Read Caption
A hypothetical reconstruction of Nanuqsaurus showing the placement of skull bones. From Fiorillo and Tykoski, 2014.

Arctic’s Tyrannosaur Was a Tiny Terror

Tyrannosaurs included some of the largest carnivores of all time, but a new Arctic tyrant was a pipsqueak compared to T. rex.

Near the end of the Cretaceous, about 70 million years ago, tyrannosaurs were North America’s apex predators. From Teratophoneus in the south to Albertosaurus in the North, these deep-skulled, tiny-armed theropods were the most menacing dinosaurs of their time.

But not all of these Late Cretaceous predators were giants.

In a study published Wednesday in PLoS One, paleontologists Anthony Fiorillo and Ronald Tykoski of Dallas, Texas’ Perot Museum of Nature and Science have named a diminutive tyrannosaur that once trod beneath the Arctic’s northern lights.

The researchers have dubbed the dinosaur Nanuqsaurus hoglundi – a title combining the Iñupiaq word for polar bear with a species name honoring geologically-minded philanthropist Forrest Hoglund.

Bones of the tyrannosaur were recovered at the Kikak-Tegoseak Quarry on Alaska’s North Slope. Even with continental shifts since the Cretaceous, the site would have been within the Arctic Circle during the heyday of Nanuqsaurus.

Relatively little of the dinosaur has been found so far. The material Fiorillo and Tykoski drew upon to name the dinosaur consists of disarticulated skull bones, including parts of the skull roof and the jaws. Some of these bones were previously thought to belong to the classic tyrannosaurs Albertosaurus and Gorgosaurus, but the new analysis by Fiorillo and Tykoski found that the North Slope fossils differ in the arrangement of the bones on the skull roof and the size of the tooth sockets. And since some of the skull bones are fused, this Nanuqsaurus was likely at or approaching adult size.

Size matters, in this case, because the most immediately striking aspect of Nanuqsaurus is the tyrannosaur’s small size. Using other tyrannosaur skulls as a guide, Fiorillo and Tykoski hypothesize that the skull of a mature Nanuqsaurus was not much more than two feet long. The skull of the largest known T. rex – the biggest known tyrannosaurs – were about five feet long. Likewise, at an estimated 25 feet long, Nanuqsaurus could have stood in the shadow of a 40-foot-long T. rex.

Still, despite the size difference, T. rex and Nanuqsaurus were relatively close relatives.

View Images
A phylogeny of North America’s Late Cretaceous tyrannosaurs. From Fiorillo and Tykoski, 2014. A phylogeny showing the relationship of Nanuqsaurus to other Late Cretaceous tyrannosaurs in North America. From Fiorillo and Tykoski, 2014.

As hypothesized by Fiorillo and Tykoski, Nanuqsaurus shares the pedigree of the most famous tyrannosaurs. The new dinosaur’s bones are more similar to the last of the tyrannosaurs, including Tarbosaurus from Mongolia and T. rex from North America, than to the sleeker, more gracile tyrannosaurs Albertosaurus and Gorgosaurus that lived at the same time.

Defining Nanuqsaurus as a unique species closely related to T. rex adds another data point to the bigger picture of how tyrannosaurs evolved and spread around the northern hemisphere during the Cretaceous.

Based on previous discoveries, including the recently-described tyrannosaurs Teratophoneus and Lythronax, paleontologists have suspected that there were mountain ranges and other geographic barriers that caused tyrannosaurs to evolve into different species in pockets of isolation. Nanuqsaurus further fits this pattern of distinct tyrannosaurs evolving along the latitudes.

The small stature of Nanuqsaurus could be a clue to such a barrier that caused the dinosaur to evolve in unique conditions. Given the harsh seasonal swings between months of darkness and light in its ancient habitat, Fiorillo and Tykoski propose, Nanuqsaurus may have evolved to be more compact to better survive on scarce resources during the long nights of the Cretaceous Arctic.

And, of course, this means that March of the Dinosaurs and Walking With Dinosaurs 3D have some revisions to make. Instead of being a towering monster, Alaska’s tyrannosaur was a modestly-sized carnivore that would have best avoided tussling with the burly herbivores of its time. Future films set the in the Cretaceous Arctic will have to find a new villain.

[Full disclosure: Part of the study funding was provided by the National Geographic Society/Waitt Grants Program.]

Reference:

Fiorillo, A., Tykoski, R. 2014. A diminutive new tyrannosaur from the top of the world. PLoS ONE. 9, 3: e91287. doi:10.1371/journal.pone.0091287