Before the formal description of Nanuqsaurus, numerous tyrannosaurid teeth were known from the Kogosukruk Tongue of the Prince Creek Formation and were first referred to the genus Gorgosaurus.[1] Later, after the locale was understood to be younger than previously thought, the consensus switched to referring to the teeth under the genus Albertosaurus.[2][3]
Holotype Specimen
In 2006, within the North Slope Borough of Alaska, the fossilized remains of what appeared to be a medium-sized theropod were located at the Kikak-Tegoseak Quarry of the Prince Creek Formation. The material was found to contain multiple fragments of the animal's skull, all of which were collected from the same area and found to most likely belong to a single individual. Among the disarticulated fragments preserved were the nasal branch of the right maxilla, a fragmentary skull roof including pieces of both frontals, parietals, a piece of the right laterosphenoid, and a fragment of the left dentary. These anatomical features were later used to determine the probable skull length of the animal, with estimates giving it a length of 600–700 mm (24–28 in).[3] Radiometric dating of nearby rock suggests that the fossils were deposited roughly 68 to 70 Ma.[2][4]
Holotype fossils as presented in the animal's initial description
It wasn't until after preparation and analysis at the Perot Museum of Nature and Science (Dallas Museum of Natural History) when the Alaskan Tyrannosaurid remains were finally recognized to represent a new taxon as opposed to being synonymous with previous genera. This prompted the creation of a new genus, Nanuqsaurus, described and named by Anthony R. Fiorillo and Ronald S. Tykoski in 2014. The initially discovered material, referred to as DMNH 21461, is now recognized as the holotype of Nanuqsaurus. As well as this, the initial discoveries of teeth were placed as more likely to be the remains of Nanuqsaurus as opposed to any other known creature, contrary to the initial proposals of the origin of the teeth.[3]
Etymology
The type species, Nanuqsaurus hoglundi, has its generic name derived from the Iñupiaq word for "polar bear", nanuq, and the Greek word sauros, meaning "lizard". The specific name honors the philanthropist Forrest Hoglund for his work on philanthropy and cultural institutions.[3]
Description
Restoration of the animal's head in life
Initially, Nanuqsaurus was estimated to have been about 5–6 meters (16–20 ft) long, a metric based on the holotype specimen, putting the animal at about half the length of Tyrannosaurus rex.[5][6][7] The length of the same specimen's reconstructed skull, based on the proportions of related animals, was 60–70 cm (24–28 in).[3] Its weight was also estimated to be 500–900 kg (1,100–2,000 lb).[5][7] This diminutive size was postulated by Fiorillo and Tykoski as being an adaptation to its high-latitude habitat.[3] However, later studies suggested that its supposed small size was unfounded and that it was likely similar in size to other North American tyrannosaurids, such as Albertosaurus, based on undescribed adult-sized teeth and postcranial elements.[8] Some of the undescribed postcranial elements scale to around 7 meters (23 feet) in length, described comparable to a juvenile Tarbosaurus.[9] If Nanuqsaurus did indeed grow to a similar length as Albertosaurus, as previously suggested, a total body length of anywhere between 8–9 meters (26–30 ft) is generally expected.[8]
Size comparison of several proposed adult body lengths of Nanuqsaurus
Nanuqsaurus would have likely resembled other large tyrannosaurines, such as Daspletosaurus, with both animals being closely related and probably serving similar roles in their respective ecosystems.[10]Nanuqsaurus itself is anatomically diagnosed by the following traits:
•frontals with a long, rostrally pointed process separating the prefrontal and lacrimal facets
•the first two dentary teeth are much smaller than the dentary teeth behind them.[3]
Classification
Phylogenetic analysis of Tyrannosauridae finds Nanuqsaurus to be a close relative of Daspletosaurus and Tyrannosaurus within Tyrannosaurinae. Below is a cladogram illustrating the relationships of the Tyrannosauridae:[10]
CT slice through the partial left dentary bone of the holotype, showing the replacement teeth present in the jaw bone
A 2023 thesis argued that its taxonomic validity and phylogenetic position is poorly resolved, and that the taxon is a nomen dubium.[11]
Paleobiology
Reproduction at polar latitudes
Analysis of material attributed to Nanuqsaurus and other Alaskan dinosaurs from the same environment has resulted in the Prince Creek Formation being recognized as having preserved an exceptionally high percentage of developmentally young dinosaurs when compared to the amount of families represented in the formation. Material from young dinosaur specimens, including birds, has been determined to be present from seven different major clades, or 70% of all the recognized families of the quarry. The families in question are Hadrosauridae, Thescelosauridae, Leptoceratopsidae, Ceratopsidae, Tyrannosauridae, Dromaeosauridae, Troodontidae, and Avialae. This evidence suggests that both Nanuqsaurus and its likely prey items remained in the paleo-Arctic yearlong and would have had to cope with ~120 days of constant winter darkness each year, as opposed to resorting to migration to escape the harsh conditions. It is proposed that the animals would likely have laid their eggs toward the beginning of the constant daylight period, around the month of April, allowing time for the eggs to incubate in the relative heat of this part of the year.[8][12]
Paleoecology
Nanuqsaurus depicted in a colder environment indicative of the high latitudes it resided in
Nanuqsaurus lived alongside many other dinosaurs during what is referred to as the Edmontonian faunal stage of the early Late Maastrichtian. Having resided at an estimated 80°–85°N paleolatitude, the area Nanuqsaurus thrived in experienced climatic extremes unlike that experienced by most other dinosaurs. The temperature of this Northern environment would've ranged from around 10 to 12°C during the warmer months and about -2°C ± 3.9°C during the colder months. As well as this, the environment would have faced 120 days of continuous low-light conditions during the Winter. In contrast to the contemporary large herbivores Edmontosaurus and Pachyrhinosaurus, which apparently preferred coastal lowland and upland environments respectively, Nanuqsaurus appears to have been fairly ubiquitous throughout the Prince Creek landscape.
^ abPaul, Gregory S. (2016). The Princeton Field Guide to Dinosaurs 2nd Edition. New Jersey: Princeton University Press. p. 114.
^ abcdDruckenmiller, Patrick S.; Erickson, Gregory M.; Brinkman, Donald; Brown, Caleb M.; Eberle, Jaelyn J. (2021-06-24). "Nesting at extreme polar latitudes by non-avian dinosaurs". Current Biology. 31 (16): 3469–3478.e5. doi:10.1016/j.cub.2021.05.041. ISSN0960-9822. PMID34171301. We note that other Prince Creek Formation tyrannosaurid material in the UAMES collection do not support the assertion that Nanuqsaurus is a diminutive, small-bodied tyrannosaur. Rather, adult-sized teeth and isolated postcranial elements suggest an adult body size more closely comparable to other North American tyrannosaurid taxa, such as Albertosaurus sarcophagus.
^ abVoris, Jared T.; Therrien, François; Zelenitsky, Darla K.; Brown, Caleb M. (2020-06-01). "A new tyrannosaurine (Theropoda:Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides insight into the evolution and biogeography of tyrannosaurids". Cretaceous Research. 110: 104388. doi:10.1016/j.cretres.2020.104388. ISSN0195-6671. S2CID213838772.
^Perry, Zackary R. (2023). "Chapter 1". A Reinterpretation of Nanuqsaurus hoglundi (Tyrannosauridae) From the Late Cretaceous Prince Creek Formation, Northern Alaska (MSc thesis). University of Alaska Fairbanks.
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