16,000 fossil footprints in central Bolivia reveal dinosaur behavior

TORO TORO, Bolivia — Legend once held that the enormous three-toed prints scattered across the central highlands of Bolivia came from monsters of supernatural strength, capable of sinking their claws even into solid stone.

Then scientists came here in the 1960s and allayed children’s fears, determining that the strange footprints actually 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 popular national park in the Bolivian Andes.

Now a team of paleontologists, primarily from Loma Linda University in California, has discovered and meticulously documented 16,600 footprints left by theropods, the group of dinosaurs that includes 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 the discovery represents the largest number of theropod footprints recorded in the world.

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

The dinosaurs that ruled the Earth and roamed this region also attempted to swim here, according to the study, scraping what was spongy sediment from the lake bottom to leave 1,378 other tracks.

They dug their claws into the mud just before the waters rose and sealed their tracks, protecting them from centuries of erosion, the 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 tracks found at Toro Toro had no precedent.

“This is a remarkable window into the life and behavior of dinosaurs at the end of the Cretaceous,” Butler added, referring to the period about 66 million years ago at the end of which an asteroid impact abruptly wiped out all dinosaurs and 75% of living species with them, according to scientists.

Although they have survived for millions of years, human life has threatened these traces. For decades, farmers threshed corn and wheat on the trays covered in imprints. Workers at nearby quarries didn’t think much of the formations as they blasted the rock layers for limestone. And just two years ago, researchers said, road crews digging tunnels into the hillside nearly erased a major site of dinosaur tracks before the national park intervened.

Such disturbances may have something to do with the striking absence of dinosaur bones, teeth and eggs in the area, experts say. For all the footprints and swimming marks found across Bolivia’s Toro Toro, there are virtually no skeletal remains of the type that litter the peaks and valleys of Argentina’s Patagonia and Brazil’s Campanha.

But the lack of bones could also have natural causes. The team said the quantity and configuration of the tracks – and the fact that they were all found in the same layer of sediment – suggests that the dinosaurs did not so much settle in what is now Bolivia as they trudged along an ancient coastal highway stretching from southern Peru to northwestern Argentina.

The range of print sizes indicated that giant creatures about 10 meters (33 feet) tall moved in flocks with tiny chicken-sized theropods, measuring 32 centimeters (1 foot) tall at the hip.

By providing insight into everyday behaviors, the prints “reveal what skeletons can’t,” said Anthony Romilio, a paleontologist at the University of Queensland in Australia, who was also not involved in the study. Using footprints, researchers can determine when dinosaurs walked, accelerated, stopped or turned around.

But why they flocked to this windswept plateau in droves remains a mystery.

“It may be that they are all regular visitors to a large, ancient freshwater lake, frequenting its vast, muddy shoreline,” Romilio suggested.

Biaggi suggested they were “running away from something or looking for a place to settle.”

What is certain is that research into this treasure trove of dinosaur footprints will continue.

“I suspect this will continue over the years and many more prints will be found just at the edge of what has already been discovered,” Biaggi said.

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DeBre reported from Buenos Aires, Argentina.