NARRATOR: About 70 million years ago, far up in the Arctic Circle, it was the dead of winter.

It was cold, around negative 10 degrees Celsius.

But the real struggle in this environment was the oppressive darkness, up to four months of it at a time.

Most of the plants there had shut down for the winter due to the lack of sunlight.

But lichens and probably horsetails still survived.

And in this dark landscape, there was what might sound like an unlikely sight, the body of a dinosaur, specifically a hadrosaur called Edmontosaurus.

It was a huge plant-eater, about as big as a school bus, with a wide duck-billed snout.

But this dead dinosaur wasn't alone.

A living theropod was there too, Troodon, covered in a thick fuzz for warmth and with big eyes to help it see in the darkness.

It feasted eagerly at the carcass, crucial calories at a time when food could be hard to find.

All told, the Arctic in the cretaceous period was a rough place to live, especially in winter.

And yet, the fossils of many kinds of dinosaurs have been discovered there.

So how were they able to survive in this harsh environment?

Like, what would a herd of hadrosaurs eat when most of the plants stopped growing?

Did they eke out an existence every winter?

Or did they migrate?

If they did, how'd they do it?

The answers to these questions are written in the bones that the Arctic dinosaurs left behind.

And it looks like these polar dinos got by in ways that are similar to how some modern Arctic mammals survive today, with a little luck and some very particular adaptations both in their bodies and behavior.

The North and South Poles are among the most challenging places on Earth for any organism to live.

It's cold and dark through most of the winter.

And food can be really limited.

So in 1961, it came as a surprise to a geologist mapping rock exposures along the Colville River in Alaska when he found big fossil bones.

These fossils weren't fully studied until the 1980s, when a paleontologist rediscovered them in storage and identified them as belonging to a hadrosaur.

And ever since then, more and more fossils of dinosaurs, mammals, and plants have been found there that give us a better picture of what the Arctic was like in the Cretaceous.

For one thing, it was actually warmer than it is today.

During the Cretaceous Period, there was no ice around the North Pole.

Mean annual temperatures were about 6.3 degrees Celsius versus around negative 12 degrees Celsius today in the northern-most parts of Alaska.

And we know this from paleosols, or fossil soils.

Experts can use the carbon and oxygen isotopes in these soils to figure out at what temperatures they formed.

They can then use that to estimate the mean annual temperature for the environment where those soils were found.

Plus, fossil plants like horsetails, ferns, and deciduous trees also provide clues about the climate at the time in things like the size and shape of their leaves.

But these warmer temperatures didn't mean life was that much easier for dinosaurs living there.

While the mean temperature during the warmer months was a balmy 14.5 degrees Celsius, winter temps could drop as low as negative 10.

And remember those four months of darkness?

Well, that's because Alaska was farther north than it is today, at almost 85 degrees latitude north.

So the winter darkness would have lasted for about 120 days.

Without sunlight, many plants would have stopped photosynthesizing, which meant less food for big herbivores like Edmontosaurus.

These hadrosaurs were probably the most abundant dinosaur in the Arctic.

But they also had close relatives further south in places like North Dakota and Wyoming.

So did these hadrosaurs actually overwinter in a place so close to the North Pole?

Well, to try to figure this out, researchers began by closely studying the dinosaur's bones, specifically looking inside them at their histology or the microscopic structure of their tissues.

By examining the internal structure of bone, you can see how and when an animal grew over the course of its life, kind of like looking at the rings of a tree.

When food is abundant or more energy dense, dinosaurs could deposit bone tissues more rapidly and grow faster.

But when food is scarce, bone tissue is laid down more slowly.

And these two types of bone look different under a microscope.

One study compared the bones of Arctic Edmontosaurus to those that lived in a more temperate climate in southern Alberta.

And the bones of Arctic dinosaurs had alternating layers of normal bone growth and slower bone growth, suggesting that they might have spent up to half the year eating lower-quality foods like ferns and horsetails.

This is something that we see in some modern mammals today.

Some subspecies of caribou that overwinter in the Arctic do this too, surviving the season on a diet of lichen.

Now, the bones of the dinosaurs from farther south also showed some variation.

But it was less consistent.

It was more like they just had a bad year for food now and then.

So this evidence suggests that Edmontosaurus in the Arctic regularly went through some pretty lean times, either by toughing it out through the winter or maybe even by migrating in search of food.

And some experts do think that hadrosaurs might have migrated either within the Arctic Circle or maybe a bit farther south.

But there's more evidence to consider.

Paleontologists have found mass bone beds of juvenile Edmontosaurus in Alaska.

But they were probably caused by rivers flooding their banks in the spring, not winter.

And some researchers think that these little dinos would have been too small to migrate very far.

So while experts differ on how exactly Edmontosaurus spent their winters, they agree that those dinos were not alone.

In some of these Edmontosaurus bone beds, paleontologists have found tooth marks on bones, as well as shed teeth.

And some of those match the teeth of Troodon.

Based on how common their teeth are in the Arctic, it looks like this dinosaur was really abundant up North.

So how did Troodon survive?

Well, for one thing, it had large forward-facing eyes that probably helped it take in as much light as possible.

Based on research from other Troodon specimens, they had binocular vision, similar to modern avian raptors and a brain size within the range of extant birds.

This means they could have probably seen well in low light.

And like other members of its extended family, the Troodontidae, it was also probably covered in feathers, which would have helped insulate it against the cold.

But there was something else that these Arctic Troodon had that their southern relatives didn't that allowed them to survive-- large bodies.

The teeth of Troodon from the Arctic are about twice as big as those of Troodon from farther south.

So instead of being about two meters long, Arctic Troodon was probably twice that, more like four meters in length.

Some researchers think a lack of other predators might have made this large size possible.

But others think it could be the result of phenotypic plasticity.

This is when an organism's behavior, morphology, and physiology change in response to its environment.

For example, in many modern animals, like the gray wolf, we see larger sizes when their prey are larger or more abundant.

And in Troodon, getting bigger may have been a response to a greater availability of food since there wasn't much competition from other predatory dinosaurs.

Or it might have just been a way for Troodon to have bigger eyes, an advantage for hunting in low-light conditions.

But, finally, there was another dinosaur that lived in the cretaceous Arctic.

And it did things a little differently.

This carnivore was Nanuqsaurus, a northern relative of Tyrannosaurus Rex.

And while Arctic Troodon survived by getting bigger than its southern cousins, Nanuqsaurus did the opposite.

It got smaller.

It was about 6 meters long, only around half the length of T. Rex.

So why did Troodon get bigger than its relatives while Nanuqsaurus got smaller?

Well, again, it has to do with how much food is available.

If Nanuqsaurus was as big as its other Tyrannosaur relatives, it would have needed a lot more food to survive, especially if it was on the warm-blooded side.

But food was limited.

And it had at least some competition from Troodon.

So a smaller body would have improved the chances that Nanuqsaurus could survive in the Arctic.

It seems like there was sort of a Goldilocks zone of body size for carnivorous dinosaurs in the Arctic-- not too big and not too small, just right.

But being adapted for life in the Arctic still couldn't save these dinos.

They became extinct, along with the other non-avian dinosaurs about 65 million years ago during the extinction event at the end of the Cretaceous Period.

They were already used to harsh conditions and low-quality diets.

But the worldwide effects of the asteroid impact and volcanism that caused the K-Pg extinction probably pushed these animals to their breaking point.

For as long as they could, though, the dinosaurs of the Arctic coped.

And they did it with the help of evolutionary strategies that resemble those of animals that live there today, changing their diets and acquiring body sizes that helped them optimize their energy use.

Just goes to show that in a similar environment, if it was a good adaptation before, it will still probably be a good adaptation millions of years later.

While the players might change, life is still playing the same game.

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