Paleontologist Paul Sereno poses with models of his Saharan discoveries, clockwise from top right: BoarCroc, PancakeCroc, RatCroc, DogCroc , DuckCroc and, in front of Serono, SuperCroc. (Mike Hettwer/National Geographic)

A six-metre croc with three sets of fangs is among the five ancient relatives of modern-day crocodiles found in the Sahara Desert, scientists said Thursday.

Three of the fossils, discovered by researchers led by Paul Sereno of the University of Chicago and Hans Larsson of McGill University, represent newly named species.

The fossil remains were found in a series of expeditions beginning in 2000.

All of the prehistoric crocs lived about 100 million years ago in the southern super-continent known as Gondwana, when the region that is now the Sahara featured dinosaurs and grassy plains criss-crossed by rivers.

"These species open a window on a croc world completely foreign to what was living on northern continents," Sereno said in a statement.

The new crocodile fossils are described in the journal ZooKeys and are featured in the November issue of National Geographic magazine and in a TV special to be broadcast Saturday on the National Geographic Channel.

Sereno is an explorer-in-residence with National Geographic. He discovered a nearly complete skeleton of a massive crocodile ancestor, dubbed SuperCroc, during expeditions in 1997 and 2000.

He has also given whimsical names to the fossils. The three new species are nicknamed BoarCroc, RatCroc and PancakeCroc. Sereno also found new fossils of two previously described species, which he's named DuckCroc and DogCroc.

"We were surprised to discover so many species from the same time in the same place," Larsson said at a news conference Thursday.

"Each of the crocs apparently had different diets, different behaviours. It appears they had divided up the ecosystem, each species taking advantage of it in its own way,"

RatCroc, DuckCroc and DogCroc were all about a metre long, while BoarCroc and Pancake Croc were six metres long.

Sereno said four of these crocs stood with their legs under their bodies. They walked like land mammals rather than sprawling with their bellies on the ground as modern crocodiles do.

PancakeCroc, on the other hand, was a squat fish-eater that likely sat motionless in the water waiting for its prey.

"My African crocs appeared to have had both upright, agile legs for bounding overland and a versatile tail for paddling in water," Sereno wrote in a National Geographic article.

"Their amphibious talents in the past may be the key to understanding how they flourished in, and ultimately survived, the dinosaur era," he wrote.