Ancient imprints: Bolivian tracksite reveals dinosaur behaviour secrets

Legend once had it that the huge, three-toed footprints scattered across the central highlands of Bolivia came from supernaturally strong monsters — capable of sinking their claws into solid stone.

Scientists dispelled those fears in the 1960s, determining that the strange footprints belonged to gigantic, two-legged dinosaurs that stomped and splashed more than 60 million years ago in the ancient waterways of what is now Toro Toro, a village and national park in the Bolivian Andes.

Now a team of paleontologists, mostly from California’s Loma Linda University, have discovered and documented 16,600 such footprints left by theropods, the dinosaur group that includes the Tyrannosaurus rex. Their study, based on six years of field visits and published last Wednesday in the journal PLOS One, reports that the finding represents the highest number of theropod footprints recorded anywhere in the world.

“There’s no place in the world where you have such a big abundance of (theropod) footprints,” said Roberto Biaggi, a co-author of the study led by Spanish paleontologist Raúl Esperante. “We have all these world records at this particular site.”

The dinosaurs that roamed this region also made awkward attempts to swim, the study found, scratching at squishy lake-bottom sediment to leave another 1,378 traces. They pressed their claws into the mud just before water levels rose and sealed their tracks, protecting them from erosion.

“The preservation of many of the tracks is excellent,” said Richard Butler, a paleontologist at the University of Birmingham who was not involved in the research. To his knowledge, he said, the number of footprints and trackways at Toro Toro had no precedent. “This is a remarkable window into the lives and behaviors of dinosaurs at the end of the Cretaceous,” he added.

Despite surviving for millions of years, the traces have been threatened by human activity. Farmers long threshed corn and wheat on the footprint-covered plateaus. Quarry workers blasted rock layers for limestone. Just two years ago, highway crews tunneling through hillsides nearly wiped out a major site of dinosaur tracks.

Such disturbances may relate to the striking absence of dinosaur bones, teeth and eggs in the region. For all the footprints found across Toro Toro, there are virtually no skeletal remains like those abundant in Argentine Patagonia or Brazil’s Campanha. But the lack of bones may have natural causes, too. The researchers said the quantity and pattern of tracks suggest that dinosaurs didn’t settle in what is now Bolivia as much as trudge along an ancient coastal superhighway stretching from southern Peru into northwest Argentina.

Footprint sizes ranged from giant creatures roughly 10 metres (33 feet) tall to tiny theropods the size of a chicken, indicating herds of mixed sizes moved together. Footprints “reveal what skeletons cannot,” said Anthony Romilio, a paleontologist at the University of Queensland. From tracks alone, researchers can tell when dinosaurs strolled or sped up, stopped or turned.

Why so many dinosaurs roamed the site remains a mystery. Romilio suggested they may have been regular visitors to a large ancient freshwater lake. Biaggi proposed they were “running away from something or searching for somewhere to settle.” What’s certain, he said, is that research into this dinosaur tracksite will continue and many more footprints will be found right there at the edges of what’s already uncovered.

Associated Press