part troi

the plesiosaur may have developed a system similar to the oils in a leatherback turtle, in that its skeleton and skin was suffused with a high concentration of polyunsaturated fatty acids. This oil would absorb the nitrogen bubbles more effectively than normal tissues, and keep the nitrogenous wastes suspended until a time when they could be excreted as waste products without harm to the animal. This would only be a secondary precautionary development,

Most importantly, the plesiosaur would only have one breath to take with it down to depth, and not constantly breathing compressed gasses like human divers do (a prehistoric scuba tank would no doubt raise more questions than answers at this point!!) the benefit of this is that the plesiosaur only takes down with it what nitrogen it got from that one breath. The 70-ish% of nitrogen it got from that one breath would hardly be enough to supersaturate its blood or tissues at any depth, and would be easily decompressed to the same volume when surfacing. The lack of a continuous supply of nitrogen and oxygen would diminish the chance of the plesiosaur developing the common diving artefacts of nitrogen narcosis, decompression sickness and more seriously, oxygen toxicity, that human divers can develop during and after a long dive.