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RE: Dinosaur diversity

I was imprecise.  "Landlocked" is not really the requirement - indeed it
would be something of a problem because you'd like to have fish in the
lake too, otherwise there is nothing for the hypothetical aquatic
mammals or dinosaurs to eat.   Some landlocked lakes have fish - many do

All it really has to be be for the argument is "mosasaur-free".  A lake
connected to the ocean by a river with waterfalls etc. which would
prevent large aquatic reptiles from entering is a sufficient condition.

The Mesozoic was long enough, and the world big enough that at least
some aquatic environments that lacked aquatic reptiles are likely to
have existed for say a million year span.   Of these, some may have been
too mineralized to support life, or otherwise unsuitable.  But still, it
seems pretty likely that a hospitable environment did occur, although it
is possible that none did.  

Islands in which major groups were missing seem even more likely - i.e.
islands without dinosaurs, but with reptiles, mammals or birds, should
be reasonably common.  The advance and retreat of the inland seaways
must have isolated some ecosystems for a million years or so.  Some of
these, by luck would have an idiosyncratic distribution of species.
Plate tectonics should have been churning up just as many island arcs as
it does today.  The same mechanisms for getting species to those new
islands must have existed.

I will grant that it may be hard to find fossilized remains of such an
isolated ecosystem.   It certainly would be fascinating if we did.

Part of the reason to speculate about such systems is that they might
illustrate .  As an example, most isolated island systems are mammal
free, so giant reptiles and birds fill the niches.  There are even
islands with giant insects (St. Helena, New Zealand).  Why does it seem
harder to get mammals to these places than everything else?   Why didn't
either mammals or dinosaurs compete in marine environments?