Ancient footprints in South Korea made by crocodiles that walked on two legs

A photo shows one of several Cretaceous era trackways left by bipedal crocodiles in what's now South Korea. Photo by Seul Mi Bae

June 11 (UPI) -- Crocodiles once walked on two legs, just like dinosaurs, according to new analysis of a unique collection of footprints discovered in South Korea.

Scientists dated the footprints to between 110 and 120 million years ago, according to the study, published Thursday in the journal Scientific Reports.

At first, researchers thought they happened upon the tracks of giant bipedal pterosaurs, but after closer examination, they determined the tracks were left by crocodiles measuring some 10 feet in length.

"One of the methods we use to identify potential track-makers is similar to Cinderella -- basically we look for a foot that fits," University of Queensland paleontologist Anthony Romilio told UPI in an email. "In the case with extinct animals, this involves comparing any with fossilized bones, but since we find more fossil tracks than fossil skeletons we have to use other approaches."

Inside dinosaur prints, the deepest portions are found in the toes. Dinosaurs walked on their toes. The tracks left among the ancient mudflats featured distinct heel prints. Primitive bipedal crocodiles walked flat-footed, making them a match for the unusual tracks.

The narrow path of the tracks made it appear as if the ancient crocodiles were walking on tightropes. This pattern, combined with the lack of tail marks, confirmed the tracks were left by crocodiles walking on their hind two legs.

"Bipedalism happened early in the crocodile evolution, even before dinosaurs, and there were many different species," Romilio said. "We had thought crocs of this type were all extinct by the end of the Triassic, and so to have evidence of this type of croc behavior [approximately] 80 million years after they were supposed to be extinct was a big surprise."

It's possible the bipedal crocodiles of the Triassic did go extinct, and the crocodiles of the Cretaceous evolved bipedalism independently.

"We don't know," said Romilio. "What we do know is that these unusual tracks are all of the same types, and so are likely from the same type of animal. But keep in mind that all of these tracks come from a very localized area."

The fossilized tracks and trackways were remarkably well-preserved, revealing details of the toe-pads and scales on their soles, researchers said

Scientists suggest the bipedal crocodiles of the Cretaceous were part of a diverse lakeside ecosystem.

"From this one track site alone, we also have pterosaurs, turtles, birds, meat-eating dinosaurs (theropods) and long-necked dinosaurs (sauropods)," Romilio said.

"And there are many track sites within that particular area that increases the faunal diversity -- frogs, baby dinosaurs, velociraptors and other types of theropods, two-legged planter dinosaurs, birds that were like spoonbills, other like ducks, and so much more, all based on the footprints they left behind in the fine muddy sediment of ancient shallow lakes," he said.

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