When we think about dinosaurs, we probably imagine them walking through a dense, tropical jungle or wandering in a warm, foggy swamp. But as a matter of fact, some dinosaur species lived in very high latitudes, as the ones found in the Prince Creek formation. This Alaskan geologic formation is one of the most important sources of arctic dinosaurs, as many fossils have been found in it. In this entry, we’ll describe some of these dinosaurs from the North Pole, and we’ll explain some of the difficulties they had to endure in order to survive in the northernmost point of the planet.
ALASKA 75 MILLION YEARS AGO
The Prince Creek formation is situated in the north of Alaska and dates from around 80-60 million years ago, at the end of the Cretaceous, the last period of the Mesozoic. At that time, North America was divided by the Western Interior Seaway; the eastern continent or Appalachia, and the western continent or Laramidia, north of which the Prince Creek formation was deposited.
At the end of the Cretaceous period, the Prince Creek formation was further north than it is today. Yet, at that time the Earth was going through a greenhouse effect phase, so the climate was a little warmer than it is today. It is thought that the mean annual temperature at Prince Creek was about 5°C, with summer maximums at about 18-20°C. Still, the difference in temperature between summer and winter would have been quite remarkable (currently, at the same latitude, it’s about 56°C).
Even if temperatures were not as low as the ones of present-day Alaska, the dinosaurs of Prince Creek had to endure long, dark winter months. Yet, the slightly higher temperatures and the proximity to the sea, produced a higher diversity of plant species. Observing the fossilized flora, we know that the landscape was that of a polar woodland, with angiosperm-dominated forests and a large number of fern, moss and fungus species, with some areas of seasonally-flooded grasslands.
As for the fauna, palaeontologists were surprised at the great number of big animals found. The fact that dinosaurs were found in such high latitudes is what makes us think that these were endotherm animals that generated their own body heat. Also in Prince Creek, there aren’t any fossils of other ectotherm reptiles like turtles, crocodiles or snakes, which are usually found in other United States deposits of the same period. Currently, dinosaurs are thought to be neither endotherm nor ectotherm, but mesotherm animals, which generated body heat metabolically, but were unable to control its temperature or keep it stable.
The relatively abundant vegetation, allowed the presence of a great diversity of plant-eating dinosaurs in such high latitudes. While the smaller herbivores had little trouble because of their low energetic requirements, the larger herbivores probably had more difficulties in order to find enough food, especially during the harsh winter months. The dinosaur fossil found at the highest latitude is Ugrunaaluk (literally “ancient grazer” in Inupiaq language from northern Alaska) a hadrosaurid or “duck-billed dinosaur”. This ornithopod measured up to 10 metres long and weighed around 3 tonnes, making it one of the largest animals in Prince Creek.
Ugrunaaluk were herbivorous animals that lived in groups. Even if many author think that these animals performed long migrations like today’s birds and mammals in order to avoid the lack of food during the winter, some others argue that young Ugrunaaluk (which had a less active metabolism than current endotherms) would had been unable to endure such long journeys. Ugrunaaluk probably moved to areas were the vegetation better tolerated the severity of winter, even if it’s thought that these great herbivores survived during the dark winters feeding on bark, ferns and probably aquatic vegetation during the coldest months.
The other great Prince Creek plant-eater was Pachyrhinosaurus (literally “thick-nosed lizard”) a ceratopsid widely-distributed through the United States, with a large protuberance on its nose which may have been used as a weapon during intraspecific combats, and a pair of laterally-facing horns on the top of its frill. Pachyrhinosaurus was the largest animal of Prince Creek, measuring up to 8 metres long and weighing up to 4 tonnes. It is possible that it used its nasal protuberance to shovel through the snow to reach the plants buried under it, similar to today’s bisons.
All the animals of Prince Creek had arduous lives. Almost all the fossils of both Ugrunaaluk and Pachyrhinosaurus, indicate that these species matured quickly and died young. Observing the growth of the different bones that have been found, it is thought that these dinosaurs rarely lived for over 20 years of age, probably due to the harsh conditions of their habitat but also to the presence of predators.
PREDATORS LARGE AND SMALL
The largest predator of the region was Nanuqsaurus (“polar bear lizard”, from Inupiaq language), a tyrannosaurid. This animal had a highly developed sense of smell which allowed it to detect their prey or animal carcasses in low light during the polar winter. Also, although there is no evidence, it was probably covered in feathers which would have protected it from the cold, as many closely-related theropods presented feathers to some extent.
What’s more surprising about Nanuqsaurus is its size, much smaller than that of its relatives. While other tyrannosaurids from the same time measured between 10 or 12 metres long and weighed up to 9 tonnes, Nanuqsaurus was more of a pygmy tyrannosaur, with an estimated length of 6 metres and a weight of 800 kg. This diminutive size was probably caused by the fact that it lived in an environment where food availability varied through the seasons. Apart from the fact that their prey’s population densities probably weren’t very high, during winter months many herbivores would migrate to other areas.
By contrast, there was another theropod that presented the opposite adaptation. Troodon (“wounding tooth”) was a relatively small dinosaur, about 2.9 metres long and 50 kg of weight. This is a pretty abundant dinosaur in many North American deposits. Troodon was a highly active carnivorous animal, with a good binocular vision and it’s also believed to be one of the most intelligent Mesozoic dinosaurs.
While Nanuqsaurus was smaller by the lack of abundant prey, Troodon specimens found at Prince Creek were characterized by their bigger size, compared with the ones from other deposits. This is what is called the Bergmann’s Rule, according to which the populations of a species that live in colder climates tend to be larger than the populations living in warmer climates, as this way they lose less body heat. Also, the larger eyes of Prince Creek’s Troodon, would give them advantage hunting during the long winter nights.
As you can see, dinosaurs not only thrived in warm and tropical environments. Even if their populations weren’t very large and their living conditions were harsher, these dinosaurs were able to adapt and survive in the polar forests of Prince Creek, and many of them surely gazed at the spectacular northern lights of 75 million years ago.
The following sources have been consulted during the elaboration of this entry:
- Susana Salazar Jaramillo (2014). Paleoclimate and paleoenvironment of the Prince Creek and Cantwell formations, Alaska: terrestrial evidence of middle Maastrichtian greenhouse event. Thesis.
- Live Science. Polar Dinosaurs Endured Cold Dark Winters.
- The Strange Lives of Polar Dinosaurs.
- North Pole Dinosaurs Lived Short, Hard Lives: Discovery News.
- Gangloff, Fiorillo & Norton (2005). The first Pachycephalosaurine (Dinosauria) from the Paleo-Arctic of Alaska and its Paleogeographic implications. Journal of Paleontology. Vol. 79. Pp: 997-1001.
- Mori, Druckenmiller & Erickson (2016). A New Hadrosaurid from the Prince Creek Formation (Lower Maastrichtian) of Northern Alaska. Acta Palaeontologica Polonica. Vol. 61. Pp: 15-32.
- Fiorillo & Tykoski (2014). A Diminutive New Tyrannosaur from the Top of the World. PLoS ONE 9(3).
- Cover Image by Julio Lacerda.