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Re: cause of Gigantism in sauropods

2011/2/14 Tor Bertin <nightimeshadow@yahoo.com>:

> Why would we assume that they would by necessity feed on the canopy? Assuming 
> there's ample vegetation at the (say) 25 foot point of a 200 foot tree that a 
> given sauropod could reach, that alone would create a browsing-height based 
> selective pressure. In this case, plant height may have been driven by 
> competition for sunlight against other trees, while still providing a 
> selective force on sauropod neck size.

True... I think that in your scenario we would see a correlation
between maximal sauropod size and height of the lower part of the
canopy. This may be grossly related to tree height, but your precision
is right. Do we have evidence of the largest sauropods co-existing
with the trees with highest lower portion of the crown?
In the case you mention, larger sauropods than necessary to harvest
the lower part of the canopy may be rewarded with yet more available
resource (namely, leaves above that point).
I also forgot to note that if neck lenght increase (which likely
implies a larger amount of mass in the neck) had a limit relative to
body mass (for example, I doubt an organism can have 50% of its mass
being neck, if it contacts the ground with the legs instead of the
neck, as in sauropods), increase in neck lenght past some point would
also imply size increase.
If sauropods were unable to raise their necks, because of
physiological problems, as some people say, the correlation would be
between sauropod mass (and related strenght) and the relationship of
height to cross-sectional area in the tree, or better even, contact
area between tree roots and soil, if these dinosaurs ate tree canopy.
A further precision: not all sauropods, or most sauropods, would be
able to use as resource the tallest, or with tallest lower part of the
crown, trees in the same region. If the forest was partially destroyed
by the largest species adults, there would be young trees and smaller
plants available for the smaller sauropods, as well as other
Of course, all this correlation between heights and size is not to be
searched if the largest trees did not significatively decrease the
primary production by smaller plants - which I do not know.

Finally, sorry the insistence on attacking the arms race Red Queen for
sauropods and theropods, but I read in a recent paper by many German
paleontologists, that the population of adult sauropods likely had a
very low density, with the juveniles having a far greater density.
This would make difficult for predators to regularly find large
sauropods for food, even if they can overpower them. Because, in
addition, the population density of large theropods would be even
lower than that of the sauropod. This may make them less experienced
in bringing down this kind of prey, and perhaps more cautious when
finding it. In addition, long time fighting at these sizes may imply
much overheating, a true problem with these beasts.