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At least one reason why dinosaurs were big

Back on the first of September Nick Longrich wrote:  

> ...why did dinosaurs get so large?  Obviously the environment has to
> select for it.  But does the environment no longer select for these
> kinds of giants, or is there simply nothing today living that can
> respond to such selectional pressures the way dinosaurs did?

Being big has advantages: immunity from predation, exploiting big
niche items such as tall trees and big prey, needing less
energy/gm of body mass (therefore making it easier to maintain
temperatures) among others.  But it also has disadvantages: blood
pressure and heart problems, mechanical stresses on tissues,
inability to hide from predators...

There have been very big mammals but by and large Nick's point is
right: without any obvious causative differences in paleo vs.
modern environments the dinos tended to bigness whereas modern
mammals tend toward medium to smallness.

I believe the main cause of this, was the reproductive style of
dinos.  The rule IT IS BETTER TO RUN THAN FIGHT can be obeyed at
all times by mammals.  Dinos could obey it only until egg-laying
season.  They were unable to hide their eggs so an untended nest
would have been a free meal for varanids, crocs, birds, other
dinos, and even, possibly, as the Cretaceous wore on, mammals. 
For this reason dinos probably guarded their nests.  But this set
up a behavioral imperative: they must stand and fight or lose
their nest.  As a group they could not be small (mammals now
precluded this with, among other things their more fit stealthy
reproduction) so their reproductive bouts could be readily
observed by those who would eat their babies.  So it was not
simply a case of just laying another clutch--every clutch was
observed and highly desirable!  This necessitated vigorous nest
defence and an obvious response would be to get bigger to defend
better.  This started an arms race which heated up over the
Cretaceous where predators got bigger, then herbivores, then
preds and so on.  But physical laws put an upper limit to this
arms race.  It couldn't go on forever.  This would be especially
true if new, differently gifted small animals, using rat-like
stealth or aerial skills, and for whom the bigger the dinos were
the easier they were to raid, stole or sucked dino eggs dry with

There is no evidence for this last idea.  But there can be no
doubt that dinosaurs had to fend off monstrous predators from
their nests.  Being bigger made this easier!