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Re: cause of Gigantism in sauropods



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.

> 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 makes a lot more sense. Growing large to defend your offspring from 
predators (and yourself, 
albeit incidentally) would seem a better source of selective pressure, since 
you have your own 
parents protecting you until you reach such a size. It would seem to require 
quite a bit of altruism 
on the part of the parents, and perhaps other herd members, though. 

It does beg the question; How long did parental care last in sauropods? Was 
defense of the young 
something the whole herd contributed to (much as elephants will help to defend 
another herd 
member's calf), with such protection extending well into adolescence?

Large body sizes would seem that have been massive over-kill for defending the 
eggs themselves 
though. Any theropod large enough to threaten the eggs or hatchlings of an 
adult sauropod would 
surely not consider such small prey as worthwhile. Defense of the nest would 
have been better 
aimed at the smaller, stealthier egg thieves (like giant pythons) - against 
which the decreased 
speed and agility of such large body sizes would have been disadvantages.

Intra-specific nest site competition, on the other hand, may very well have 
contributed to increases 
in body size. The largest and strongest sauropods could have claimed the 
central territories of 
nesting sites, leaving the smaller and weaker parents to futilely battle 
stealthy egg thieves on the 
periphery of the colony.

-- 
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Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia               http://home.alphalink.com.au/~dannj
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