When Dinosaurs Chilled in the Arctic

About 70 million years ago, high above the Arctic Circle, it was the middle of winter. It was cold, about -10 degrees Celsius. But the real struggle in this environment was the oppressive darkness: up to 4 months at a time. Most of the plants had closed for the winter due to lack of sunlight, but lichens and probably horsetails still survived. And in this dark landscape, there was what might seem an unlikely sight: the body of a dinosaur, specifically, a hadrosaur called Edmontosaurus. It was a huge herbivore, about the size of a school bus, with a wide, duck-billed snout. But this dead dinosaur was not alone.

There was also a living theropod, Troodon, covered in thick down for warmth and with large eyes that helped it see in the dark. He enthusiastically feasted on the carcass – crucial calories in a time

when

food could be difficult to find. Ultimately, the Arctic during the Cretaceous period was a difficult place to live, especially in winter. And yet, fossils of many types of

dinosaurs

have been discovered there. So how were they able to survive in this harsh environment? What would a herd of hadrosaurs eat

when

most plants stopped growing? Did they eke out a living each winter?

Or did they migrate? If they did, how did they do it? The answers to these questions are written in the bones left behind by Arctic

dinosaurs

. And it seems that these polar dinosaurs managed in a similar way to how some modern Arctic mammals survive today: with a little luck and some very particular adaptations, both in their bodies and in their behavior. The north and south poles are among the most difficult places on Earth for any organism to live. It is cold and dark for most of the winter, and food can be very limited. Then, in 1961, a geologist mapping rock exposures along the Colville River in Alaska was surprised when he found large fossil bones.

These fossils were not fully studied until the 1980s, when a paleontologist rediscovered them in storage and identified them as belonging to a hadrosaur. And since then, more and more fossils of dinosaurs, mammals and plants have been found there, giving us a better idea of ​​what the Arctic was like in the Cretaceous. For one thing, it was actually warmer than today. During the Cretaceous there was no ice around the North Pole. Average annual temperatures were about 6.3 degrees Celsius, compared to the current -12 degrees C in the northernmost parts of Alaska. And we know this from paleosols or fossil soils.

Experts can use the carbon and oxygen isotopes of these soils to determine at what temperatures they formed. They can then use it to estimate the average annual temperature of the environment where those soils were found. In addition, fossil plants, such as horsetails, ferns and deciduous trees, also provide clues about the climate at that time, in aspects such as the size and shape of their leaves. But these warmer temperatures didn't mean life was much easier for the dinosaurs that lived there. While the average temperature during the warmer months was a pleasant 14.5 degrees, winter temperatures could drop as low as -10 degrees.

And do you remember those four months of darkness? Well, that's because Alaska was further north than it is today, at almost 85 degrees north latitude, so the winter darkness would have lasted about 120 days. Without sunlight, many plants would have stopped photosynthesizing, which meant less food for large 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 really spend the winter in a place so close to the north pole? Well, to try to figure this out, researchers began by closely studying dinosaur bones, specifically looking at their interior, their histology, or the microscopic structure of their tissues.

By examining the internal structure of the bone, you can see how and when an animal grew throughout its life, sort of like looking at the rings of a tree. When food is abundant or has more energy density, dinosaurs can lay down bone tissue more quickly and grow faster. But when food is scarce, bone tissue is laid down more slowly and these two types of bone look different under the microscope. One study compared the bones of the Arctic Edmontosaurus with 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 they could have spent up to half the year eating lower-quality foods, such as ferns and horsetails.

This is something we see today in some modern mammals. Some subspecies of caribou that winter in the Arctic also do the same: they survive the season on a diet of lichens. Now, dinosaur bones from further south also showed some variation, but it was less consistent. It was more like they had had a bad year when it came to food from time to time. So this evidence suggests that Edmontosaurus in the Arctic regularly had some pretty tough times, either weathering the winter or perhaps even migrating in search of food. And some experts believe that hadrosaurs could have migrated, either within the Arctic Circle or perhaps a little further south.

But! There is more evidence to consider! Paleontologists have found huge beds of juvenile Edmontosaurus bones in Alaska, but they were probably caused by rivers overflowing their banks in spring, not winter. And some researchers believe these little dinosaurs would have been too small to migrate very far. So while experts differ on how exactly Edmontosaurus spent their winters, they agree that those dinosaurs weren't alone. In some of these Edmontosaurus bone beds, paleontologists have found tooth marks on the bones, as well as fallen teeth. And some of them match Troodon's teeth. Based on how common its teeth are in the Arctic, it seems that this dinosaur was very abundant in the north.

So how did Troodon survive? Well, for starters, it had large forward-facing eyes, which probably helped it capture as much light as possible. According to research on other Troodon specimens, they had binocular vision similar to that of modern birds of prey and a brain size within the range of extant birds. This means they could probably see 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 from the cold. But there was something else these Arctic Troodons had that their southern relatives didn't that allowed them to survive: larger bodies.

The teeth of the Arctic Troodon are about twice as large as those of the southern Troodon. So instead of being about 2 meters long, Arctic Troodon was probably twice that, more like 4 meters long. Some researchers believe that the lack of other predators could have made this large size possible, but others think it could be the result of phenotypic plasticity. This occurs when an organism's behavior, morphology, and physiology change in response to its environment. For example, in many modern animals, such as the gray wolf, we see larger sizes when their prey is larger or more abundant. And in Troodon, the growth may have been a response to greater food availability, since there wasn't much competition from other predatory dinosaurs.

Or it could have simply been a way for Troodon to have larger eyes, an advantage for hunting in low-light conditions. But finally, there was another dinosaur that lived in the Cretaceous Arctic and did things a little differently. This carnivore was Nanuqsaurus, a northern relative of Tyrannosaurus rex. And while Arctic Troodon survived by becoming larger than its southern cousins, Nanuqsaurus did the opposite. It became smaller: it was about twenty feet long, only about half the length of a T. rex. So why did Troodon get bigger than its relatives while Nanuqsaurus got smaller? Well, again, it has to do with the amount of food available.

If Nanuqsaurus were as large 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 at least he had some competition from Troodon. So a smaller body would have improved Nanuqsaurus' chances of surviving in the Arctic. There appears to have been a sort of “Goldilocks zone” of body size for carnivorous dinosaurs in the Arctic; Not too big, not too small, just right. But adaptation to life in the Arctic still could not save these dinosaurs. They went extinct along with other non-avian dinosaurs about 65 million years ago during the extinction event at the end of the Cretaceous Period.

They were already accustomed to harsh conditions and low-quality diets. But the global effects of the asteroid impact and volcanism that caused the K-Pg extinction likely pushed these animals to their breaking point. But as long as they could, Arctic dinosaurs managed, and they did so with the help of evolutionary strategies that resemble those of animals living there today: They changed their diets and acquired body sizes that helped optimize their use of energy. energy. It just goes to show that, as far as evolution is concerned, if it was a good adaptation before, it will still be a good adaptation millions of years later.

While the players may change, life continues to play the same game. Want more Eons content? Be sure to follow Eons on social media! You can find us on Instagram, Twitter and Facebook. And you can join me on Instagram at Fossil_Librarian. I have to thank this month's eontologists for chilling with us: Lucan Curtis-Mahoney, Sean Dennis, Jake Hart, Jon Davison Ng, Patrick Seifert, and Steve. Remember that all levels of pledge have access to our Discord, so join us and become an Eonite at patreon.com/eons. Also thank you for joining me in the Konstantin Haase study. And if you like what we do here, be sure to subscribe at youtube.com/eons.