Nanuqsaurus

Nanuqsaurus (meaning "polar bear lizard") is a genus of tyrannosaurine theropod dinosaur known from the Late Cretaceous period (middle Maastrichtian age) Prince Creek Formation of the North Slope of Alaska, having lived roughly 70-68 million years ago. It contains a single species, Nanuqsaurus hoglundi, known only from a partial skull and multiple undescribed postcranial and teeth elements.

Initial Material
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. Later, after the locale was understood to be younger than previously thought, the consensus switched to referring to the teeth under the genus Albertosaurus.

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 –. Radiometric dating of nearby rock suggests that the fossils were deposited roughly 68 to 70 Ma. 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.

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.

Description
Initially, Nanuqsaurus was estimated to have been about 5 – long, a metric based on the holotype specimen, putting the animal at about half the length of Tyrannosaurus rex. The length of the same specimen's reconstructed skull, based on the proportions of related animals, was 60 –. Its weight was also estimated to be 500 –. This diminutive size was postulated by Fiorillo and Tykoski as being an adaptation to its high-latitude habitat. 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. Some of the undescribed postcranial elements scale to around 7 m in length, described comparable to a juvenile Tarbosaurus. If Nanuqsaurus did indeed grow to a similar length as Albertosaurus, as previously suggested, a total body length of anywhere between 8 – is generally expected. 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. Nanuqsaurus itself is anatomically diagnosed by the following traits:

•a thin, rostrally forked, median spur of the fused parietals on the dorsal skull roof that overlaps and separates the frontals within the sagittal crest

•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.

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:

A 2023 thesis argued that its taxonomic validity and phylogenetic position is poorly resolved, and that the taxon is a nomen dubium.

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.

Paleoecology
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.

Other animals that lived alongside Nanuqsaurus include the following: unnamed leptoceratopsid, the hadrosaurid Edmontosaurus (similar to E. regalis), an unnamed lambeosaurine, an unnamed thescelosaurine (mentioned to be similar to Parkosaurus and Thescelosaurus), an unnamed orodromine (mentioned to be similar to Orodromeus), the ceratopsid Pachyrhinosaurus perotorum, the pachycephalosaurid Alaskacephale, a large troodontid assigned to the dubious genus Troodon, the dromaeosaurids Dromaeosaurus and Saurornitholestes, along with an unnamed saurornitholestine, an unnamed ornithomimosaur, and multiple avialans. As well as this, several mammals, including the metatherian Unnuakomys, the eutherian Gypsonictops, both an unnamed and named multituberculate, the latter being Cimolodon, and finally an indeterminate marsupial. Interestingly, due to the cooler conditions of this habitat, many otherwise common ectothermic clades lack representation entirely in the Prince Creek Formation, suggesting that all the animals that did thrive in these extreme latitudes were indeed endotherms to some degree.