Judyth Sassoon of the University of Bristol discovered this pliosaur had arthritis. Image: Simon Powell.

The pliosaur, a prehistoric whale-like animal, may have been menacing to other creatures of the sea in the Jurassic Era, but at least one had a very modern-sounding malady.  Judyth Sassoon, a paleontologist at the University of Bristol in the UK, found that it actually suffered from arthritis.

The reptiles lived about 150 million years ago. This particular pliosaur was about 26 feet long and had eight-inch teeth, but Sassoon said the fossil's left jaw joint was eroded and the jaw was shifted to one side. The jaw showed signs that the pliosaur had dealt with the arthritic jaw for many years, with tooth marks from the upper jaw in the bone of the lower jaw. Similarly, a tooth from the lower jaw apparently caused an infection in a tooth socket of the upper jaw.

"It may be surprising that the pliosaur could survive with a deviated jaw, and yet the erosive marks on the jaw indicate a prolonged misalignment so the animal must have been able to feed in spite of its disease," Sassoon told ABCNews.com. "It is also surprising that other pliosaurs did not take advantage of it. One would expect such deadly predators to hunt weakened specimens of their own kind. Perhaps the sheer size of our specimen put the others off?"

Pliosaurs had crocodile-like heads, whale-like bodies, short necks and giant flippers. The creatures were the top predators in the marine environment, so other animals would not have hunted them, and they likely lived to "ripe old age."  Sassoon said conditions like arthritis are rarely seen in fossils.

An estimated 50 million adults in the U.S. have some sort of arthritis or related disease, according to the CDC.

"Our findings show that, as these animals aged in years, they, like humans, who are also the top predators, succumbed to diseases of old age," said Sassoon. "And that is an interesting and new observation."