The fossilised bones of a giant sea monster nicknamed Godzilla have been unearthed at the foot of the Andes mountains in Argentina.
Scientists believe that the extinct creature was a marine predator between 65 million and 250 million years ago when dinosaurs ruled the land.
The beast measured 13ft from nose to tail and had jaws a foot and a half long which were armed with four-inch serrated teeth that could interlock to form a vice-like grip.
Diego Pol, a researcher at Ohio State University who helped to identify the find, said that the sea monster belonged to a crocodile-like group of reptiles that spent all their time in water.
"The animal is the most bizarre marine crocodile known to date," he said. "This species was very unusual, because other marine crocodiles that were around at the same time had very delicate features long, skinny snouts and needle-like teeth for catching small fish and molluscs. But this croc was just the opposite. It had a short snout, and large teeth with serrated edges. It was definitely a predator of large sea creatures."
Dr Pol added: "This was a top predator that probably swam around using its jagged teeth to bite and cut its prey, like dinosaurs and other predatory reptiles did."
Crocodiles evolved during the late Permian period and became widespread during the Cretaceous period from 146 to 65 million years ago.
Details of the find, released by the journal Science, reveal that Dakosaurus andiniensis had a hefty jaw capable of crushing large prey a marine equivalent of Tyrannosaurus rex.
Zulma Gasparini and Luis Spalletti of the National University of La Plata in Argentina found the fossilised bones at a site in Patagonia.
"This sea croc reminded me of the famous Godzilla coming out of the sea, " Dr Gasparini told the National Geographic Society, which helped to fund the expedition.
Unlike modern crocodiles, Dakosaurus had fins instead of legs but still used its immensely powerful tail to swim like today's members of the crocodile family.
Scientists believe that no other crocodile living at the time of Dakosaurus were as large as the reptile, or as robust. At the time that Dakosaurus lived, the area that is now Patagonia formed a deep tropical bay in the Pacific Ocean which was richly populated by fish, molluscs and smaller reptiles.
Fossils of all other crocodile-like animals alive at the time show that they had long, slender snouts and needle-like teeth, rather than the thick-set head and jaws and blunt, peg-like teeth of Dakosaurus,
"This is the most remarkable change in the size of the teeth and snout in the history of marine crocs," Dr Pol said, adding that the results indicated a greater diversityof the crocodiles in the Jurassic period than was thought.
Excavations in the desert region of Neuquén province have resulted in three specimens of Dakosaurus being uncovered so far, including an intact skull and several vertebrae.
The shape of the fossils at first presented a puzzle, but Dr Pol was finally able to map the creature's lineage using sophisticated software, which marked the Dakosaurus out as a hunter of large marine reptiles rather than a fish.