The paleontologists who work the site have, to put it mildly, a difficult commute. It begins with a 500-mile drive north from the University of Alaska Fairbanks along the oil pipeline to Prudhoe Bay.

“From there we fly a few hundred miles to the river in a plane with balloon tires that can land on gravel bars,” Dr. Erickson said. “Then we use inflatable boats to get around. It takes seven or eight flights to get all the equipment in.”

The climate when U. kuukpikensis flourished was much warmer than today, with average temperatures in the low 40s.

“These animals were living in a very strange world,” said another member of the team, Patrick Druckenmiller, earth sciences curator at the University of Alaska Museum of the North in Fairbanks. “They probably had freezing and snow in the winter, and they had to survive four months of complete darkness. Finding food would be difficult. The plants are not growing at this time, and they would have to live on low-quality forage: ferns, twigs and bark.”

The find was described in the journal Acta Palaeontologica Polonica.

“The neatest thing is that our work is showing that there were dinosaurs thriving above the Arctic Circle, and all the ones we’re finding are unique to Alaska,” Dr. Druckenmiller said. “These are not the same species as at lower latitudes. What we have is a unique community of dinosaurs that lived in the polar regions when the world was a very different place.”