In the dark and cold northernmost part of the earth, a group of dinosaurs once thrived

The name Dinosauria was formally proposed by British paleontologist Richard Owen in 1842. In the following century, dinosaurs were portrayed as cold-blooded, slow-moving lizards that lived in a warm environment all year round.

Since the 1950s, fossils of (non-avian) dinosaurs have been discovered in the Arctic and Antarctic, such as the famous Antarctic Ankylosaur and Cryolophosaurus.

As dinosaur research continues to advance, scientists have also discovered that at least some dinosaurs were active endotherms (commonly known as warm-blooded animals), which raises a question - did polar dinosaurs live at the poles all year round, or did they migrate seasonally?

Imaginary picture of dinosaur ecology in the Prince Creek Formation (Polar) (Photo credit: Julio Lacerda)

The "migration" hypothesis holds that small dinosaurs stayed in the Arctic to overwinter; large herbivorous dinosaurs (hadrosaurs and triceratops) became the main force of migration, traveling 3,200 kilometers every autumn, heading south to the warm low latitudes, and returning to the Arctic in the spring of the following year; and carnivorous dinosaurs (such as Tyrannosaurus) would follow the migrating herbivores, making long journeys together, over and over again, which was a spectacular sight.

Imaginary picture of the migration of Pachyrhinosaurus (Photo source: Screenshot from the BBC film "Walking with Dinosaurs")

Since there is very little evidence that dinosaurs lived in the polar regions all year round, the grand and epic dinosaur "migration" hypothesis is often mentioned and brought to the screen.

However, a study in May 2021 strongly supports the hypothesis that dinosaurs "lived at the poles all year round":

Scientists have discovered the Earth's northernmost dinosaur fossil site to date in Alaska, which contains abundant juvenile dinosaur bones that can provide new insights into the reproduction, development and life of polar dinosaurs.

The northernmost dinosaur on Earth

Back in the late Cretaceous period (about 72 million years ago), North America was still divided by shallow seas:

The eastern continent was called Appalachia and the western continent was called Laramidia, the latter of which stretched from Mexico to Alaska, where the Prince Creek Formation, the northernmost known dinosaur fossil site on Earth, was formed.

Location and paleoenvironmental conditions of the Prince Creek Formation: (A) North America 72 million years ago (B) Polar night time of strata at different latitudes (Image source: modified from reference 1)

Although there were no polar ice sheets during the Late Cretaceous, fossil evidence suggests that the Prince Creek Formation was an Arctic woodland composed of coniferous forests and ferns.

However, the ancient latitude here is about 80°N-85°N, located within the Arctic Circle. There is no sunlight for about 120 days a year, and the average annual temperature is only 6.3°C ± 2.2°C. It is shrouded in darkness and cold all year round.

After entering the long winter night, the dinosaurs here will not only face the dilemma of plant withering and food shortage, but also have to endure the invasion of the cold climate.

If that’s the case, then the dinosaurs had no choice but to migrate south to spend the winter?

However, judging by the fossil assemblage of the Prince Creek Formation, the dinosaurs appear to have chosen to stay.

Arctic "Maternity Ward"

The sediments of the Prince Creek Formation were formed in a flood environment, with hundreds of dinosaur bones piled up together. Through 30 years of field research, scientists carefully selected and identified 7 different groups of dinosaurs (in addition to a bird), including duck-billed dinosaurs, ceratopsians, tyrannosaurus, dromaeosaurs, troodonts, etc. They vary in size and diet, which reveals the diversity of polar dinosaurs.

In addition to dinosaurs, many mammals were also found here, but there were no ectothermic animals such as turtles, tortoises, and squamates. This also supports two viewpoints: the temperature of the Prince Creek Formation is not suitable for the survival of ectothermic animals; these seven types of dinosaurs are endothermic animals like mammals.

What is most surprising is that scientists discovered hundreds of tiny dinosaur bones and teeth by using very fine sieves (≥500μm).

According to bone histological analysis, they are all dinosaurs in the incubation and juvenile stages - there is no doubt that in the late Cretaceous period, this coastal plain within the Arctic Circle was favored by at least seven types of polar dinosaurs, where they reproduced, built nests, and hatched...

Seven types of dinosaur fossils from the Prince Creek Formation, including the size comparison of the tooth fossils of juvenile dinosaurs and adult dinosaurs. (AG) They may belong to the troodontidae, sauropoda, Parkosaurinae, scaphoceratops, hadrosauridae, ceratopsianidae, and tyrannosauridae, respectively (Image source: Reference 1)

This is direct evidence that dinosaurs reproduced in the Arctic, because dinosaur cubs could not withstand large-scale migration and could not move here from other places with their parents while still in their infancy.

Then think about it, would the dinosaurs hatch in the Arctic, develop for a period of time, and then migrate south to hibernate before the polar night arrives?

Only in the Arctic

Past studies have shown that the incubation period for dinosaur eggs is approximately 2.5 to 6 months, depending on their size.

For dinosaurs living at low latitudes, the temperature might have been suitable for hatching eggs throughout the year, so they could reproduce all year round. However, for dinosaurs in the Prince Creek Formation within the Arctic Circle, due to the influence of the polar night, the temperature suitable for hatching eggs only lasted for about 7 months each year (March to September), which limited them to only one litter of viable cubs each year.

For larger dinosaurs (such as hadrosaurs), the hatching of this nest of cubs may not be completed until September at the latest, when the leaves have already fallen in the Arctic, and a month later it enters the polar night that lasts about four months; for smaller dinosaurs (such as leptoceratops), even if the cubs can be hatched in June, the results of growth and development studies show that they will not be able to grow to a physique that can support long-distance migration before the arrival of autumn.

Dinosaur incubation period of the Prince Creek Formation (Image source: modified from reference 1)

It can be seen that due to the long incubation period, short breeding window, smaller dinosaur cubs and growth and development limitations, the newly born dinosaurs each year did not have enough time to migrate to the warm south and were forced to stay in the Arctic, accompanied by darkness and cold. Therefore, these seven types of dinosaurs did live in the Arctic all year round.

While the fossil evidence from the Prince Creek Formation provides new insights that support the hypothesis that dinosaurs nested and lived in the Arctic year-round, it also raises an important and intriguing mystery:

The average polar night temperature in the Prince Creek Formation is −2.0°C ± 3.9°C, occasionally dropping to −10°C, with darkness lasting about 120 days. Food resources are also very limited in winter, making it a challenging and dangerous environment.

So, how did these dinosaurs survive?

Cave dwelling? Fasting period? Feathers?

Limited by fossil information, we do not yet know what other adaptive habits the dinosaurs of the Prince Creek Formation evolved to cope with the cold, darkness, and seasonal food shortages so that they could live in the Arctic Circle year-round, in addition to the necessary endothermy.

We can only make the following guesses based on fossils from other places and the physiological habits of modern animals:

Cave dwelling

For small dinosaurs, burrowing is a good wintering strategy. When winter comes, adult dinosaurs use natural caves or dig caves, curl up in them to protect young dinosaurs, and wait for the first rays of sunshine of the next year.

Evidence of dinosaur caves from the Cretaceous period has been discovered in Montana, USA. Deep in a two-meter-long cave, there are three skeletons of Diplodocus dinosaurs, one large and two small.

Suspected dinosaur cave structures have also been found in high-latitude sediments in Victoria, Australia, so scientists believe that burrowing may be an adaptive evolution of dinosaurs to the polar environment.

Photo and diagram of the Diggerosaurus Cave in Montana, with three Diggerosaurus fossils at the deepest part of the cave (Image source: modified from reference 2)

Fasting period

For large herbivorous dinosaurs, burrowing was not so friendly. Putting on weight in the autumn to cope with the food shortage in the winter may be a possible strategy. During the long fasting period, they could only make do with a few bites of bark, ferns, and mosses occasionally.

As for carnivorous dinosaurs? Just look for other animals in the ice and snow...

feather

Don't forget that dinosaurs could have feathers, too, especially theropods.

Although no dinosaur feather fossils have been found in the Wangzixi Formation, since various feathered dinosaurs have continued to emerge in western Liaoning, the image of many dinosaurs has changed from "bald lizards" to "furry walking chickens." They rely on feathers to fly, camouflage, attract the opposite sex... and of course, the most basic warming function is indispensable.

The climate in western Liaoning was cold in the early Cretaceous period, with an average annual temperature of about 10°C. Even the 9-meter-long predator Yutyrannus was covered with feathers.

Large theropod dinosaur - Yutyrannus magnificent (Photo credit: Brian Choo)

In addition, a study in 2019 reported for the first time the fossils of polar dinosaur feathers. 118 million years ago, southeastern Australia was still within the Antarctic Circle and was also shrouded in cold and darkness during the winter. The dinosaurs (including birds) living here accidentally shed their feathers by the lake and sank to the bottom of the lake and were buried for hundreds of millions of years. Now they have reappeared in the form of fossils.

Polar dinosaur feathers in Australia (Image source: Reference 4)

Combined with these feather fossils, we can try to imagine the dinosaurs of the Prince Creek Formation:

As snowflakes fell and polar night approached, adult dinosaurs and their newborn cubs slowly moved forward, looking for a suitable shelter. Unconsciously, their feathers were dyed whiter and whiter...

Unfortunately, imagination is just imagination after all. We are still a long way from uncovering the life history of polar dinosaurs. Confirming that at least seven types of dinosaurs lived in the Arctic all year round is only a small step. More answers can only be entrusted to the dinosaur fossils buried deep in the Arctic Circle, waiting to be discovered.

References:

[1] Druckenmiller, PS, Erickson, GM, Brinkman, D., Brown, CM, and Eberle, JJ (2021). Nesting at extreme polar latitudes by non-avian dinosaurs. Curr. Biol. 31, 3469– 3478.

[2] Varricchio David J, Martin Anthony J and Katsura Yoshihiro 2007First trace and body fossil evidence of a burrowing, denning dinosaurProc. R. Soc. B.2741361–1368.

[3] Xu,

[4] Kundra´t, M., Rich, TH, Lindgren, J., Sjo¨vall, P., Vickers-Rich, P., Chiappe, LM, and Kear, BP (2019). A polar dinosaur feather assemblage from Australia. Gondwana Res. 80, 1–11.

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