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Re: no easy (mammal) answer

Why were dinosaurs big and mammals little?  How about body heat?  For an
endotherm, staying warm depends on metabolism, mass, and insulation.

I suppose the smallest dinosaurs were about the size of chickens or
human babies.  Babies are capable of incredibly high metabolism.  A hen
has enough metabolism to survive a freezing, windy night even when
molting leaves skin exposed.  In such a situation, a smaller bird would
probably perish.

There is controversy about whether dinosaurs had respiratory
turbinates.  Some say the absence of RTs proves dinosaurs were not
endotherms.  Others say dinosaurs must have had RTs because there is
other evidence that they were endotherms.  

A third possibility is that dinosaurs were endotherms but didn't breathe
in the same way as any modern animal.  After all, birds and mammals
don't breathe the same.

Perhaps dinosaurs needed a certain body temperature, but the dinosaur's
aerobic system could not be adapted to the extremely high metabolism
necessary for a shrew or a hummingbird.  That would explain the absence
of tiny dinosaurs.

Why didn't mammals get bigger?  In all those millions of years, there
were bound to be times and places where dinosaurs left niches for
mammals to get toeholds as incrementally larger species.  Mammals
stayed  small.  For an animal over a few pounds, perhaps the dinosaur's
aerobic system had an important advantage over that of a mammal.

Say... about 30 years ago, did Jack Horner work as a jarhead in a
tropical paradise?

- Stephen Throop