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Dinosaur size determinants.

I think it was Jim Kirkland who proposed the idea that dinos may have been
big because of the benefits of the need for less energy/gram of body mass
in big animals.  This would, presumably, allow the utilization of poor
quality forage.  This selective pressure was at first very attractive to
me (though it got in the way of _my_ idea that the need to defend a
fixed-site baby drove an arms race of inceasing size).  But now I have a
question.  The graph familiar to all of us which shows a straight line
from shrew->elephant has (I think) both axes on a logarithmic scale.  When
you convert this to a real number scale, you see that the
line hugs the x axis pretty much after horses.  There would be only a very
very tiny difference between elephant and, say, a sauropod.  On the other
hand tiny diferences in selective pressures can, over time, lead to big
changes in morphology.  And big creatures may have an advantage in
processing food that also helps.