Proneusticosaurus

Proneusticosaurus had very dense, heavy bones, especially in its legs and ribs, which helped it sink and swim underwater.2

Scientists counted seven growth rings in its thigh bone, showing it grew slowly and lived for several years.2

Its vertebrae were shaped like barrels, a condition called pachyostosis, which made its spine extra heavy.2

Proneusticosaurus is one of the oldest known eosauropterygians, a group that includes plesiosaurs.2

Only a few fossils of Proneusticosaurus have been found, and many were lost during World War II.2