What could possibly make a cranky crocodylomorph more terrifying? How about one that chases you on its hind legs?

Oh snap! Evidence of new bipedal alligator ancestor found in South Korea

New evidence from South Korea suggests that three-metre ancestors of modern-day crocodiles and alligators walked upright on their back legs.

According to a research paper in the Nature Scientific Reports published today, large, well-preserved footprints from the Lower Cretaceous period (between 145 million years ago and 100.5 million years ago) show the ancient monster was bipedal.

Found in the Sacheon Jahye-ri region near Sacheon City in South Korea, the prints belong to a previously unknown species of crocodylomorph. The Batrachopus grandis, as the authors coined it, made tracks more than twice the size of those belonging to the same family: footprint lengths between 18 and 24cm suggest a body up to three metres long.

Kyung-Soo Kim, of Chinju National University of Education, and Martin Lockley, of the University of Colorado, and colleagues found the narrow trackways were made entirely by the back limbs, with clear heel-to-toe impressions and skin traces in some areas.

Though the rear footprints may have covered those made by the front feet or the tracks could be poorly conserved, the research suggests an upright gait not previously seen from trackway specimens in the same family, the authors said.

"The trackway evidence of a large crocodylomorph with bipedal or facultative bipedal gait in the Lower Cretaceous is surprising, but consistent with terrestrial or semi-terrestrial adaptation," they wrote.

The discovery has led the researchers to take another look at footprints elsewhere in South Korea. Paw-marks in the Haman Formation from the Albian age were thought to have been made by giant pterosaurs, those famous flying reptiles, which gave the impression they were walking on two legs to preserve their wings. But this evidence has now been reinterpreted to suggest they were instead made by ancient crocodile relatives.

"This interpretation helps solve previous confusion over interpretation of enigmatic tracks of bipeds from... Haman Formation sites by showing they are not pterosaurian as previously inferred," the paper said. "Rather, they support the strong consensus that pterosaurs were obligate quadrupeds, not bipeds."

If the team's discoveries are on point, Batrachopus grandis may be one of the youngest bipeds in the crocodile family, but certainly not the first.

Carnufex carolinensis ("Carolina butcher") roamed swamps between 237 million years ago and 228 million years ago when North America was a part of the supercontinent Pangaea and North Carolina was near the equator, roughly 100 million years before Batrachopus grandis although it was roughly the same size.

If the thought of upright walking alligators could inspire nightmares, don't dwell on the fact that Batrachopus grandis was a relative tiddler in the crocodile family. Even modern examples of the saltwater variety living in Australia have been known to reach nearly twice the length at 8.6 metres.

Deinosuchus, thought to be the largest crocodile relative in history, measured up to 12 metres or about 39 feet. After hunting in what is now the United States, it died out 73 million years ago. ®