Scientists document over 16,000 footprints in the world’s most extensive dinosaur tracksite

By Mindy Weisberger, CNN

(CNN) — A high-traffic “dinosaur freeway” may have once stretched across a shoreline in what is now Bolivia. Traveling along this busy route were theropods — three-toed, bipedal meat-eating dinosaurs, which left behind thousands of fossil footprints. Paleontologists have now described their tracks for the first time, offering a rare glimpse into dinosaurs’ movements through their habitat.

Scientists recently counted 16,600 theropod tracks — more than any other trackway site — at the Carreras Pampas tracksite in Bolivia’s Torotoro National Park. There the theropods stamped their feet into the soft, deep mud between 101 million and 66 million years ago, toward the end of the Cretaceous period.

This study is the first scientific survey of the footprint-covered areas, which extend roughly 80,570 square feet (7,485 square meters). Some tracks were isolated, but many formed trackways, or multiple impressions left by the same animal, researchers reported Wednesday in the journal PLOS One.

“Everywhere you look on that rock layer at the site, there are dinosaur tracks,” said study coauthor Dr. Jeremy McLarty, an associate professor of biology and director of the Dinosaur Science Museum and Research Center at Southwestern Adventist University in Texas.

Most of the tracks were traveling north-northwest or southeast, McLarty told CNN. They were likely made over a relatively short time span, indicating that this area was a popular thoroughfare for theropods and could have been part of a larger dinosaur freeway that spans Argentina, Bolivia and Peru.

Print shapes and the distance between the footprints revealed how the animals were moving; some strolled at a leisurely pace, while others sprinted through the muddy shoreline, and more than 1,300 tracks preserved evidence of swimming in shallow water, the researchers reported.

Several trackways included drag marks from the theropods’ tails, and varying lengths and widths of the footprints suggested that the dinosaurs ranged greatly in size: from a hip height of about 26 inches (65 centimeters) to more than 49 inches (125 centimeters). Several hundred additional tracks at the site were made by birds that shared the shoreline with the dinosaurs.

‘Incredible implications’

Identifying thousands of individual prints and describing the different gaits “has incredible implications for reconstructing these ancient environments and how dinosaurs and birds used them,” said paleontologist Sally Hurst, who was not involved in the new study. Hurst is an adjunct fellow in the School of Natural Sciences at Macquarie University in Australia.

The tracks are preserved at varying depths in what was previously soft, deep mud, “which can often end up recording a lot about how these animals moved their feet,” Dr. Peter Falkingham, a professor of paleobiology at Liverpool John Moores University in the UK, told CNN in an email.

“It’s the deeper tracks that preserve more of the foot’s motion, which is what I’m interested in, and they have quite long trackways of such tracks,” said Falkingham, who studies dinosaur trackways but was not involved in the new research.

For example, swimming tracks “look distinctly different from the normal walking tracks,” McLarty said. When a theropod was buoyed up by water, its middle toe pressed more deeply into the mud, and the other two toes and heel left a much lighter impression behind.

“Tracks are a record of soft tissues, of movements, and of the environments the dinosaurs were actually living in,” Falkingham added. This site, with its abundant tracks of different-size animals moving in various ways, “really brings these lost ecosystems to life in a way the bones don’t.”

Leaving an impression

Since the 1980s, Carreras Pampas has been known for its dinosaur tracks, but the scope and number had never been studied in detail, McLarty said. His team’s work raises new questions about this preserved slice of South American Cretaceous life, such as why nearly all the footprints belong to theropods and why there are so many of them, McLarty said.

Many sites around the world preserve multiple trackways of sauropods, the plant-eating and long-necked dinosaurs that grew to be larger than any land animal alive today. Sauropods were known to travel in herds, as do many types of large modern herbivores. In comparison, theropods are predators, which typically do not roam in large groups.

Bolivia is known for its numerous trackway sites, dating to the Triassic, Jurassic and Cretaceous periods, the study authors reported. Before the mapping of Carreras Pampas, the site with the most dinosaur tracks in the world was also in Bolivia: Cal Orck’o in Sucre, dating to about 68 million years ago and containing an estimated 14,000 prints.

“How does what we’re finding at Carreras Pampas relate to these other sites in Bolivia?” McLarty asked. “What kind of large-scale picture arises when we start comparing across different sites?”

These thousands of footprints provide important clues about dinosaurs that fossil skeletons can’t, because trackways reveal how living animals moved, said paleontologist Dr. Anthony Romilio, a research associate at the University of Queensland in Australia, who was not involved in the research.

“A skeleton shows what an animal could do; trackways show what it actually did, moment to moment,” Romilio told CNN in an email. “They record speed, direction, turning behaviour, slipping, posture, and sometimes group movement.”

The Carreras Pampas tracks are significant because of the different theropod sizes represented, Romilio said. “This could reflect multiple species, multiple age classes, or a combination of both.”

And unlike body fossils, trackways preserve a dinosaur’s connection to a specific location when it was alive. Bones can be transported after an animal’s death, “so where you find a dinosaur bone may not be the exact place the dinosaur was,” McLarty noted. In comparison, trackways offer a direct snapshot of an ancient moment in time — in this case, when scores of scampering theropods crisscrossed a shoreline.

“Tracks don’t move,” McLarty said. “When you visit Carreras Pampas, you know you are standing where a dinosaur walked.”

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