How did plesiosaurs breathe?
Total Layman here, but it occured to me, how did plesiosaurs, especialy elasmosaurs and kin, breathe? The head just seems to small to admit enough air for an animal that spend smost of its time underwater
The simple answer is that they must have had big lungs!

It's certainly a problem in imagining how an elasmosaur with a 7 meter long neck could breathe. The volume of air needed to fill a trachea over 7 meters long must be considerable, and one can imagine a situation in which the process of breathing was simply shunting the same air up and down the neck. Of course, reducing the diameter of the trachea reduces this problem - but then we have the problem of fitting enough air through a narrow tube.

My best guess is that they had, as I said, big lungs. There are other possibilities such as a system of air sacs as are found in modern birds which allow air to be circulated rather than simply shifted back and forth, or that the skin had some respiratory function. There is however no fossil evidence to support either of these hypotheses.