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cause of Gigantism in sauropods



Reading: Biology of Sauropod Dinosaurs, P. Martin Sander, et al.,
(2010) I was flummoxed by its logic. They offer a simple explanation
of Cope's rule: "...as life diversifies, there is always room for body
size to expand in one direction: to the top. As habitat is partitioned
and ecospace becomes crowded, one way out is evolution towards larger
body size" (pg. 8). They call this a "...simple (if not simplistic)
explanation..." and then go on to use it as total justification for
their thesis: "Given that Cope's Rule in its most general formulation
is valid...the question with regard to sauropod dinosaurs must be what
limited their body size...not what drove body size increase" (pg.20).
Then follows a description of the marvelous adaptations which
successfully removed those limits.

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.
This had to put a premium on defense, a factor that mammals with baby
inside do not have to worry about, i.e., they can simply run away. As
a hypothesis it can stand in line behind all the others, but it should
not be ruled out on the basis of the above arbitrary claim. And 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.

So my question is: how do the relative sizes of lions vs. elephants
compare to that of therapods vs. large sauropods? At least with the
taxa shown in the paper, it looks to be in favor of therapods. (I
understand that predator/prey relationships were diverse over the
years)