Found in Dorset, the giant sea monster that was armed to the teeth

A prehistoric sea monster with gigantic jaws that could swallow a man whole and snap a car in half has been unearthed by an amateur fossil hunter at the foot of a cliff on the Jurassic Coast of southern England.

The creature's skull was nearly 8ft long and was armed with a fearsome array of massive teeth. It was a member of the pliosaurs, an extinct group of marine reptiles which lived about 150 million years ago as top predators in the sea, at a time when dinosaurs dominated the land.

It took several years for the collector, 62-year-old Kevan Sheehan, to carefully amass portions of the fossilised jawbones and skull as they emerged from the cliff-face after a succession of rock falls. His patience has been rewarded by selling the fossil to Dorset County Council for £20,000, using money from the Heritage Lottery Fund.

"In 40 years of collecting, I have often been green with envy at some of the finds other people have made. But now, when someone shows me a find, I can say: 'That's not a fossil – this pliosaur, that's a fossil'," Mr Sheehan said.

Richard Edmonds, the council's earth science manager for the Jurassic Coast, said: "This part of the coastline is eroding really rapidly, and that means the fossils that are trapped and buried are constantly tumbling out on to the beach.

"The collector was lucky enough to come along on the day a large piece fell out of the cliff, and that gave him the clue to keep on looking. He spent the next four years coming back day after day, and as a result he has uncovered this absolutely incredible fossil. It was an amazing effort."

The rest of the beast's massive body is probably still buried in the cliff. "The ground is dipping very steeply, and as it is such a huge specimen it will be buried beneath layer upon layer of rock, so we will have to patiently wait for the next big landslide," Dr Edmonds said.

Pliosaur fossils were known about in Victorian times, and often came from the same region of Dorset and east Devon. The latest find, probably the most complete set of jaws and skull to date, suggests that this particular specimen grew up to 50ft long.

"This new skull is the biggest complete skull of a pliosaur discovered so far, but there are some tantalising fragments suggesting even larger animals," said Dr David Martill, a paleontologist and pliosaur expert at the University of Portsmouth.

Pliosaurs had relatively short necks and swam using their four paddle-like limbs. Their long snouts had sharp teeth on the end for gripping prey, which would have been swallowed quickly into the back of the mouth where sets of ratchet-shaped teeth ensured there was no escape for the creature.

"Pliosaurs had big muscles on their necks, and you would have imagined that they would bite into the animal and get a good grip, and then with these massive neck muscles they probably would have thrashed the animal around and torn chunks off – it would have been a bit of a blood bath," Dr Martill said.

One specimen, known as "Predator X", was discovered last year in the permafrost on the Norwegian archipelago of Svalbard. Scientists estimated that its teeth could have been clamped together with a pressure of 33,000lbs per square inch, compared to Tyrannosaurus rex's bite strength of 3,000lb per square inch, and the 2,500lb square inch "death grip" of today's alligators.

Richard Forrest, a paleontologist and pliosaur expert who has analysed the specimen, said it made T-rex look harmless. "If we look at the lower jaw, this is the point at which the muscles attach, and then you've got the great beam coming forward – that bone is roughly the strength of steel, so think of the strength of a steel girder that size," Dr Forrest said. "These things were powerful enough to bite a small car in half. It would take a human in one gulp. It would take T-rex in one gulp. Compared to this beast, T-rex was a kitten," he said.

There were many kinds of pliosaurs in the seas during the Jurassic period. They fed on a range of prey, including fish, dolphin-like ichthyosaurs and smaller species of pliosaurs. The specimen unearthed on the Dorset coast is exceptional in that it has not been crushed flat like so many other pliosaur fossils.