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The age of dinosaurs may have been dominated by dinosaurs, but they certainly weren't the only fearsome creatures around. A series of remarkable discoveries from a team led by Paul Sereno, a paleontologist at the University of Chicago and a National Geographic explorer-in-residence, has made it clear that another, less celebrated group of animals lived alongside the dinos, and sometimes even dined on those better-known cousins.

The animals in question were crocodiles, which thrived in the wetlands of the ancient Sahara 100 million years ago. Sereno found his first specimens of these prehistoric monsters about a decade ago, a species called Sarcosuchus, nicknamed SuperCroc: it was some 40 ft. long and weight 8 tons.

Now, reporting in the journal ZooKeys, Sereno's team has announced the discovery of fossils from three brand-new species and new fossils from two known species. Along with SuperCroc, they add up to a virtual menagerie of ancient crocodiles that inhabited a range of ecological niches — species nicknamed BoarCroc, RatCroc, PancakeCroc, DuckCroc and DogCroc.

The strangest thing about these animals isn't their names, though; it's the fact that many of them weren't flat to the ground, like modern crocs, but stood upright and walked on their legs, like modern mammals. "We have an idea of what a crocodile should be and what a mammal should be," says Sereno, "but you have to break down these categories to see what was going on in Africa back then." BoarCroc, for example, was 20 ft. long and had three rows of fangs, like a boar from hell, which made it what Sereno calls a "dinosaur slicer." With its agile legs, he says, "that thing probably came out of the water and charged up the bank to attack dinosaurs."

DogCroc, by contrast — dog-size, with a doglike nose — mostly ate plants and grubs. It could run too, but, Sereno suspects, "it probably ran down the bank to escape from dinosaurs." Bucktoothed RatCroc was also small and ate a similar diet. DuckCroc, about 3 ft. long, had a broad snout for rooting in shallow water and onshore, ducklike, for fish and frogs. And PancakeCroc was named for its wide, flat head, which it kept low, jaws open, waiting for an unsuspecting dinosaur to step into the mouth. "Modern crocs can take prey three times their size, if necessary," says Sereno — which means that the 20-ft.-long PancakeCroc could have taken down some reasonably large dinosaurs, like a multiton, long-necked sauropod, for instance. And SuperCroc, which was probably too heavy to run and likely lurked at the water's edge, could have taken even bigger ones.

Oddly enough, it was a modern crocodile — an Australian freshwater croc known as a "freshy" — that helped Sereno figure out how some of the ancient crocs behaved. "It's able to get up and gallop, unlike the saltwater crocodiles that live nearby," he says. Since many of the ancient crocodiles have legs like the freshies but tails like the salties, he figures they were both good swimmers and good runners — a lethal combination that may explain something intriguing about dinosaurs.

"[Dinosaurs] never went into the water in a serious way," he says. "[They] never radiated into the oceans the way mammals did after the asteroid hit." Maybe that's because dinos were simply afraid of what lurked in the waters, waiting for them.

• Michael D. Lemonick
• An odd new species of ancient crocodiles stood on two legs, lived alongside dinosaurs and sometimes even hunted them