Even during the Cretaceous, roughly 70 million years ago, the High Arctic saw months of darkness. That’s offered paleontologists a longstanding puzzle. Various dinosaur species—from tyrannosaurs to armored dinosaurs—have been found in rocks from around this age along Alaska’s North Slope and certainly had to contend with the long winter nights. Did these dinosaurs migrate with the seasons, paleontologists have wondered, or did they stay in place through the harshest season?
The answer has come from a surprising source. During decades of explorations and excavations in Alaska’s Prince Creek Formation, paleontologists have slowly gathered a collection of bones from very young dinosaurs—and not just one species, but many. Described in Current Biology today, these tiny fossils speak volumes about the dinosaurs of the High Arctic. Instead of migrating to warmer regions to raise their young, polar dinosaurs stayed in ancient Alaska year-round and raised their offspring there.
Finding the remains of any hatchling or juvenile dinosaurs is a reason for paleontologists to celebrate. These fossils are very rare. Baby dinosaurs are rarely preserved in the fossil record, likely owing to their small size and the fact that young dinosaurs were likely the favored prey of large carnivores. All the same, over the course of a three-decade search for such fossils, University of Alaska paleontologist Pat Druckenmiller and colleagues have found the delicate remains of baby hadrosaurs, horned dinosaurs, raptors and tyrannosaurs. Surprisingly, the research even found a small tooth belonging to a small horned dinosaur—known as a leptoceratopsid—that is not yet known from adult remains, indicating that there is certainly more to discover in the Prince Creek Formation.
“The discovery of these tiny bones and teeth was no small accomplishment,” Druckenmiller says. Searching for fossils in the Prince Creek Formation is not like walking the rocky exposures of Montana or Utah. Alaska’s famous dinosaur-bearing formation is only exposed as cliffs along the Colville River, and in this setting Druckenmiller and colleagues looked for thin layers of rock that might contain accumulations of microfossils, or spots where teeth, small bones and other fossil tidbits have been preserved together. These deposits often act like a fossil census, giving paleontologists an idea of which species were around in the environment at the time the rocks were laid down. The process took exacting care and patience, almost like sifting sand grains in an effort to save any fossils larger than half a millimeter. “We didn’t expect to find babies, much less remains of seven different species that clearly reproduced in the Arctic,” says Druckenmiller.
To find so many infant and juvenile dinosaurs from one formation is unexpected. “Baby bones are rare in most places, even those that produce lots of dinosaur egg fossils like the southern regions of Canada, Mongolia and the U.S.A.,” says University of Calgary paleontologist Darla Zelenitsky, who was not involved with the new study. Despite the fact that paleontologists have only been searching the Prince Creek Formation since the 1950s, paleontologists now have more baby dinosaur fossils from this place than any other Cretaceous formation in North America.
The discovery that most of the known dinosaur groups from the Prince Creek Formation nested and started their lives as hatchlings in the High Arctic resolves the question of whether Alaska’s ancient dinosaurs stayed in place or migrated with the seasons. The hatchlings and infants wouldn’t have been able to travel long distances, after all, and so they stuck it out through the harsh months. Other fossil finds, such as a juvenile raptor announced last year, have added evidence to the same idea, with the new paper providing the strongest evidence yet that Alaska’s dinosaurs stayed year-round.
Up until now, fossils of young dinosaurs from Alaska have been of small species. Paleontologists still wondered whether the larger species, such as the shovel-beaked hadrosaurs, would have ventured away from Cretaceous Alaska in the cold months. But finding the infants of large species, as well as small ones, hints that the whole dinosaur community remained in the High Arctic through the year. “Our baby dinosaurs are not the first evidence to support the idea of them being year-round denizens, but I’d argue it is some of the most compelling evidence to date,” says Druckenmiller.
Based upon what paleontologists and geologists have been able to piece together about the ancient environment of the Prince Creek Formation, Druckenmiller and colleagues suggest that parent dinosaurs would have been building their nests and laying eggs at the transition between winter and spring, around March. The size and other biological factors played a role in how long the baby dinosaurs had to incubate, but many of the hatchlings would likely appear between June and September. The babies had to get a start before winter set in again, meaning that the young of large dinosaurs likely entered an autumn landscape with precious little time to get their bearings before winter set in. “It seems like the whole ecosystem was staying put,” says University of Edinburgh paleontologist Greg Funston, who was not involved in the new study. If any dinosaurs migrated back and forth with the Cretaceous seasons, they were in the minority.
The question now is how the likes of the horned dinosaur Pachyrhinosaurus and the tyrannosaur Nanuqsaurus survived the cold and dark months. Even though the area was a bit warmer than it is today, the fact that paleontologists haven’t found turtles, crocodiles, amphibians or other “cold-blooded” animals indicates that this place truly could get chilly. From dinosaur growth rates, bone tissue structure and other clues, paleontologists know that dinosaurs had elevated body temperatures. This no doubt helped dinosaurs make it through the cold months. But Druckenmiller wonders what other traits or behaviors these animals had that helped them through the polar night. “How feathered were some of these critters? What food helped the herbivores survive in the winter? Did the small species burrow and hibernate?”
How dinosaurs navigated the harsh polar winters has major implications for their biology. “The residency of dinosaurs in the Arctic must have had impacts on their growth, diets and senses,” Funston says. Some studies of how dinosaurs grew, such as the horned Pachyrhinosaurus, indicate that polar dinosaurs had to grow rapidly in the warm months and stopped growing during the cold ones—creating rings in their bones, similar to tree rings—but that’s just the tip of the paleobiological iceberg.
Finding more answers to questions about dinosaurs that lived in the High Arctic will take additional discoveries and analyses to parse. Nevertheless, Zelenitsky notes, “dinosaurs were probably more adaptable than we previously realized, able to reproduce, rear young and likely thrive in the most northerly landscapes of the Cretaceous.” Somehow these impressive reptiles found a way to carve out a living in an environment unlike any other known from the Cretaceous, which is only inspiring a new hunt to understand the survival secrets of these polar dinosaurs.