Re: sauropods: homotherm,heterotherm or gigantotherm?

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> Firstly there is evidence that many sauropods ate tree leaves. Including 
> (well... mainly) conifer needles. This plentiful food resource is today 
> exploited only by insects (and the occasional sloth, specialized monkey or 
> weird bird), and the sauropods hardly seem to have had competition in that.

What about large ornithopods or therizinosaurs?  Couldn't they have munched 
on conifers too?

> And fourthly, make sure you don't overestimate the body masses of 
> sauropods. Their vertebral columns usually contained more air than bone...

And to emphasize David's Point, for diplodocoids (like _Seismosaurus_) most 
of the body length was made up of the ridiculously long neck and tail.

> > Do naked mole rats and ,possibly,crocodiles count as
> > heterotherms descended from homotherms?
>
> Naked mole rats are about half homoiothermic. :-) Quite little research has 
> been done on Jurassic and Triassic crocodyl...omorphs...

There is one recent study that argues that the first crocodylomorphs were 
endothermic, and that the ectothermic metabolism of modern crocodilians is 
secondary:

Seymour, R.S., C.L. Bennett-Stamper, S.D. Johnston, D.R. Carrier, and G. C. 
Grigg (2004).
Evidence for endothermic ancestors of crocodiles at the stem of archosaur 
evolution.  Physiological and Biochemical Zoology 77: 1051?1067.

Abstract: "Physiological, anatomical, and developmental features of the 
crocodilian heart support the paleontological evidence that the ancestors of 
living crocodilians were active and endothermic, but the lineage reverted to 
ectothermy when it invaded the aquatic, ambush predator niche.  In 
endotherms, there is a functional nexus between high metabolic rates, high 
blood flow rates, and complete separation of high systemic blood pressure 
from low pulmonary blood pressure in a four-chambered heart.  Ectotherms 
generally lack all of these characteristics, but crocodilians retain a 
four-chambered heart.  However, crocodilians have a neurally controlled, 
pulmonary bypass shunt that is functional in diving.  Shunting occurs 
outside of the heart and involves the left aortic arch that originates from 
the right ventricle, the foramen of Panizza between the left and right 
aortic arches, and the cog-tooth valve at the base of the pulmonary artery.  
Developmental studies show that all of these uniquely crocodilian features 
are secondarily derived, indicating a shift from the complete separation of 
blood flow of endotherms to the controlled shunting of ectotherms.  We 
present other evidence for endothermy in stem archosaurs and suggest that 
some dinosaurs may have inherited the trait."

> There is one known living gigantotherm: the leatherback turtle 
> (*Dermochelys coriacea*).

Tuna and some sharks (e.g., great white shark, makos) also have some kind of 
endothermy ("regional endothermy"), and the muscles (and perhaps overall 
body mass) play a role.  This gigantothermy (= mass homeothermy) issue tends 
to be very murky and complicated, especially given that these particular 
vertebrates (leatherback, tuna, lamnid sharks) spend either all or most of 
their time underwater.

Cheers

Tim