal burrower) 
occupied by animals we suspect may be secondarily ectothermic.  Thus, there is 
no reason to think that dinosaurs produced secondarily ectothermic forms.

At least that's my take.

Nick Pharris
Department of Linguistics
University of Michigan