Arctic fossils suggest dinosaurs migrated across the Bering Land Bridge

A version of the Bering Land Bridge existed in the Cretaceous.

2075
Ancient Arctic dinosaurs, such as those in the genus Edmontosaurus, might have also used an earlier version of the Bering Land Bridge. (Masato Hattori illustration)

A new look at a Cretaceous dinosaur discovered in Arctic Alaska casts doubt on whether it should be classified as a separate species and genus — but that in turn points to a different exciting development.

The dinosaur previously called Ugrunaaluk kuukpikensis, after Iñupiat terms for meaning “ancient grazer of the Colville River,” might be a variation on the Edmontosaurus, a genus of duck-billed hadrosaurs with a very wide distribution over northern North America, according to a new study published in the journal PLOS ONE.

If so, the new analysis adds to some evidence supporting the theory that dinosaurs — like the mammals and humans that followed long after them — used a Cretaceous-era land bridge to cross between Asia and North America, said Anthony Fiorillo, co-author of the study and an expert in Alaska dinosaurs.

[The COVID-19 pandemic has halted most US Arctic field research for 2020]

There are similarities to a different dinosaur recently discovered in Japan, the Kamuysaurus japonicas. Its nearly complete skeleton was found in 2013, and analysis by a scientific team that included Fiorillo determined that it was a type of hadrosaur — and that it looked a lot like an Edmontosaurus.

“We talked about the possibility — is this really the first Asian Edmontosaurus?” said Fiorillo, chief curator of the Perot Museum of Nature and Science in Dallas.

The answer was no, but the similarities were striking, he said. “The beauty of this one is that this is as close to having the same dinosaur on both sides as I think we’ve found.”

Versions of the Bering Land Bridge have come and gone over geologic eras. There is evidence of such bridges going back to about 100 million years ago, Fiorillo said.

The similarities between Kamuysaurus japonicas and Edmontosaurus add to another important piece of evidence supporting the idea of Cretaceous Bering Land Bridge crossings — the mingling of hadrosaur footprints in Alaska with footprints a long-clawed dinosaur that appears to be those of an Asian-type dinosaur, the therizinosaurus. Those mingled prints were found in Denali National Park, where Fiorillo has done extensive work.

When it comes to Ugrunaaluk kuukpikensis, Fiorillo and his research partners base their opinion on the fact that the specimen discovered and used for the identification was a juvenile.

It was one of the many discoveries made in a fossil-rich area of the Colville River on Alaska’s North Slope. There have been so many Cretaceous fossils found there that the site is known as the Liscomb Bonebed, named after Robert Liscomb, a Shell Oil geologist who in 1961 was the first to discover dinosaur bones there.

The trove of polar dinosaur remains at that site has reshaped thinking about dinosaurs in general. The existence of dinosaurs in such a far-north area suggests that the ancient animals were partially warm-blooded, not cold-blooded reptiles as previously believed.

A challenge to analyzing the Liscomb Bonebed discoveries, Fiorelli said, is that most of them have been of juvenile animals. Juvenile and adults can vary a lot, making it difficult to pinpoint species, he said.

Why does the bone bed hold the remains of so many ancient juveniles? Fiorillo said one theory is that is one theory spring floods from high-altitude snowmelt caught more juvenile dinosaurs, resulting in their remains being deposited into the same place.

In the Cretaceous era, what is now Alaska’s North Slope was even farther north than it is now. It was also warmer than it is now, though not exactly hot; it likely had a climate similar to the today’s Pacific Northwest. Meanwhile, an earlier version of the Brooks Range was probably even taller than the current mountain range, Fiorillo said. That means there were snowfields on the mountaintops that melted in spring, creating flood “kill events,” he said.

Analysis of the bones discovered to date backs up the idea that the dinosaurs died in the spring, after a winter of little food availability, he said.

“It looked like the dinosaurs had died just as they were coming out of a period of slow growth.” That would match the springtime, after a winter of little food availability. The theory is that smaller and weaker juveniles were killed in the floods, but the adults were able to survive.

Duck-billed hadrosaurs were plentiful across what is today’s North Slope, scientists say. They did not migrate vast distances like caribou do now, but they didn’t sit still, Fiorillo said.

“Otherwise, they would eat everything in sight,” he said. “They probably ambled, if you will, zig-zagging across the landscape.”

Fiorillo himself won’t be ambling across the Alaska landscape this year. The coronavirus pandemic has halted field expeditions around the Arctic and subarctic, including his.

“It’s kind of weird to think that this is the first time in 23 years I won’t be traveling to Alaska,” he said.