Were there any fresh water plesiosaurs?
I am a senior at Pennsylvania State University doing my thesis on the paleoclimate of southeastern Australia during the early Cretaceous. Sediments in the area are interpreted as being fluvial/lacustrine while my thesis advisor and I believe them to be marine.

Plesiosaur teeth have been found by Patricia and Thomas Rich in these "freshwater" sediments and they have interpreted theses plesiosaurs to be freshwater. Plesiosaur bones have also been found in the Judith River Group in Montana/Alberta in fluvial sediments.

A friend of my thesis advisor mentioned that large marine whales swim up rivers at times to free themselves of a number of parasites that depend on normal marine salinities.

My question to you is: Do you think that plesiosaurs could have inhabited freshwater? If not, could you offer any explanation to this "freshwater" plesiosaur phenomena?
Cruickshank (1997) reports that several plesiosaurs, especially those from southern continents, have originated from non-marine sediments, and quotes Bartholomai (1966), Molnar (1982, 1984) and Rich et. al. (1989 (note that Cruickshank mistakenly gives the year as 1991)) in support. The remains of Leptocleidus are known from fresh water deposits from the Isle of Wight, though nothing has been published. I have recently submitted for publication an paper on a fauna including small plesiosaurs from lagoonal or estuarine deposits from north Lincolnshire which straddle the Jurassic/Cretaceous boundary.

I can see no reason why plesiosaurs could not have inhabited fresh water - after all (and following a quick mental review) all modern secondary marine adapted animals have freshwater as well as saltwater forms. I suspect that many collections contain plesiosaur bones from freshwater deposits which have been misidentified. I've come across them labeled as dinosaur and pterosaur, as well as the classic 'reptilia indet'.