Here’s what you’ll learn when you read this story:

• The Carreras Pampa tracksite at Torotoro National Park is now the largest recorded site for dinosaur tracks.
• Despite a record-breaking amount of tracks, dinosaur bones in the area are scarce, making it difficult to determine what species the tracks belonged to.
• The majority of the prints are examples of “ghost tracks,” which suggest the dinosaurs that made them were some variety of theropod.

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70 million years ago, the arid landscape of central Bolivia used to be lush and humid. What are now dry sandstone beds were once lakes, and at the edge of one of these shallow, freshwater lakes, theropod dinosaurs used to thrive. Eventually, they left behind long trails of tracks and tail marks that would slowly be covered by sediments and frozen in time.

The Carreras Pampa tracksite in what is now Torotoro National Park is now the largest recorded site for dinosaur tracks. Among its 1,321 trackways are 16,600 three-toed tracks and 289 lone prints (including tail drag marks), with an additional 1,378 swim tracks possibly left by theropods or ancient crocodilians in 280 of those trackways. The nearly 18,000 individual tracks discovered at the Upper Cretaceous site have now broken records.

Paleontologist Raúl Esperante from the Geoscience Research Institute led a team of researchers in identifying the footprints of the creatures that used to wander through the mud at this lake shore.

“Bolivia has one of the most extensive and diverse records of dinosaur tracksites in the world, spanning the Triassic, Jurassic, and Cretaceous,” Esperanto said in a study that was recently published in PLOS ONE. “However, despite the abundance of tracksites, few scientific studies have been published.”

Despite the amount of tracks in Torotoro, dinosaur bones are scarce, as the fossilization conditions that favored the preservation of footprints in soft sediments were not ideal for preserving skeletal remains. Carcasses near the water are thought to have been gnawed by scavengers and eroded, disarticulated, or scattered by waves before they could be buried. (There are no such risks for footprints, which formed ichnofossils after the mud hardened and was covered by more sediment before the tide could erase them.) And because the Carreras Pampa trackways were probably made by migrating dinosaurs, it is unlikely that many died along the way anyway.

An absence of bones makes it difficult to determine what species the tracks belonged to, and most of the footprints are what Esperanto calls “ghost tracks.” Tracks of this type feature claw marks evenly spaced out without a heel impression. They alternate from left to right, had enough distance between them to suggest bipedal walking, and were found alongside more defined theropod tracks—all of which support them having been left by actual theropods as opposed to being random markings on the rock. Some associated with tail drags were deep and elongated without much detail. Other tracks were found to be exceptionally well-preserved, usually with claw marks and impressions of the heel, padding, and hallux (the innermost toe). Morphotypes of the tracks varied, but all seemed to suggest three-toed theropods.

Though it is unclear which theropods left the tracks, there are some species that can potentially be inferred. The team found similarities to the morphology of adult *Tyrannosaurus *and *Eubrontes *feet, while smaller prints may have belonged to juvenile Grallators. But whatever species of dinosaurs passed by the lake were apparently stable on their feet. Their speed was evidenced by the spreading of the toes—they weren’t too close together, but they also weren’t spread too far apart, which suggests intermediate strides that would be expected of medium theropods. Wider spaces between toes are the result of running or other rapid movements, but this could also mean the toes spread out to provide stability on mud or other soft and unstable sediments.

Theropods traversing this stretch of shoreline at Carreras Pampa tended to walk in curved, meandering paths, which is something animals often do when they need to cover long distances. There were also changes in direction associated with pauses, interpreted as a dinosaur trying to avoid an obstacle (potentially another dinosaur, if they were moving in packs). Found among these footprints were the tracks of ancient birds, and (in some of the dinosaur tracks) rosette-shaped invertebrate burrows.

Besides tracks on land, there are swim tracks that appear as elongated scratches or grooves, resulting from swimming motions that involve clawed feet touching down on the rippled lake bottom. Whether they were left by dinosaurs or ancient crocodilians (or both) is unknown, but considering the large number of theropod tracks at the site (without the tracks of other tetrapods), it could mean that the theropods could also swim.

“The swim tracks, tail traces, and avian tracks are remarkably well preserved, and most tracks are found in continuous trackways,” Esperanto said. “The abundance and exceptional preservation of these tracks and traces make the Carreras Pampa tracksite an ichnologic concentration and conservation Lagerstätte.”

Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.