The finding, published in Thursday's online issue of the journal Science, sheds new light on an animal that millions of years ago dominated Earth's seas.
Scientists are calling the beast "Godzilla." Its discovery will be highlighted in the December issue of National Geographic magazine.
The animal is "the most bizarre marine crocodile known to date," said Diego Pol of The Ohio State University, a co-author of the study.
Alerted by a group of farmers who stumbled across several fragments, a team of paleontologists led by Zulma Gasparini of Argentina's La Plata University collected a skull and parts of a vertebrae of the animal in the Argentine province of Neuquen in 1996.
Scientists named the previously unknown species Dakosaurus andiniensis, after a European crocodile from the same period that shares some similar characteristics.
With flipper-like feet and a tail like those found on fish, the crocodile is believed to have been about 12 feet (4 metres) long.
But its most unusual characteristic, said Pol, was a short and high snout, unlike the long, thin snouts found on many of today's crocodiles.
The animal also had large sharp teeth - some 13 in all - that paleontologists say were more often found in dinosaurs.
The fossil acquires 'flesh' as a
digitised model in this image
"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," Pol said.
The Dakosaurus, paleontologists say, was an animal that had four paddle-like limbs instead of legs, using its fish-like tail to propel through water.
It likely fed on reptiles and other large sea life when it roamed the Pacific Ocean off southwestern South America millions of years ago, they say.
"We find these results extremely interesting because they indicate that the diversity of crocodiles bank in the Jurassic was much greater than expected," Pol said.