Paleontologists have found tens of thousands of dinosaur tracks in South America, which offers evidence as to which species were traveling via an ancient coastline.

A total of nearly 18,000 tracks — including 16,600 footprints as well as 1,378 swim tracks and several tail traces — have been located along the Carreras Pampa track site, an ancient coastline located in Torotoro National Park in central Bolivia, according to a paper published in the journal PLOS One on Wednesday.

The ripple marks extend in a northwest-southeast direction, which probably indicate how the dinosaurs and other animals moved along the paleocoastline, according to the paper.

Most of the tracks belong to theropods — a clade of dinosaurs from the Cretaceous period known for their bipedal mode of walking that includes the Tyrannosaurus rex, according to the study. Some of the preserved tracks, estimated to have been created between 145 million and 66 million years ago, also belong to birds, the researchers said.

View of “Ciudad de Itas” rocky outcrop at the Toro Toro National Park, in Bolivia on October 31, 2018.
Aizar Raldes/AFP via Getty Images

Carreras Pampa, which spans about 80,570 square feet, is home to the most preserved dinosaur footprints and highest number of dinosaur swim trackways in the world, as well as the highest number of preserved continuous dinosaur swim trackways in the world, according to the paper.

The plethora of tracks shows how the dinosaurs were walking, running and swimming but also how they dragged their tails or took sharp turns amid their journeys, according to the paper.

The swim tracks were likely imprinted when the theropods scratched the bottom of the water with their middle toe, resulting in grooves that appear straight or curved, like a comma, the researchers noted.

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Evidence from more than 1,275 trackways were collected and analyzed, according to the paper.

While most sites that feature dinosaur swim tracks only contain individual tracks, Carreras Pampa shows swim tracks that alternate between the right and left foot, according to the paper.

A variety of shapes and sizes of the tracks also indicate that several types of dinosaurs wandered along the ancient coastline as a prehistoric highway, the researchers determined.

Sauropods, theropods and ankylosaurus footprints from the Cretaceos period are seen at the Toro Toro National Park, where about 3.500 footprints were found turning the place into a new palaeontological attraction, in Bolivia on October 31, 2018.
Aizar Raldes/AFP via Getty Images

Some tracks had footprints that measured less than 4 inches, which is not common in the fossil record, according to the paper. Researchers hypothesize that those footprints belonged to either a smaller theropod species, like Coelophysis, or by juveniles of larger species.

Mid-size theropod dinosaurs, such as Dilophosaurus or Allosaurus, are likely responsible for the largest footprints, which measured more than 12 inches, according to the paper. Larger theropods, such as T. Rex and Giganotosaurus, usually create footprints larger than 16 inches, the researchers said.

Track depths vary from very shallow to very deep, and researchers were able to estimates of the speeds, gaits, and sizes of the track-makers, according to the paper.

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While paleontologists were previously aware of the mass existence of ancient footprints, this is the first time the track site was properly studied, according to the paper.

Researchers examined the footprints and swim tracks more closely by sweeping debris off the imprints and clearing sediment from regions where they expected to find additional tracks.

Bolivia is also home to the Cal Orck’o track site, a fossil bed located about 250 miles southeast of Carreras Pampa that also contains a high concentration of dinosaur tracks.

The country has one of the most extensive and diverse records of dinosaur track sites in the world, spanning the Triassic, Jurassic and Cretaceous periods, according to the paper.