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Re: cause of Gigantism in sauropods



2011/2/7 Dann Pigdon <dannj@alphalink.com.au>:
> On Mon, Feb 7th, 2011 at 2:30 PM, John Bois <mjohn.bois@gmail.com> wrote:
>
>> ... I would have thought it would rate a mention as an important difference
>> between extant mega herbivores and those of the Mesozoic. Indeed,
>> comparisons are frequently made between the two groups: early in the
>> paper predation is given credit for being the potential major
>> selective value of large size, i.e., it drives selection. This
>> apparent contradiction with the paper's thesis is resolved by saying
>> that sauropods, like elephants, outsized their predators, and that is
>> was limits on predator size (possibly due to bipedal body plan) that
>> made this no longer a driver of selection.
>
> The problem with growing large to outgrow *your own* predators is that you 
> first have to survive
> long enough for that to happen. This would have been especially difficult for 
> sauropods who started
> off so much more relatively smaller than their adult form, and who didn't 
> have the benefit of super-
> rich milk (a la whales) to grow fast on.

It seems, as far as I read, that histology suggests fast growth rates
for large dinosaurs, especially sauropods. Fast bone growth rates for
hadrosaurs have actually been seen as a way to avoid predation. I read
a paper (I think from Erickson) where it is claimed that Tyrannosaurus
grew faster than modern elephants.

>> Now, I have been nursing my own pet hypothesis for a score of years, a
>> hypothesis that argues that large size _was_ driven; driven by the
>> fact that dinosaurs above a size that inhibited concealment, had
>> to--if nest attendance was in operation--defend or abandon the nest.

What about exploiting a previously unexploited resource? (tree tops,
for example). Generally speaking, larger animals can, also, bite
harder, and are thus perhaps able to eat thougher stuff, although this
is not likely the case in sauropods. Large size can also expand the
preferred range of food in the case of predators, for tigers can hunt
from locusts to gaur, and I suppose the range is much lower for
smaller cats (may there be something similar in herbivores, accounting
for greater possible forces and thus an increase in the potential
range of size and thoughness of the exploited resource?).