Not long after bulldozers began pushing aside dirt for a new waterfront housing complex on South Korea’s southern coast last year, construction was halted and paleontologists were called in.
South Korean law mandates that new construction sites in the region, already famous for the fossilized tracks of dinosaurs, ancient turtles, frogs and lizards, make accommodations for fossil discoveries — and this was a big one.
Underneath the topsoil, the bedrock yielded large, pristine footprints over 100 million years old. Kyung Soo Kim, a professor of paleontology at Chinju National University of Education in South Korea, took samples back to his laboratory. Initially, he thought the tracks were made by a pterosaur, one of the large winged reptiles that ruled the skies during the age of dinosaurs.
But in November, he picked up his colleague, Martin G. Lockley, at the airport and they drove straight to Dr. Kim’s lab. There, Dr. Lockley, a specialist in fossilized tracks at the University of Colorado Denver, quickly determined the tracks belonged to an undiscovered species, a large prehistoric ancestor of modern crocodiles that moved about on only two feet.
“I was very surprised, and I did not believe him at first,” Dr. Kim said. “But he was right. The tracks prove they were bipedal.”
Today’s semiaquatic crocodiles are decidedly quadrupedal — low to the ground, durable, fearsome and fast. But strong evidence showed that this prehistoric creature got around on two feet. While the tracks for the back feet were clearly defined from heel to toe and are found in regular formation, no tracks for the front limbs were discovered. That suggests bipedalism, much like you and me.
Dr. Lockley and Dr. Kim estimate that the type of ancient crocodylomorph that made the tracks were roughly nine feet long from snout to tail, with their heads likely raised a few feet up from the ground, able to see prey and predators from a distance. They lived and hunted alongside dinosaurs, and based on the low frequency of fossilized track evidence, they were not a dominant species of the era.
“For that time period, in that part of the world and the size of the animal,” said Dr. Norell, who did not participate in the study, “it’s definitely a new species.”
The species itself has no name yet, as there are no fossilized bones to confirm its existence. But the researchers named the tracks Batrachopus grandis. Unlike today’s crocodile tracks, which are made from animals with feet splayed more to the outside, the track way of this ancient biped is narrow and well defined.
“If you put down a chalk line, these things would pass the sobriety test,” Dr. Lockley said.
Some could argue that the reason there are no front footprints is that the animal may have been wading through a body of water. But Dr. Lockley countered that tracks left by a partially submerged animal would only have shown the front claws of their feet as they pushed themselves along, not the heels.
These imprints clearly show the full foot, Dr. Kim said, and in some the cases the impressions of skin are seen, too.
When first presented with the tracks, Dr. Lockley noticed they were the same shape as tracks made by another crocodylomorph, which was the size of a house cat, over 100 millions years before. But a quadruped made those tracks.
“All we’ve done is find exactly the same type of track 100 million years later with two differences,” Dr. Lockley said. “One, the tracks are huge. And try as we might, we can find no evidence for the front footprints. The tracks are proof that they were bipedal.”