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RE: Dinosaur diversity



This brings up an interesting point.  It is usually argued that
dinosaurs "prevented" mammals from getting much bigger than a cat, by
occupying the niches that would otherwise go to large mammals.   This is
a partial motivation for Bakker's "superiority of the dinosaurs"
argument.

As Nick points out aquatic environments are a case where reptiles seem
to have kept both dinosaurs and mammals out!

There must have been isolated Mesozoic ecosystems in which these
constraints are broken.

Today on earth we know that isolated islands (Galapagos, Aldabra) exist
where reptiles (giant tortises) are the largest terrestrial herbivores
or islands (Komodo, Flores...) where giant reptiles (Oras or Komodo
Monitors) are the largest terrestrial predators.  This is not limited to
terrestrial environments - there are plenty of places where crocodilians
are the largest aquatic predators.   

The same is true for isolated environments of the recent past - i.e. the
extinct megafauna of Australia, which included giant reptiles (monitors)
and marsupials. The extinct Moas of New Zealand and Elephant Birds of
Madagascar.

So, by extension, I wonder if there weren't some isolated terrestrial or
aquatic ecosystems in the Mesozoic (for example islands or landlocked
lakes...) in which you found:

-  Aquatic dinosaurs or mammals.   This might happen in a land locked
lake which could never get mosasaurs etc..  After all IF competition was
the problem, why not fill the niche? 

-  Large Mesozoic mammals.   Presumably these would occur in ecosystems
where you had mammals, but which there were no . 

-  Finally, you could also postulate giant birds or reptiles (much like
today or the recent past) in ecosystems that lacked both dinosaurs and
mammals.

In either case there would seem to be sufficient time.   An early
mutation rate calculation done to support evolution showed that a mouse
sized mammal could (under the right circumstances) be expected to evolve
into elephant size in a miminum of 60,000 years, and rather comfortably
in 100,000.   Although elephant size is not necessarily expected on a
island, at the very least you could imagine an early mammal stranded on
a Mesozoic island to reach a few hundred kilograms (like the extinct
giant birds and tortises).    During the Mesozoic there was AMPLE time
for isolated ecosystems to evolve all sorts of "exceptions that prove
the rule" like this.

The most likely answer to this is that it might well have happened, but
we won't know unless we could survey the fossil record for isolated
ecosystems of the Mesozoic.  This is tough due to spotty preservation
and many other problems.  Still, it is an interesting thought.

Nathan