Dinosaur footprint “coliseum” unearthed in Alaska

A 70-million-year-old dinosaur track site about the size of a soccer field and described as “beautiful” has been discovered in Alaska.

Dubbed “The Coliseum” by the researchers who discovered it, the site in the Denali National Park and Preserve is the largest single dinosaur track site in the US state.

The Coliseum contains the footprints of multiple species of dinosaur spanning many generations who roamed interior Alaska toward the end of the Cretaceous period (145–66 million years ago), just before the demise of dinosaurs. The researchers’ findings are published in the journal Historical Biology.

“It’s not just one level of rock with tracks on it,” says lead author Dustin Stewart, a former University of Alaska Fairbanks graduate student who published the paper as part of his master’s thesis. “It is a sequence through time. Up until now, Denali had other track sites that are known, but nothing of this magnitude.”

Denali supported a river system and was much warmer than today. Modern average temperatures in the region are below freezing, -2.2°C, while scientists estimate that 70m years ago the average was a balmy 10°C. Due to continental drift, Denali would have been further north in the late Cretaceous. So, it was hardly a tropical paradise. Some regions would have supported permafrost and ice fields.

The Coliseum tracks give a sense of the kinds of dinosaurs that called this habitat home. Among them are hardened impressions in ancient mud and casts of tracks created when sediment filled the track and then hardened.

“They are beautiful,” says senior author Dr Pat Druckenmiller, University of Alaska Museum of the North director. “You can see the shape of the toes and the texture of the skin.”

3d heat map dinosaur footprint
A single large meat-eating dinosaur track, likely from a tyrannosaur. The image was made by taking multiple photos from different angles to produce a three dimensional view of the track, accentuated by colours. Credit: Dustin Stewart.

A variety of juvenile to adult dinosaurs walked through the area over thousands of years. Most common among the tracks were those left behind by hadrosaurs – duck-billed dinosaurs – and horned dinosaurs. There were also meat-eating dinosaurs including a tyrannosaur and raptors, and even small wading birds.

“It was forested and it was teeming with dinosaurs,” Druckenmiller comments. “There was a tyrannosaur running around Denali that was many times the size of the biggest brown bear there today. There were raptors. There were flying reptiles. There were birds. It was an amazing ecosystem.”

The team also found fossilised plants, pollen grains, and evidence of freshwater shellfish and invertebrates.

“All these little clues put together what the environment looked like as a whole,” Stewart notes.

Preserving the fossil site is an important part of the National Park Service’s mission, says the park’s geologist Denny Capps.

“On one hand, we must protect world-class fossil sites like The Coliseum from disturbance and theft,” Capps says. “On the other hand, we encourage visitors to explore for fossils in their geologic context to better grasp the evolution of landscapes and ecosystems through time, while leaving them undisturbed for others to appreciate.”

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