Fossil footprints show some crocodile ancestors walked on two legs

Puzzling nearby tracks were likely also made by a bipedal croc ancestor, not a giant pterosaur

illustration of crocodile ancestors walking on two legs

About 106 million years ago, a large crocodile ancestor (shown here in an artist’s conception) walked on two legs in what is now South Korea, leaving behind a series of footprint impressions now preserved in the rock.

Anthony Romilio/The University of Queensland

Fossil tracks preserved in a South Korean rock formation are the first footprint evidence that some ancient ancestors of modern crocodiles walked on two legs. The size and spacing of the 106-million-year-old tracks suggest the crocodylomorph was 2 to 3 meters long — a fearsome predator similar in size to its modern descendants, researchers report June 11 in Scientific Reports.

The tracks were found in the fossil-rich Jinju Formation, home to the remains of a wide variety of animals including dinosaurs. It’s tough to identify a species from footprints, says paleontologist Martin Lockley of the University of Colorado Denver. “Short of finding the animal dead in its tracks, there’s always a little bit of uncertainty.” But the beautifully preserved prints made it possible to attribute them to Batrachopus, a genus of fossil tracks known to be made by crocodylomorphs.

Perhaps the most surprising feature of the tracks is the utter absence of any manus, or hand, prints — strong evidence that the creature walked on only its hind legs, Lockley says. “We have dozens of these things, and not one sign of a front footprint, so we’re pretty convinced.”

fossil footprints
These three fossil footprints, made by hind feet, were preserved in the fossil-rich Jinju Formation. They belong to the fossil track genus Batrachopus, known to be made by crocodylomorphs.Kyung Soo Kim/Chinju National University of Education
fossil footprints
These three fossil footprints, made by hind feet, were preserved in the fossil-rich Jinju Formation. They belong to the fossil track genus Batrachopus, known to be made by crocodylomorphs.Kyung Soo Kim/Chinju National University of Education

A bipedal crocodylomorph might have also been responsible for an enigmatic set of tracks found in the nearby, similarly aged Haman Foundation. In 2012, the same team of researchers speculated in Ichnos that the bipedal Haman tracks might have been made by pterosaurs, winged reptiles that lived alongside dinosaurs. However, most researchers — including Lockley and colleagues — are now convinced that pterosaurs were quadrupedal (SN: 10/19/08).

The new tracks aren’t the first hint that some crocodile ancestors walked on two legs. About 231 million years ago, Carnufex carolinensisnicknamed the Carolina Butcher — may have prowled North Carolina on its hind legs (SN: 3/19/15).

But that suggestion was based on skeletal reconstructions of the creature, not actual footprints, Lockley says. “The real punchline of our story is we have proof of large bipedal crocs.”

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