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Re: sauropods: homotherm,heterotherm or gigantotherm?



> Date: Thu, 08 Sep 2005 22:26:04 +0200
> From: Brian Lauret <zthemanvirus@hotmail.com>
> 
> Homothermy [in sauropods] would seem allmost impossible to me. How
> could an animal as large as a Seismosaurus have met its food
> requirements for one day within that very same day?

Well.  If a sauropod was endothermic, it could gather food much more
quickly than an ectothermic counterpart could.  More important, it
would have much more efficient digestion (recall that in general a ten
degree difference in temperature doubles the speed of all chemical
reactions).  Would these factors outweigh the increased food
requirements of the endothermic version?  I don't know, and neither
does anyone else, as no-one's ever published any numbers on this.

However, Greg Paul's 1998 paper _Terramegathermy and Cope's Rule in
the land of titans_ (Modern Geology 23:179-217) makes a strong case
from simple observation of extant and fossil critters that _only_
endotherms can get big: apart from dinosaurs, whose metabolic strategy
is still controversial, the only big terrestial animals (defined as
"more than a tonne in mass") have been mammals, hence endothermic.
The biggest tortoises and lizards (e.g. _Geochelone_ and _Megalania_)
may have approached a tonne, but we have no evidence that the
significantly exceeded it.  In contrast, we have extant elephants
around ten tonnes, extinct ones that were probably closer to fifteen
tonnes, and twenty-tonne extinct rhinos.  Not to mention oddities like
_Megatherium_, the giant ground sloth.  Paul's argument boils down to
"If ectotherms can grow larger than endotherms for metabolic reasons,
why don't they?" which I find pretty darned convincing.

As if that weren't enough there is the histological evidence of Martin
Sander's 2000 paper _Longbone histology of the Tendaguru sauropods:
implications for growth and biology_ (Paleobiology 26:466-488), which
seems to show pretty conclusively that sauropods grew _fast_.  In
particular, one well-preserved femur of _Janenschia_ (femur Ja2) was
determined to have attained "sexual maturity" (i.e. a levelling off of
growth) in 11 years, and maximum size around 26 years.  Table 1 (on
p. 468) shows that this femur was 127cm long, which is indicative of a
mass around 20-30 tonnes.  So this thing was averaging a tonne of
growth per year, or nearly 3 kg per day.  Show me an ectotherm that
can do that.

In conclusion, the balance of evidence seems to indicate endothermic
sauropods, or at least a metabolism elevanted in comparison to that of
extant reptiles.  But, be warned, plenty of people -- including Paul
Upchurch -- disagree with me.

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor  <mike@miketaylor.org.uk>  http://www.miketaylor.org.uk
)_v__/\  "If you really want to be a writer, develop calluses on your
         pride" -- Adrian Bedford.