As reported this week in the journal PLOS ONE, a team of paleontologists from the Perot Museum of Nature and Science in Dallas first discovered the fossilized remains of the new tyrannosaur species when preparing for a dig in 2006 at northern Alaska’s Kikak-Tegoseak quarry. On a rocky bluff overlooking the Colville River, almost 400 miles north of Fairbanks, they uncovered the bones of Pachyrhinosaurus perotorum, a new species of horned dinosaur. Back in their laboratory, however, the scientists realized they had also unearthed fragments of the skull, lower and upper jaw bones of a tyrannosaur.
Though Tyrannosaurus rex is by far the most famous, many different species of tyrannosaur roamed Asia and North America during the Late Cretaceous Period, some 70 million years ago. Through analysis of its remains, the scientists determined this particular one was much smaller than other tyrannosaurs: It probably measured around two meters tall at the hips and seven meters from nose to tail, around half the size of a T. rex. They were also able to identify the bones as belonging to a full-grown adult, thanks to the distinctive sockets along the edge of the upper jaw, only seen in jawbones of other adult tyrannosaurs.
According to Ron Tykoski, fossil preparator at the Perot Museum, “It wasn’t until the past few years, with more work being done on growth rates, that we were able to look at these pieces in finer detail and realize that they weren’t a youngster of a known species, but a mature individual of something new”: a pygmy tyrannosaur. Tykoski and his colleagues named the new species Nanuqsaurus hoglundi; “Nanuq” means polar bear in the language of the local Inupiat people, and “hoglundi” is a nod to Forrest Hoglund, a Dallas businessman who helped raise funds to build the Perot Museum.
Certain similarities, including a distinctively shaped ridge on its head, indicate that N. hoglundi was a close cousin of T. rex. When they looked back at the bones of the horned dinosaur species uncovered in 2006, the scientists found tooth marks and deep grooves on some of them, indicating that N. hoglundi preyed on local plant-eating dinosaurs. According to Tony Fiorillo, the Perot’s curator of earth sciences, “We feel pretty confident that this pygmy tyrannosaur was eating the herbivorous dinosaurs around at the time.” Such discoveries, Fiorillo says, shed new light on what life might have been like in the prehistoric Arctic. “By seeing tooth marks, it makes them the animals that they really were,” Fiorillo said, “instead of just a cool collection of objects.”
Yet the question remained: why was N. hoglundi so small compared to other tyrannosaurs? According to Fiorillo and Tykoski, the answer lies in the cool climate and high latitude of its Arctic home. While the weather above the 70th parallel in Alaska would have been warmer during the Late Cretaceous than it is today, there still would have been far less sunlight and fewer resources than the typical T. rex climate, and long seasons during which food might not have been readily available.
Still, such a conclusion seems to contradict a common evolutionary axiom, which holds that animals closest to the poles tend to be larger (as larger mass to surface area ratio helps them keep warmer). As this so-called “Bergmann’s Rule” doesn’t hold true in the case of the pygmy tyrannosaur, the new species remains something of a mystery–one that scientists will undoubtedly continue to explore. “We’d certainly like to know more about this animal,” said Fiorillo. “And whatever else might be out there.”