The ancient Jurassic seas would have been a terrifying place to take a swim. Now there's one more reason to be thankful that you weren't around some 160 million years ago: the so-called "Melksham Monster."

Paleontologists at the University of Edinburgh have identified the new species by studying a heavily damaged fossil that had been sitting in the Natural History Museum’s archives for almost 150 years. Due to the complicated state of its preservation, the specimen had remained something of a mystery until now.

“The specimen was completely enclosed in a super-hard rock nodule with veins of calcite running through, which had formed around it during the process of fossilization, explained Mark Graham, Senior Fossil Preparator at the Natural History Museum, in a press release. “The work took many hours over a period of weeks, and great care had to be taken to avoid damaging the skull and teeth as they became exposed.”

Distinctive features of the creature's skull, lower jaw and, in particular, its teeth, made it clear to researchers that they were dealing with a new species. The Melksham Monster, now officially named Ieldraan melkshamensis, was about 10 feet long with powerful jaws and serrated teeth. It would have been an apex predator in the warm, shallow seas that covered much of what is now Europe.

The discovery re-writes the book on the evolution of prehistoric crocodiles to which the new species belongs, known as Geosaurini. Previously, it was believed that this group of marine reptiles originated in the Late Jurassic period, between 152 and 157 million years ago. But the Melksham Monster dates to around 163 million years ago, turning back the clock to at least the Middle Jurassic.

"It’s not the prettiest fossil in the world, but the Melksham Monster tells us a very important story about the evolution of these ancient crocodiles and how they became the apex predators in their ecosystem," said Davide Foffa, at Edinburgh's School of GeoSciences. "Without the amazing preparation work done by our collaborators at the Natural History Museum, it would not have been possible to work out the anatomy of this challenging specimen."