The bizarre-looking therizinosaurs, which had "hands like Freddy Krueger", and the duck-billed hadrosaurs may have made for an odd couple, but recently discovered tracks in Denali National Park, Alaska, suggest they co-existed in Cretaceous-era America. Not only that but the presence of these two dinosaurs in The Last Frontier supports a theory that the state acted as a landbridge – or "superhighway" – between Asia and the Americas. 

The latest findings, published in the journal Scientific Reports, follow on from a 2012 discovery involving a single footprint from a therizinosaur, normally found in Asia, in Denali National Park in the US. Subsequent excavations unearthed many more tracks made from therizinosaurs – and, unexpectedly, some made from hadrosaurs. While there have been single tracks of either a therizinosaur or a hadrosaur, this is the first time the two have been spotted together in one bedding plane of rock.

"Hadrosaurs are very common and found all over Denali National Park. Previously, they had not been found alongside therizinosaurs in Denali," Anthony Fiorillo, chief curator and vice president of research and collections at the Perot Museum of Nature and Science in Dallas, Texas, said in a statement. 

However, it is not the first time the fossil record has linked the two herbivores.

"In Mongolia, where therizinosaurs are best known... 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," said Fiorillo. 

The evidence here, Fiorillo hypothesized, could indicate that this part of Alaska acted as a thoroughfare or, in his words, a "superhighway" for animals like the therizinosaur and hadrosaur between western North America and Asia. To find out, he led an international team of palaeontologists and other geoscientists who studied the ancient ecosystem of the area as well as the tracks themselves to confirm whether or not they were indeed those of a therizinosaur (which it turned out, they were).

Fossils, including one that appears to have been from a water lily, revealed that the location of the tracks was an area of Denali National Park that once had a wet, marsh-like climate, an environment that would have suited both the therizinosaurs and the hadrosaurs. In fact, it is this watery habitat that Fiorillo and his team suspect led these two beasts to co-habit, rather than "sophisticated multi-taxonomic herd behavior".   

"Given the similarity of track preservation, it seems likely that these two taxa occupied the same environment at the same time," the study authors write.

"It would appear though that the taxa were not linked by complex mutually beneficial behavior [such as predator avoidance or sharing food]. Rather it is more likely that an aspect of the environment led to the co-occurrence of these taxa and their tracks."

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