Scientists Document More than 16,000 Dinosaur Footprints in Carreras Pampas

Dinosaur tracks in Carreras Pampas. (Source: Jeremy McLarty via CNN)

JAKARTA - A bustling "dinosaur highway" may have once stretched along the coast in what is now known as Bolivia, where theropods - three-legged, upright-walking meat-eating dinosaurs - left behind thousands of fossil footprints.

Paleontologists have now described their tracks for the first time, offering a rare glimpse of dinosaur movement in their habitat.

Not long ago, scientists counted 16,600 theropod footprints - more than any other footprint site - at the Carreras Pampas footprint site, Torotoro National Park, Bolivia.

There, theropods set foot in soft, deep mud between 101 million and 66 million years ago, towards the end of the Cretaceous period.

The study is the first scientific survey of the footprint-covered area, which stretches about 80,570 square feet (7,485 square meters).

Some tracks were isolated, but many formed trail paths, or multiple tracks left by the same animal, researchers reported in the journal 'PLOS One' last month.

"Wherever you look in the rock layers at the site, there are dinosaur tracks," said study co-author Dr. Jeremy McLarty, a biology professor and director of the Museum and Center for Dinosaur Science Research at Southwestern Adventist University in Texas, CNN reported (20/1).

"Most of the tracks are oriented north-northwest or southeast," McLarty told CNN. The tracks were likely made within a relatively short time frame, suggesting this area was a popular route for theropods and may have been part of a larger dinosaur migration route that stretched across Argentina, Bolivia, and Peru.

The shape of the tracks and the distance between the footprints reveal how the animals moved; some walked leisurely, while others ran briskly along the muddy shoreline, and more than 1,300 tracks preserve evidence of swimming in shallow waters, researchers reported.

Several footprints include the remains of a theropod tail drag, and the varying length and width of the footprints suggest that the dinosaurs were of a very diverse size: from about 26 inches (65 centimeters) high at the hips 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.

"Identifying thousands of individual footprints and describing the various walking styles "has tremendous implications for reconstructing this ancient environment and how dinosaurs and birds used it," said paleontologist Sally Hurst, who was not involved in the new study.

Hurst is an adjunct researcher at the School of Natural Sciences at Macquarie University in Australia.

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

Dinosaur tracks in Carreras Pampas. (Source: Raúl Esperante via CNN)

"It's the deeper tracks that have more of a footprint of the foot movement, and that's what interested me, and the tracks are quite long," said who studies dinosaur tracks but was not involved in this new study.

For example, the swimming footprint "looks very different from a regular walking footprint," McLarty said.

When a theropod was lifted by water, its middle finger pressed deeper into the mud, and the other two toes and heel left a much lighter footprint.

"Tracks are records of the soft tissues, movements, and environments that dinosaurs actually lived in," added Falkingham.

The site, with traces of animals of various sizes moving in a variety of ways, "really brings this lost ecosystem back to life in a way that bones can't."

"Since the 1980s, Carreras Pampas has been known for its dinosaur tracks, but their coverage and number have never been studied in detail," said McLarty. His team's research raises new questions about the remains of this preserved Cretaceous life in South America, such as why almost all the footprints belong to theropods and why their number is so large, said McLarty.

Many sites around the world preserve many sauropod footprints, long-necked plant-eating dinosaurs that grew larger than any land animal alive today.

Sauropods are known to live in groups, as are many modern large-sized herbivores. By comparison, theropods are predators, which do not usually roam in large groups.

Bolivia itself is known for its many footprint sites, which date back to the Triassic, Jurassic, and Cretaceous periods, the study's authors report. Prior to the Carreras Pampas mapping, the site with the most dinosaur footprints in the world was also in Bolivia: Cal Orck'o in Sucre, which dates back to about 68 million years ago and is estimated to contain 14,000 footprints.

"How do our findings at Carreras Pampas relate to other sites in Bolivia?" asked McLarty.

"What kind of large-scale picture emerges when we start comparing different sites?," he continued.

"These thousands of footprints provide important clues about dinosaurs that cannot be given by fossil skeletons, because footprints reveal how living animals move," said paleontologist Dr. Anthony Romilio, a researcher at the University of Queensland in Australia, who was not involved in the study.

"Skeletons show what animals can do; footprints show what they actually do, over time," Romilio told CNN in an email.

"Footprints record speed, direction, turning behavior, slipping, posture, and sometimes group movement," he said.

"The footprint of Carreras Pampas is significant because of the different sizes of theropods," said Romilio.

"This can reflect different species, different age classes, or a combination of both," he said.

And unlike body fossils, footprints preserve dinosaurs' relationship with a particular location while they were alive. Bones can be transported after an animal dies, "so the place you find a dinosaur bone may not be the exact place the dinosaur was," said McLarty.

By comparison, footprints offer a direct glimpse of an ancient moment, in this case, when dozens of theropods scampered across a shoreline.

"The footprints are not moving," McLarty said.

"When you visit Carreras Pampas, you know you're standing where dinosaurs once walked," he said.