Fossil dinosaur tracks show Alaska was lush and warm

A large group of fossilised dinosaur footprints, plants and tree stumps in Alaska’s far northwest reveal a very different environment 100 million years ago.

Fossilised tree trunk and tape measure
Fossilised tree trunk. Credit: Geophysical Institute, Anthony Fiorillo / University of Alaska Fairbanks.

The new find is detailed in a paper published in the journal Geosciences.

Alaska is home to several beautifully preserved dinosaur trackways. But the newly discovered tracks are much older.

“We’ve done work in three other formations – in Denali, on the North Slope and in Southwest Alaska – and they’re about 70 million years old,” says co-author and University of Alaska Fairbanks geology professor Paul McCarthy. “This new one is in a formation that’s about 90 to 100 million years old.”

The tracks were found in the Nanushuk Formation. The rocks from this period formed about the same time geologists believe a connection between Asia and North America called the Bering Land Bridge began to form.

“We want to know who was using it, how they were using it and what the conditions were like,” says lead author Anthony Fiorillo, executive director of the New Mexico Museum of Natural History & Science.

Dinosaurs laid down these tracks during the middle of the Cretaceous period (145–66 million years ago).

“The mid-Cretaceous was the hottest point in the Cretaceous,” McCarthy explains. “The Nanushuk Formation gives us a snapshot of what a high-latitude ecosystem looks like on a warmer Earth.”

They found 75 fossil tracks and fossilised tree stumps about 60 cm in diameter. Most abundant among the different dinosaurs were two-legged plant eaters.

Bipedal herbivores make up 59% of the tracks. Four-legged plant eaters account for 17%, while birds are 15% and non-avian, mostly carnivorous, bipedal dinosaurs are 9% of the tracks.

“It was just like we were walking through the woods of millions of years ago,” Fiorillo says. “For at least 400 yards we were walking on an ancient landscape. On that landscape we found large upright trees with little trees in between and leaves on the ground. We had tracks on the ground and fossilised faeces.”

During the mid-Cretaceous, the area would have been a warm riverine or delta setting.

Theropod dinosaur fossilised track diagram scale
Theropod track. Credit: Anthony Fiorillo / Geophysical Institute, University of Alaska Fairbanks.

Carbon isotope analysis suggests the region received about 1,800 mm of rain annually.

The site is 10–15° further north in latitude than it was during the Cretaceous. Today, temperatures in the North Slope County where the fossils were found are about 2 to 13 °C in summer and −21 to −29 °C in winter.

“The temperature was much warmer than it is today, and what’s possibly more interesting is that it rained a lot,” Fiorillo said. “The samples we analysed indicate it was roughly equivalent to modern-day Miami. That’s pretty substantial.”

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