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 even into solid stone.
Then scientists came here in the 1960s and dispelled children's fears, determining that the strange footprints in fact belonged to gigantic, two-legged dinosaurs that stomped and splashed over 60 million years ago, in the ancient waterways of what is now Toro Toro, a village and popular national park in the Bolivian Andes.
Now, a team of paleontologists, mostly from California’s Loma Linda University, have discovered and meticulously documented 16,600 such footprints left by theropods, the dinosaur group that includes the Tyrannosaurus rex. Their study, based on six years of regular field visits and published last Wednesday in the peer-reviewed journal PLOS One, reports that this 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 Raul Esperante. "We have all these world records at this particular site.”
The dinosaurs that ruled the earth and roamed this region also made awkward attempts to swim here, according to the study, scratching at what was 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 centuries of erosion, scientists said.
"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. He said that, to his knowledge, the number of footprints and trackways found in Toro Toro had no precedent.
"This is a remarkable window into the lives and behaviors of dinosaurs at the end of the Cretaceous,” Butler added, referring to the period around 66 million years ago at the end of which an asteroid impact abruptly extinguished all dinosaurs and 75% of living species along with them, according to scientists.
Despite nature’s success at preservation, humans have too often threatened these traces. For decades, farmers threshed corn and wheat on the footprint-covered plateaus. Nearby quarry workers didn’t think much of the formations as they blasted rock layers for limestone. And just two years ago, researchers said, highway crews tunneling through hillsides nearly wiped out a major site of dinosaur tracks before the national park intervened.
Such disturbances may have something to do with the area's striking absence of dinosaur bones, teeth and eggs, experts say. For all of the footprints and swim traces found across Bolivia’s Toro Toro, there are virtually no skeletal remains of the sort that litter the peaks and valleys of Argentine Patagonia and Campanha in Brazil.
But the lack of bones could have natural causes, too. The team said the quantity and pattern of tracks – and the fact they were all found in the same sediment layer – 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.
The range in footprint sizes indicated that giant creatures roughly 10 meters (33 feet) tall moved in a herd with tiny theropods the size of a chicken, 32 centimeters (1 foot) tall at the hip.
In presenting a snapshot of everyday behavior, footprints "reveal what skeletons cannot,” said Anthony Romilio, a paleontologist at the University of Queensland in Australia who also did not participate in the study. Just from footprints, researchers can tell when dinosaurs strolled or sped up, stopped or turned around.
But the reason they flocked in droves to this wind-swept plateau remains a mystery.
"It may have been that they were all regular visitors to a large, ancient, freshwater lake, frequenting its expansive muddy shoreline,” offered Romilio.
Biaggi suggested that they were "running away from something or searching for somewhere to settle.”
What's certain is that research into this treasure trove of a dinosaur tracksite will continue.
"I suspect that this will keep going over the years and many more footprints will be found right there at the edges of what’s already uncovered," Biaggi said.
