[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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 
us something.>

     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 
AEC behavior?
     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.


Scott Hartman