Nanuqsaurus: Beast of the Week

This week we shall be checking the arctic dinosaur, Nanuqsaurus hoglundi!  Nanuqsaurus was a tyrannosaurid, closely related to Tyrannosaurus, Tarbosaurus and Lythronax, that lived in what is now Alaska, North America, during the late Cretaceous period, about 69 million years ago.  Even though only parts of the skull, a few teeth, and a few other bones have been discovered, it can be estimated that Nanuqsaurus would have measured roughly 22 feet (6.7m) long from snout to tail when alive.  Like all known tyrannosaurids, it would have been a meat-eater.  The genus name, Nanuqsaurus, translates to "Polar Bear Lizard" because it was discovered in Alaska...where Polar Bears live.  


When Nanuqsaurus' fossils were initially published on in 2014, it was described as a "dwarf tyrannosaur" with an adult size estimate at about 18 feet long.  The thinking behind this was that since it lived at a higher elevation in an environment that was at least seasonally colder parts of the year, a smaller size would have aided it in requiring less food to stay alive, especially in the winter, when prey was likely more scarce.  A more recent study, however, suggests Nanuqsaurus was at least a little larger, more like 22 feet long, possibly even closer to 30 feet long as an adult, due to the observation that the original fossil material was likely from a juvenile that wasn't done growing at the time of its death, plus additional teeth found that were clearly from a larger animal.

Nanuqsaurus watches as a family of Edmontosaurus pass through.  Watercolor reconstruction by Christopher DiPiazza

A common misconception is that since Nanuqsaurus was found in Alaska, it must have been adapted to extremely cold temperatures, often depicted by artists with a thick coat of snow white feathers trudging through an icy wasteland.  This would make sense if Alaska was exactly the same during the Cretaceous as it is now...except it almost certainly wasn't.  During the Cretaceous, the global temperature was much warmer than it is today, so even the poles would have been noticeably greener.  That being said, what is now Alaska still would have been coolER at the time, experiencing definite colder seasons during winter and fall months, but probably with only occasional snow.  One thing about that environment that was consistent to today, however, was how the poles would have experienced extended periods of daylight in the summer and extended periods of darkness in the winter.  So winter still would have been challenging for most dinosaurs.    

The known remains of Nanuqsaurus from the paper by Fiorillo and Tykoski.  Despite the fact that it isn't much, they were able to determine after close examination that this is indeed a new genus and species of tyrannosaurid.

So how did a dinosaur like Nanuqsaurus survive in a cooler environment when tyrannosaurids elsewhere at the time thrived in more tropical climates?  We know that tyrannosaurs belonged to the broad branch of dinosaurs called, coelurosaurs, many of which were discovered with feathers.  It is therefore totally plausible to infer that all tyrannosaurs had at least some feathers. (Yes, even T. rex.)  Maybe Nanuqsaurus happened to have a denser coat of feathers for better insulation, much like birds in the arctic do today?  

Nanuqsaurus' size may have helped keep it warm as well.  An animal with a larger body has less surface area to volume ratio and therefore more heat conservation.  This is called Bergman's Rule.  You can see examples of Bergman's Rule today in the largest modern reptiles, like crocodiles, which can remain active for longer periods of time without direct access to heat than smaller kinds of reptiles like most lizards/snakes.  In fact, there is another dinosaur known from Nanuqaurus' habitat, a still unnamed species of troodontid, that was the largest known member of its typically small-bodied family, which probably evolved that way for the same reason.  

Animals that live in colder environments also often have shorter limbs and other extremities, which helps prevent as much heat from leaving the body as possible, called Allen's Rule.  We don't have much of Nanuqsaurus' skeleton past the jaws, but it wouldn't be unreasonable to guess it may have had a stockier build for this reason.  

Allen's Rule states that animals typically evolve shorter limbs and extremities to better adapt to consistently colder environments, as demonstrated with the cold adapted, Arctic Fox (Vulpes lagopus) compared to the more generalist, Red Fox (Vulpes vulpes).

When alive, it is likely Nanuqsaurus was the top predator of its environment.  It would have shared its habitat with the large ceratopsian, Pachyrhinosaurus perotorum, and the hadrosaur Edmontosaurus, and probably at least hunted their young.  It also crossed paths with the unusually large (but still smaller than it) Troodon species mentioned earlier, plus at least two kinds of dromaeosaurs.  

That is all for this week!  Feel free to comment below!

References

Druckenmiller, Patrick S.; Erickson, Gregory M.; Brinkman, Donald; Brown, Caleb M.; Eberle, Jaelyn J. (2021-08-23). "Nesting at extreme polar latitudes by non-avian dinosaurs". Current Biology. 31 (16): 3469–3478.e5.

Fiorillo, A. R.; Tykoski, R. S. (2014). "A Diminutive New Tyrannosaur from the Top of the World". PLoS ONE 9 (3): e91287. doi:10.1371/journal.pone.0091287.

 Fiorillo, Anthony R.; Gangloff, Roland A. (2000). "Theropod teeth from the Prince Creek Formation (Cretaceous) of Northern Alaska, with speculations on Arctic dinosaur paleoecology". Journal of Vertebrate Paleontology 20 (4): 675–682. doi:10.1671/0272-4634(2000)020[0675:TTFTPC]2.0.CO;2.