Footprints of Ancient 10-foot Crocodile That Walked on Two Legs Discovered in South Korea

Researchers have discovered nearly one hundred large animal footprints in South Korea dated to between 110 million and 120 million years ago, which they say belong to a previously unknown ancient ancestor of modern crocodiles that walked on two legs, like many dinosaurs.

The footprints, which were discovered during excavations at the Sacheon Jahye-ri site, near Sacheon City in the south of the country, measure 18-24 centimeters (around 7-9 inches) in length, indicating that the animals that made them could have been nearly 10 feet long with legs similar in length to adult human legs, according to a study published in the journal Scientific Reports.

The exceptionally well-preserved footprints appear to have been made entirely by the back limbs, with the researchers even reporting the discovery of skin traces in some tracks.

"Typical crocodiles walk in a squat stance and create trackways that are wide," Kyung Soo Kim, lead author of the study from Chinju National University of Education, South Korea, said in a statement.

"Oddly, our trackways are very narrow looking—more like a crocodile balancing on a tight-rope. When combined with the lack of any tail-drag marks, it became clear that these creatures were moving bipedally."

"They were moving in the same way as many dinosaurs, but the footprints were not made by dinosaurs. Dinosaurs and their bird descendants walk on their toes. Crocodiles walk on the flat of their feet leaving clear heel impressions, like humans do," he said.

The researchers say the animal that made the tracks represents a previously unknown species of crocodylomorph, a large group of animals containing modern-day crocodilians—such as crocodiles and alligators—as well as their extinct relatives.

The fact that the researchers found no "handprints" led them to question this conclusion given that these animals tend to be four-legged. However, they found no evidence to suggest the tracks were made by a "four-legged" crocodylomorph.

"As an animal walks, the hind feet have the potential of stepping into the impression made by the hand and 'over-printing' it, but we find no evidence of this at these Korean sites," Anthony Romilio, another author of the study from the University of Queensland, Australia, said in the statement.

"It isn't due to poor preservation either, because these fossils are spectacular, they even have the fine details of the toe-pads and scales on their soles preserved."

crocodile ancestors
Artist's illustration of tracks being made by prehistoric crocodile ancestors. Anthony Romilio

The latest findings also help to shed new light on other sites in South Korea where scientists have previously found footprints that were originally thought to belong to giant pterosaurs—a group of prehistoric flying reptiles—walking on land, according to the researchers.

"At one site, the footprints were initially thought to be made by a giant bipedal pterosaur walking on the mudflat, we now understand that these were bipedal crocodile prints," Romilio said.

"Fossil crocodile tracks are quite rare in Asia, so finding an abundance of nearly one hundred footprints was extraordinary."

Animals like the bipedal crocodile ancestor that can only be identified from trace fossils, such as footprints or fossilized excrement, are classified using the "ichnospecies" system, rather being described as a traditional "species."

In the case of animal tracks, researchers use this system to avoid confusion because one particular shape of print can be produced by several different species. Furthermore, individual species are capable of making different types of tracks.

The researchers say the newly identified footprints of the crocodile ancestor belong to the ichnospecies Batrachopus grandis. This is significant because not only are the footprints of Batrachopus grandis more than twice as large as any of its closest ichnospecies relatives, but it is also the only one known to walk on two legs.

"I love the fossil record—it is always revealing new and exciting things. This discovery and analysis is very interesting. It adds to our developing understanding that crocodiles were much more diverse functionally and ecologically than previously thought," Spencer Lucas, a researcher from the New Mexico Museum of Natural History & Science, who was not involved in the latest study, told Newsweek.

"About a decade ago, I made a presentation at a meeting, arguing that the track record shows that crocodiles were much more important predators during the age of dinosaurs than previously thought. It seems that crocodiles made a real bid to rival the dinosaurs, and I see bipedality as another part of their attempt to grab ecospace from the terrible lizards."

Lucas said the study is also an excellent example of how the footprint record tells us about aspects of vertebrate evolution that the bone record, alone, has not provided.

"The paper is very rigorous and does a good job of eliminating other possible explanations (than bipedality) of why only hind foot impressions are preserved in the trackways," he said.

"We do well to remember that the earliest crocodiles were terrestrial hunters, not the amphibious predators known today. So, it would seem those terrestrial hunters evolved some bipedal forms. The question is what did they do with their freed up forelimbs? Grab prey? This opens doors to new possibilities in the evolutionary history of crocodilian hunting."

This article was updated to include additional comments from Spencer Lucas.