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Re: Big = Old = Advanced?

In a message dated 97-08-21 07:43:03 EDT, garrison@efn.org (Garrison
Hilliard) writes:

<< >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.
 Crocodilians excluded? >>

Actually, crocs are a good example to support the contention that large size
brings on limited diversity. They've had a very long evolutionary history,
but basically they're unchanged from their Triassic ancestral forms. Sure,
there have been a few offshoot groups from time to time with suitably bizarre
anatomies, but these are all undeniably crocs and don't depart significantly
from the basic croc body plan. There were never thousands of species of crocs
alive at the same time, either, as there are with passerine birds, rodents,
lizards, and snakes.

Large animals needs lots of territory, and have smaller populations, longer
gestation and/or growth-to-maturity periods, and smaller litters/hatchings.
All these factors place clear limits on their potential for diversification.