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Re: gigantism as liability
It depends on what sort of metric one uses - large size invokes special
mechanical constraints, and thus represents a sort of biological
"solution" to interesting problems. At the same time, I agree with your
sentiment that "bigger is better" is a subjective tendency that probably
clouds some analyses. Particular size ranges are only "best" in the
context of specific circumstances.
Sizes larger than those achieved by terrestrial mammals have two manifest
advantages, off the top of my head: animals with such sizes can live off
very poor fodder by simply eating more of it, and "humungous animals get a
lot less predator attention than large ones" (Mark Witton).
One of the primary size effects is probably seen in neonate size, in fact,
more than adult size - sauropods apparently maintained a relatively
"r-selected" type breeding strategy in terms of juvenile size, juvenile
growth, and clutch size. This probably works very well under a wide range
of conditions, but may become a liability under specific circumstances.
Such an r-strategy allows a species to survive the complete extinction of
all adults. Maybe that's why the sauropods were such a success story,
surviving the Triassic-Jurassic, Jurassic-Cretaceous, Aptian-Albian,
Cenomanian-Turonian and probably a few smaller mass extinction events.