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RE: Dinosaur diversity

land-locked lakes are features of glacial landscapes, but very rare
in non-glaciated landscapes or those without lava-dams.  even so 
the latter only last a few thousand years.  only in glaciated landscapes
do large landlocked lakes occur with any regularity.  and even they are
occasionally invaded by the sea.  to have enough time for a species to
evolve, you must have the landlocked lake around for a few million years.
very rare for any lake, even the size of lake ontario or superior. 

that leaves one other possibility and that is fault bounded lakes like
lake baikal or the east african rift lakes.  these, however, are usually
rather small in area limiting the availibility of primary food (algae,
etc.) and hence productivity.  but at least these do persist for million
of years.  the problem is that many tend to be very salty, much more
so that seawater, which again limits possiblities for larger life forms.
shallow lakes in tropical regions like lake victoria seem to dry out 
repeated during their histories and that precludes large life forms

so the problem for large lakes not attached to the sea is lack of suitable
conditions, time, area, salinity, food sources.  
Bonnie A.B. Blackwell,                          bonn@qcvaxa.acc.qc.edu
Dept of Geology,                                off: (718) 997-3332
Queens College, City University of New York,    fax: (718) 997-3299
Dept of Earth \& Environmental Sciences,        messages: (718) 997-3300
The Graduate Center, CUNY,                      
Flushing, NY, 11367-1597, USA