Were they warm-blooded?
I am doing an essay which involves plesiosaurs, and I was wondering were they warm blooded. I know the great white, and one of the large tunas , and I think, one of of the sea turtles is.
I wish I knew the answer to this question!

You're right about tunas and sea turtles, though I believe that their homothermy is not as stable as that of mammals and birds. Chris McGowan wrote about it is one of his books - "Dinosaurs, Spitfires, and Sea Dragons" (I think). The difficulty is that there is little evidence in the skeleton to resolve the argument one way or another. In modern animals such as turtles, morphologially similar animals can have very different temperature control mechanisms. Tuna and Marlin are a good example of animals of a similar size, with similar lifestyles, one of which is endothermic the other homothermic.

I am hoping at some stage to be able to look at the varaition in isotope ratios along the spinal column of a specimen of Muraenosaurus we have in Leicester Museum, but this will depend on being able to raise the funds to do so. Such ratios can tell us about the temperature gradient along the body. If it is relatively uniform, the likelihood is that the animal was warm-blooded.

Similar work was done on a skeleton of Tyranosaurus about five years ago, and concluded that it was warm-blooded. I haven't been able to track down the reference.