About 70 million years ago, far up in the
Arctic Circle, it was the dead of winter.
It was cold - around -10 degrees Celsius.
But the real struggle in this environment
was the oppressive darkness - up to 4 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 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 -12 degrees C today
in the northernmost 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 that time, in things like the
size and shape of their leaves.
But these warmer temperatures didn’t mean
life was that much easier for the dinosaurs
living there.
While the mean temperature during the warmer
months was a balmy 14.5 degrees, winter temps
could drop as low as -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’ve 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 dinosaurs’
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 can deposit bone tissue 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 the Arctic dinos had alternating
layers of normal bone growth and slower bone
growth, suggesting that they might’ve 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 dinosaurs from farther south
also showed some variation, but it was less
consistent.
It was more like they just had the 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 bonebeds of
juvenile Edmontosaurus in Alaska, but they
were probably caused by rivers flooding their
banks in the spring - not winter.
And some researchers think these little dinos
would’ve 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 bonebeds, paleontologists
have found tooth marks on the 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, 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 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’ve 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: larger bodies.
The teeth of Troodon from the Arctic are about
twice as big than those of Troodon from farther
south.
So instead of being about 2 meters long, Arctic
Troodon was probably twice that, more like
4 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’ve 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 six meters long,
only around half the length of a T. rex.
So why did Troodon get bigger than its relatives
while Nanuqsaurus got smaller?
Well, again, has to do with how much food
is available.
If Nanuqsaurus was as big as its other tyrannosaur
relatives, it would’ve 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’ve improved the
chances that Nanuqsaurus could survive in
the Arctic.
It seems like there was a sort of “Goldilocks
zone” of body size for carnivorous dinosaurs
in the Arctic; not too big, not too small,
juuuust 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 optimize their energy use.
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 might change, life is still
playing the same game.
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Gotta thank this month’s Eontologists for
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Sean Dennis, Jake Hart, Jon Davison Ng, Patrick
Seifert, and Steve!
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