How do we know plesiasours (sorry if I misspelled) were/are cold blooded?

Question

How do we know they are cold blooded, I think the title said it all.
If all we have found are fossils, which is all that claims to have been found, how would we possibly know they are cold blooded, it would just be an assumption.

Please only genuin answers.
I am wondering because of other research I am doing.
Thank you so much

Answer 1

Plesiosaurs are reptiles. All reptiles are 'cold-blooded', thus we assume plesiosaurs were. It is actually unknown whether dinosaurs were homeotherms ('warm-blooded; maintain a fairly constant internal temperature despite environmental temperatures) or ectotherms ('cold-blooded'; unable to maintain constant internal temperatures despite enviornmental temperatures). I suspect that they were less homeothermic than modern birds and mammals, but more so than modern reptiles.

Answer 2

CTVS gave a good answer (except he should have said "all _modern_ reptiles are cold-blooded"). One way to assess the 'blooded-ness' of animals that we only know from fossils is to look at the bone structure and compare it to that of modern cold-blooded and warm-blooded animals. That gives you a better educated guess. I don't off-hand know if that has ever been done with the marine reptiles, but I would not expect them to be 'warm-blooded'. Fish, after all, (including sharks) manage just fine despite their cold-bloodedness.

Answer 3

"They" say they are cold-blooded because "most" dinosaurs are also considered to be reptiles. Ever heard of a warm-blooded reptile?