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Re: New refs (medium)
Actually, Nick's take is basically correct for the one part of my arguement.
Go back to my first post; I'm not claiming that placental RMR variation is
proof of dinosaur metabolic variation, I'm using them as part of a two-pronged
strategy to falsify sloppy research that attempts to provide evidence that
dinosaurs used disperate metabolic stratagies. The claim is that because
modern animals (in the most recent case, crocs)show fewer fluctuations in body
temperature when they are large, that we should expect big dinosaurs to have
lower metabolic rates. than smaller ones. My response is:
1) They are confusing RMR constants (which tell us about metabolic capability
and better predict aerobic excercise capacity) with mass-specific RMR rates.
In point of fact, big crocs have the same RMR constant as small crocs, rhinos
the same as small ungulates, etc. The idea that big animals reduce their
mass-specific RMR rate is a fact, the idea that true RMR levels (i.e. RMR
constants) scale with size is a MYTH.
2) Modern animal groups do not provide good support for the idea that we should
"expect" to see so much metabolic variation in clades of similar adaptive
variety. If elephants and rhinos were ectotherms (or even had lower-mammalian
levels of RMR constants), that would be good positive support for the idea that
large extinct terrestrial tetrapods should demonstrate a similar range. In the
absence of modern examples that match the hypothesized metabolics of dinosaurs,
they provide ....no good evidence to support it; my original statement.
Again, I'm not claiming that dinosaurs _couldn't_ have had disperate
metabolic stratagies, just that modern analogs don't. Umless you want to get
back to lamniforms, monotremes, or insects being good analogs for dinosaurs.
<Land mammals never got as large as the larger sauropods. I think that tells
What do you think it tells us? Remember that indricotheres got larger
than the vast majority of dinosaur taxa. Also remember that no extant
terrestrial tetrapods reduce their RMR constant based on size. So why on earth
should we expect dinosaurs to have done so? We know that dinosaurs grew in a
way fundamentally different from even muti-tonne crocs. Should we imagine that
gigantothermy conveyed some growth advantage to big dinosaurs but not to big
crocs? How could this work? Only organisms with elevated RMR constants grow
quickly in the wild, regardless of temperature (nile crocs do not grow faster
just because it's warmer) what alowed this to be different in the past?
Shouldn't there be some burden of proof on those who propose that extinct
animals have some unobserved metabolic strategy? Shouldn't they at least
propose coherent mathematical models that fit into our understanding of RMR and
It may well be that dinosaurs had intermediate (or highly variable)
metabolic strategies, but most of the models presented so far have been based
on intuitive mush rather than empirical evidence. They may not be wrong, but
they need to provide better evidence.