Recognition of avascular necrosis through propodial head subsidence in fossils indicates that plesiosaurs were susceptible to decompression syndrome and implies deep, prolonged or repetitive diving behavior for these animals. Contrary to the situation in mosasaurs, plesiosaur vertebrae were not targets of avascular necrosis. This suggests that the anastomosing internal vascularization of plesiosaur vertebral centra was functionally adaptive. No phyletic trend is evident in the susceptibility of plesiosaurs to the pathology, although stem group sauropterygians were almost never affected (not surprising for near-shore, shallow water animals). Cryptoclididae were the only plesiosaur family not regularly afflicted with decompression syndrome. The difference between this and other groups is most likely to have been behavioral and/or ecological.