Bones and skeletons can only teach us so much about ancient animals – fossil footprints can reveal how different creatures interacted and lived together. Now one of the most unusual examples has been uncovered in Alaska, with hadrosaur and therizionosaur tracks found side-by-side in what may have been the "dinosaur superhighway" connecting Asia to North America.

Hadrosaurs were basically the cows of the dinosaur age. This group of "duck-billed" herbivores was very common across the globe, and likely grazed in herds by watering holes. Therizinosaurs, on the other hand, are rarer and much weirder-looking. They kind of look like a cross between a sloth and a swan, weighed five tonnes and sported the longest claws of any known animal ever. But despite their intimidating appearance, they were herbivores too.

Individually, fossils of therizinosaurs and hadrosaurs have previously been found across Asia and North America, and in Mongolia they've turned up in the same layers of rock, indicating they likely lived together. But the new find is the first time this co-existence has been discovered in footprint form.

"Hadrosaurs are very common and found all over Denali National Park (Alaska)," says Anthony Fiorillo, lead author of the new study. "Previously, they had not been found alongside therizinosaurs in Denali. In Mongolia, where therizinosaurs are best known – though no footprints have been found in association – skeletons of hadrosaurs and therizinosaurs have been found to co-occur from a single rock unit so this was a highly unusual find in Alaska, and it prompted my interest. From our research, we've determined that this track association of therizinosaurs and hadrosaurs is currently the only one of its kind in North America."

The discovery played out over a few years. In 2012, the researchers studied a lone footprint that they determined belonged to a therizinosaur, and more detailed examination of the area in the following years turned up dozens more, as well as many hadrosaur tracks. Closer studies of the rocks and plant fossils suggest that the landscape at the time was wet and marshy.

But the discovery has wider implications than just determining dinosaur roommates. These two creatures aren't the only ones to have been discovered in both Asia and North America, and it's long been believed that ancient Alaska served as a land bridge connecting the continents. These tracks support that hypothesis.

"This discovery provides more evidence that Alaska was possibly the superhighway for dinosaurs between Asia and western North America 65 to 70 million years ago," says Fiorillo.

Tens of millions of years later, and it seems like our own ancestors crossed that same tract of land as they migrated out of Africa, through Asia and into the Americas.

The research was published in the journal Scientific Reports.