Modern crocodiles are known for their sprawling four-legged gait, but this Cretaceous crocodilian walked on two legs as it prowled prehistoric shores, as evidenced by well-preserved footprints it left behind.
This remarkable trace fossil, called Batrachopus grandis, was discovered at the Sacheon Jahye-ri tracksite in South Korea, which dates back to around 110 million to 120 million years ago. As shown by its footprints, this animal featured distinctly crocodile-like hind feet, yet it left no trace of its forelimbs or a dragging tail, leading an international team of researchers to conclude that the creature that left these tracks was a bipedal crocodilian. This research was published today in Scientific Reports.
Crocodilians, as the name suggests, refer to a diverse group of crocodile-like creatures, to which modern crocodiles and alligators are descended. The animal that left these tracks, however, was very un-crocodile-like in that it resembles bipedal theropod dinosaurs, with their long hind legs and narrow gait. Modern crocodiles, by contrast, feature short legs and leave behind wide trackways when they walk.
“Typical crocodiles walk in a squat stance and create trackways that are wide,” explained Kyung Soo Kim, the lead author of the study and a paleontologist at Chinju National University of Education, in a press release. “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.”
Footprint and gait analysis suggests this two-legged croc was quite large, measuring nearly 3 metres long, but with its horizontal stance, it stood no higher than a human’s hip. The authors of the study believe it frequented the shores of large lakes.
Nearly 100 footprints from multiple sites were found at the Sacheon Jahye-ri tracksite, which is extraordinary given how rare it is to find crocodilian fossil evidence in Asia. The tracks were so well preserved that some still exhibited skin patterns. These animals walked heel-to-toe, but no handprints were found, nor any evidence of a dragging tail. Importantly, the researchers ruled out the possibility that these track-makers were doing a kind of half-swim, half-walk through water, as that would have resulted in toe-tip tracks.
Interestingly, this discovery shows that ancient footprints found elsewhere in South Korea are not what they initially appeared to be.
Trackways found in the Haman Formation, which dates back to the earlier Jurassic period, were assigned to a very different animal: pterosaurs. Paleontologists, not sure what to make of these tracks, figured the aerial pterosaurs landed on two feet and then waded through shallow waters to protect their wings. This doesn’t really hold up, however, as pterosaurs are “obligate quadrupeds,” meaning they can only walk on all fours. These tracks, as shown in the new research, are more appropriately assigned to the crocodilian Batrachopus. The prints found at Sacheon Jahye-ri are twice the size of those found at Haman, measuring over 24 centimetres — a size far more consistent with a crocodilian origin.
“The footprints are very well preserved and the structure is spot on for crocodiles,” Martin Lockley, a co-author of the new paper and a researcher at the University of Colorado Denver, wrote in an email to Gizmodo.
The new research potentially shows that bipedal crocodiles emerged some 200 million years ago during the Jurassic period and lasted until at least the Cretaceous. “So their design was successful,” said Lockley.
It’s worth noting that these interpretations are based on footprints, as opposed to fossilized bones. Batrachopus grandis remains an ichnospecies, in the parlance of paleontologists, as it’s a “species” described solely by trace fossils, which in this case are a series of well-preserved footprints. Ideally, scientists will find some bones to go with these prints and further bolster the case for the existence of these extraordinary, terrifying two-legged crocodiles.