What dinosaur tracks tell us about life in ancient Alaska

The researchers had to wait for the right lighting conditions to reveal the tracks, which are a combination of impressions and casts in the ancient mud. “When the sun angles itself perfectly with those beds, they just blow up,” Stewart said. “Immediately, all of us were just flabbergasted, and then Pat said, ‘Get your camera.’ We were freaking out.”

The tracks belong to a variety of dinosaurs that thrived in what is now Interior Alaska in the Late Cretaceous Period. The most common ones were large herbivorous duck-billed and horned dinosaurs, but there were also rare carnivores such as raptors and tyrannosaurs, as well as small wading birds. The tracks show the size, shape, and texture of the dinosaur feet and skin.

Fossils of past ecology

The researchers also found fossilized plants, pollen grains, and freshwater shellfish and invertebrates at the site, which provide clues about the environment and climate of the area. The site was part of a large river system, with ponds and lakes nearby. The area was warmer than today, more like the Pacific Northwest. There were coniferous and deciduous trees and an understory of ferns and horsetails.

Pat Druckenmiller, senior author of the paper and director of the University of Alaska Museum of the North, said that the site offers a glimpse into a fascinating ecosystem that existed in Denali millions of years ago. “It was forested and it was teeming with dinosaurs,” he said. “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 park’s geologist, Denny Capps, said that preserving fossil sites like The Coliseum is an important part of the National Park Service’s mission. He said that, on the one hand, they must protect world-class fossil sites like The Coliseum from disturbance and theft. On the other hand, he said they encourage visitors to explore fossils in their geologic context to better grasp the evolution of landscapes and ecosystems through time while leaving them undisturbed for others to appreciate.

Druckenmiller said that he plans to continue collaborating with the National Park Service to study The Coliseum and other track sites. He said their track research in the park compliments their work on dinosaur bones they collect in northern Alaska along the Colville River. He said, “Denali National Park and Preserve is a world-class area for dinosaur tracks. There is a lifetime of exploring left to do, and I can only wonder what other surprises await.”

The study was published in Historical Biology

Study abstract:

The Upper Cretaceous Cantwell Formation in Denali National Park and Preserve (DENA) preserves an abundant and diverse ichnofossil record of high latitude dinosaurs. Field-based investigations of the formation remain limited due to its wide areal extent and remoteness, leaving questions concerning its temporal and faunistic relationships to other units unaddressed. Here we describe the largest tracksite known in DENA and all of Alaska – a 7500 square metre outcrop of steeply dipping beds known as the Coliseum. The site exposes 66.3 metres of vertical section consisting of laterally extensive fine- to medium-grained sandstone, mudstone, calcareous shale, and bentonite. Vertebrate trace fossils include true tracks, transmitted tracks, natural casts, and trackways documented via handheld and UAV-assisted photogrammetry, enabling three-dimensional mapping of the site. Facies analysis reveals a depositional environment of crevasse splay and overbank deposits within a fluvial floodplain. U-Pb dating of bentonite collected from the site returned an age of 69.3 ± 0.9 Ma, improving the limited temporal constraints of the formation. We document several ichnotaxa, some new to the formation, representing ornithopods, ceratopsids, and large and small-bodied avian and non-avian theropods. The Coliseum provides a rare window into a high latitude, Late Cretaceous forested ecosystem deposited in a Greenhouse World.