Pachycostasaurus

Pachycostasaurus had ribs and belly bones that were so thick they looked like sausages. This condition is called pachyostosis, and it also happens in modern dugongs and manatees.1

Its flippers were small compared to its body, and its neck was short. This made it a slow swimmer, unlike many other pliosaurs.1

The only known fossil of Pachycostasaurus was found by Alan Dawn, an amateur geologist and museum volunteer, in a quarry in Peterborough, England.1

Its head was about 20% of its total body length, which is typical for pliosaurs of its time.1

Pachycostasaurus lived about 165 to 161 million years ago, during the Middle Jurassic period.2

The bones of its backbone had a large canal for the spinal cord, and some parts were not fully hardened, suggesting the animal might not have been fully grown when it died.1