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Big = Old = Advanced?

Beware: Watch out for falling massive generalizations.

First let me say that I'm aware that great chronological extent <>
large size <> evolutionarily advanced (otherwise, planet-sized Ligula
would rule the Internet), but it gets the point across.

Looking at the Mesozoic from the perspective of a casual observer, it
seems to me that the dinosaur history is one in which spurts of diversity
are followed by periods of size increase.  All of the major groups
(actually, let's make that "groups that maintain high popularity as
colored marshmallows"  I have no idea which groups are actually "major")
seem to appear as small-medium sized animals, then show a general trend
toward increasing size before settling at a size that may not match the
peak, but greatly surpasses the initial state.

Sauropods certainly follow the medium-large-huge-OBoythat'sBig-huge trend.  
Ceratopsians seem to go the same route, as do Tyrannosaurs, Nodosaurs, and
several others.

My question is: does large size represent such a specialization in its
own right, that it limits future diversity from this stock?  Or, turning
it the other way around, must a dinosaur be small in order to be
generalized enough to spawn a new major group?  
Offhand, I can't think of any "big-animal" group that comes from
big-animals.  (Hadrosaurs?) If this is actually the case, it
has some interesting consequences.  Among other things, it would suggest
that once all members of a group were Living Large, the future diversity
of that group was limited.

Of course, this is most likely a case of law generated through ignorance
of facts.

Mark Sumner
DEVIL'S TOWER sampler at http://www.inlink.com/~range