Images of dinosaurs as cold-blooded creatures needing tropical temperatures could be a relic of the past.
University of Alaska Fairbanks and Florida State University scientists have found that nearly all types of Arctic dinosaurs, from small bird-like animals to giant tyrannosaurs, reproduced in the region and likely remained there year-round.
Their findings are detailed in a new paper published in the journal Current Biology.
"It wasn't long ago that people were pretty shocked to find out that dinosaurs lived up in the Arctic 70 million years ago," said Pat Druckenmiller, the paper's lead author and director of the University of Alaska Museum of the North. "We now have unequivocal evidence they were nesting up there as well. This is the first time that anyone has ever demonstrated that dinosaurs could reproduce at these high latitudes."
The findings counter previous hypotheses that the animals migrated to lower latitudes for the winter and laid their eggs in those warmer regions. It's also compelling evidence that they were warm-blooded.
For more than a decade, Druckenmiller and Gregory Erickson, a Florida State University professor of biological science, have conducted fieldwork in the Prince Creek Formation in northern Alaska. They have unearthed many dinosaur species, most of them new to science, from the bluffs above the Colville River.
Their latest discoveries are tiny teeth and bones from seven species of perinatal dinosaurs, a term that describes baby dinosaurs that are either just about to hatch or have just hatched.
"One of the biggest mysteries about Arctic dinosaurs was whether they seasonally migrated up to the North or were year-round denizens," said Erickson, a co-author of the paper. "We unexpectedly found remains of perinates representing almost every kind of dinosaur in the formation. It was like a prehistoric maternity ward."
Recovering the bones and teeth, some no larger than the head of a pin, requires perseverance and a sharp eye. In the field, the scientists hauled buckets of sediment from the face of the bluffs down to the river's edge, where they washed the material through smaller and smaller screens to remove large rocks and soil.
Once back at their labs, Druckenmiller, Erickson and co-author Jaelyn Eberle from the University of Colorado, Boulder, screened the material further. Then, teaspoon by teaspoon, the team, which included graduate and undergraduate students, examined the remaining sandy particles under microscopes to find the bones and teeth.
"Recovering these tiny fossils is like panning for gold," Druckenmiller said. "It requires a great amount of time and effort to sort through tons of sediment grain-by-grain under a microscope. The fossils we found are rare but are scientifically rich in information."
Next, the scientists worked with Caleb Brown and Don Brinkman from the Royal Tyrrell Museum of Palaeontology in Alberta, Canada, to compare the fossils to those from other sites at lower latitudes. Those comparisons helped them conclude that the bones and teeth were from perinatal dinosaurs.
Once they knew the dinosaurs were nesting in the Arctic, they realized the animals lived their entire lives in the region.
Erickson's previous research revealed that the incubation period for these types of dinosaurs ranges from three to six months. Because Arctic summers are short, even if the dinosaurs laid their eggs in the spring, their offspring would be too young to migrate in the fall.
Global temperatures were much warmer during the Cretaceous, but the Arctic winters still would have included four months of darkness, freezing temperatures, snow and little fresh vegetation for food.
"As dark and bleak as the winters would have been, the summers would have had 24-hour sunlight, great conditions for a growing dinosaur if it could grow quickly enough before winter set in," said Brown, a paleontologist at the Royal Tyrrell Museum.
Year-round Arctic residency provides a natural test of the animals' physiology, Erickson added.
"We solved several long-standing mysteries about the dinosaur reign, but opened up a new can of worms," he said. "How did they survive Arctic winters?"
"Perhaps the smaller ones hibernated through the winter," Druckenmiller said. "Perhaps others lived off poor-quality forage, much like today's moose, until the spring."
Scientists have found warm-blooded animal fossils in the region, but no snakes, frogs or turtles, which were common at lower latitudes. That suggests the cold-blooded animals were poorly suited for survival in the cold temperatures of the region.
"This study goes to the heart of one of the longest-standing questions among paleontologists: Were dinosaurs warm-blooded?" Druckenmiller said. "We think that endothermy was probably an important part of their survival."
This research was supported by the National Science Foundation.