Arctic life was tough on dinosaurs, with many not making it to their 20th birthday.

THE GIST

By counting tree-like bands on femur fossils, paleontologists can estimate dinosaur growth stages.

Some North Pole dinosaurs grew fast as juveniles, became sexually mature at age 9, and died at around 19.

Northern Alaska during the latter Dinosaur Age was slightly warmer than it is now, but still experienced freezing temperatures during long, dark winters.

The winter holiday season often portrays the North Pole as a cozy fantasyland, but new research on dinosaurs that lived there shows that Arctic life has been tough for millions of years, with North Pole dinos finding it hard to reach their 20th birthday.

The findings, published in the journal Historical Biology, offer a rare look at dinosaur life stages. Fossils from high latitudes better express growth bands that reveal how these animals grew up. Scientists can then analyze them similar to how they study tree rings.

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"We determine growth rates by looking at the number and spacing of the growth bands in a cross section of a femur," co-author Patrick Druckenmiller explained to Discovery News. "We measure the distance of each band from the center of the bone as a proxy for body size. In this case, it's presented as a percentage of total length. Growth banding becomes narrower as the growth rate progressively tapers off later in life."

Following this process, the researchers determined some polar dinosaurs grew rapidly as juveniles, became sexually mature at about age 9, and died at around age 19 (assuming they didn't bite the dust due to disease, an accident, or for some other reason).

Druckenmiller, curator of Earth Sciences at the University of Alaska Museum, and colleague Gregory Erickson focused their research on Pachyrhinosaurus femur bones excavated from the early Maastrichian (about 65 to 70 million years ago) of Prince Creek Formation in Northern Alaska.

"Pachyrhinosaurus is a member of the horned dinosaur family Ceratopsidae," Druckenmiller said. "It was a large, probably gregarious, herbivore. Instead of having thick horns over the eyes and nose area (think of the iconic Triceratops) it had large 'bosses,' which are bony growths that give the skull a very thick appearance."

This explain the dinosaur's name: Pachy, meaning "thick and heavy," and rhino, meaning "nose."

This dinosaur was far from being alone in the Arctic, however. The North Slope of Alaska was home to numerous other plant-eating and carnivorous dinosaurs. The most common was a duck-billed dino very similar to Edmontosaurus. The region was also home to a large tyrannosaurid, a few dromaeosaurs, and the human-sized Troodon.

Pachyrhinosaurus would have been preyed upon by the carnivores, but because of its size -- about 26 feet long and weighing around 4 tons -- Druckenmiller suspects few hunters "took on an adult-sized healthy animal."

Animals that would be expected from this area, such as lizards, crocodilians and turtles, have never been found. One reason could be that they had trouble getting to Alaska.

Hendrik Poinar, a McMaster University anthropologist who has also studied North Pole animals, told Discovery News that the Bering Land Bridge, which joined Alaska to eastern Siberia, may have been more of a barrier than a gateway.

"I think it is increasingly clear that the bridge was indeed a filter more than a bridge," Poinar said. "It certainly was not a freeway, and it makes us think about what ecological function it clearly played over the last several millennia."

Somehow dinosaurs did make it to Alaska, but they faced "annually freezing winter temperatures," according to Druckenmiller, who explained that scientists determine past climates based on plant remains. The Arctic, however, was somewhat warmer than it is today at certain latitudes.

Nevertheless, he said, "Long, dark winters were probably a major influence on anything that lived there."

Druckenmiller and other researchers hope future studies on North Pole dinosaurs might reveal information about migratory behaviors, overwintering strategies, and dinosaur physiology. The world's largest collection of Arctic dinosaurs is housed within the Earth Science Collection at the University of Alaska Museum in Fairbanks.