Few data are available that show exactly how deep plesiosaurs dived. That they dived to some considerable depth is proven by traces of decompression sickness. The heads of the "humeri" and "femora" with many fossils show necrosis of the bone tissue, caused by a too rapid ascent after deep diving. However, this does not allow to deduce some exact depth as the damage could have been caused by a few very deep dives, or alternatively by a great number of relatively shallow descents. The vertebrae show no such damage: they were probably protected by a superior blood supply, made possible by the arteries entering the bone through the two "foramina subcentralia", large openings in their undersides.