|An artist's impression of the Melksham Monster|
The marine predators were 10ft-long with strong jaws and serrated teeth that allowed it to feast on large prey, such as prehistoric squid.
The Ieldraan melkshamensis, named the Melksham Monster after the Wiltshire town where the fossil was found, once roamed the seas of Europe.
But the fossil was acquired by the Natural History Museum in 1875 and sat in its archives for almost 150 years before a closer look found it held information about the origins of the distant relatives of today's crocodiles.
It was thought that the sub-family of prehistoric crocodiles, including the new species, came from the Late Jurassic period, between 152 and 157 million years ago.
But palaeontologists at the University of Edinburgh investigated further and found it was actually much older: around 163 million years.
They used the monster's distinctive skull, lower jaw and teeth to identify it as a new species.
Mark Graham, senior fossil preparator at the Natural History Museum, explained the long and difficult process, saying the animal was "one tough old croc in life and death".
"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 fossilisation.
"This unyielding matrix had to be removed by force, using carbon steel tipped chisels and grinding wheels encrusted with industrial diamonds.
"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."
Dr Steve Brusatte, from the university's School of GeoSciences, said: "The Melksham Monster would have been one of the top predators in the oceans of Jurassic Britain, at the same time that dinosaurs were thundering across the land."
The study is published in the Journal of Systematic Palaeontology.