Naturalist and TV presenter David Attenborough unveiled the fossil at the Dorset County Museum, the Dorset Echo reported.
Only the skull of the pliosaur was discovered by a fossil collector in a cliff at Weymouth, the BBC reported. The area is so rich in fossils it is known as the Jurassic Coast.
Scientists estimate the pliosaur may have been up to 60 feet long. The skull is one of the most complete ever found.
Kevan Sheehan told the BBC he spent five years between 2003 and 2008 gathering bits of the skull as they worked their way out of the cliffs.
"It was sheer luck -- I was sitting on the beach, and saw three pieces," he said. "I had no idea what they were, but I proceeded to drag them back. Then over several years, I'd go back every year and find a new piece. I'm a beach magpie."
The discovery was announced in 2009. The museum paid Sheehan 10,000 pounds ($16,000) with the owner of the land where it was found getting the same amount.
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