The Cretaceous era was a truly terrifying place, with galloping crocodiles capable of giving the better-known dinosaurs a run for their money. A documentary on the giant crocodiles of the era will screen on December 20th.

In 2009, National Geographic expeditions to Morocco and Niger revealed three new species of 100-million-year-old crocodiles, formally described in Zookeys.

The first specimen unearthed of Kaprosuchus saharicus would have been 6.5-meters-long (21 feet). Combined with three sets of sharp tusks and a snout that could probably have been used as a sort of battering ram, this must have been the stuff of nightmares for the beasts of the time. While it is thought that K. saharicus fed mainly on dinosaurs, our ancestral mammals probably had plenty of reason to be afraid.

Mike Hettwer/National Geographic. Kaprosuchus saharicus model and original skull.

Laganosuchus thaumastos grew to a similar size, but its flat head appears to have been more suited to ambush attacks on passing fish.

Mike Hettwer/National Geographic. NIcknamed PancakeCroc, Laganosuchus thaumastos probably had a diet of mostly fish.

Both species were closely related to crocodiles, and lived a similar semi-aquatic lifestyle, but had legs beneath their bodies, rather than sprawling sideways as modern crocodillians do. "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 National Geographic Magazine at the time of the discoveries. "These species open a window on a croc world completely foreign to what was living on northern continents."

Todd Marshall/National Geographic. This cretaceous crocodile nicknamed DogCroc was probably a fast and agile runner.

The third species identified for the first time on the same expedition, Araripesuchus rattoides, was only a meter long and probably lived on roots and insects.

None of the three compare to Sereno's best known discovery, the 12-meter-long (39 feet) Sarchosuchus imperator, dubbed “SuperCroc,” that roamed (and shook) the Earth 112 million years ago.

Mike Hettwer/National Geographic. Paul Sereno with models of six of the crocodile species he helped discover and describe.

"We were surprised to find so many species from the same time in the same place," University of Montreal palaeontologist Hans Larsson told The Guardian when the discoveries were announced. "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."

National Geographic has animated Sereno's discoveries and interviewed him extensively for the soon-to-be released documentary.