Re: New refs

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

"Brian Lauret" <zthemanvirus@hotmail.com> wrote:

> Considering "hysilophodontids" were the ancestral Iguanodontoids it seems 
> rather strange they were endotherms while their 
> descendants,hadrosaurs,weren't.


Perhaps not so strange.  It helps if you avoid thinking of "endothermy" and 
"ectothermy" as polar opposites, and think of them as two ends of the same 
spectrum.  Among modern mammals, small mammals can be regarded as "more" 
endothermic than big mammals, since their high surface area to volume ratios 
renders them more prone to loss of body heat.  Also, some fishes (tuna, some 
sharks) show a degree of endothermy.

In the course of dinosaurian evolution, individual lineages may have slid 
across the endothermic-ectothermic spectrum, depending on many factors: body 
size, required activity levels, diet, climate etc.  The small size and 
cursorial habits of hypsilophodontids may have favored a greater emphasis on 
endothermy than their bulkier relatives (e.g. hadrosaurs), who could rely 
more on their sheer body mass to retain heat.



Tim

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