Fossil footprints show some crocodile ancestors walked on two legs

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 quadrupeda.

The new tracks aren’t the
first hint that some crocodile ancestors walked on two legs. About 231 million
years ago, Carnufex carolinensis — nicknamed the Carolina Butcher — may have prowled North Carolina on its hind legs.

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.”