Nanuqsaurus

An artist’s restoration of the animal's head

Initially, Nanuqsaurus was estimated to have been about 16 to 20 feet long, based on the holotype specimen, which put 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 24 to 28 inches.[3] Its weight was also estimated to be 1,100 to 2,000 lbs.[5][7] This diminutive size was postulated by Fiorillo and Tykoski as being an adaptation to its high latitude arctic 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 23 feet in length, described as 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 26 to 30 feet is generally expected.[8]

A size comparison of several proposed body lengths of Nanuqsaurus to a human

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 1) 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, 2) frontals with a long, rostrally pointed process separating the prefrontal and lacrimal facets, and 3) that the first two dentary teeth are much smaller than the dentary teeth behind them.[3]

Phylogenetic analysis of Tyrannosauridae finds that Nanuqsaurus is a close relative of Daspletosaurus and Tyrannosaurus within Tyrannosaurinae. The cladogram below illustrates the relationships of Tyrannosauridae.[10]

This CT slice through the partial left dentary shows the replacement teeth present in the jaw bone

A Nanuqsaurus in its environment

The holotype specimen assigned to Nanuqsaurus comes from the mid Maastrichtian age Prince Creek Formation. At an age of 70 to 68 million years ago, the Kikak-Tegoseak Quarry region if the Prince Creek Formation dates to the mid Maastrichtian. An average age found in dating rocks from the formation is 69.1 ± 0.3 million years ago, so it is likely that Nanuqsaurus is from around that age. The formation is along the Colville River on the North Slope Borough, and is made up of alluvial sediments. It is one of a few dinosaurs to live at very high latitude arctic areas.[3][12]

Nanuqsaurus lived alongside many other dinosaurs in the Prince Creek formation region during the Edmontonian faunal stage in a climate that was around 50 to 53.6° F during summer and about 27° F ± 39° F during the winter.[8] Other animals alongside Nanuqsaurus include an unnamed leptoceratopsid, the hadrosaurid Edmontosaurus (similar to E. regalis),[13] 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 referred to the dubious genus Troodon, the dromaeosaurids Dromaeosaurus and Richardoestesia (similar to R. isosceles), an unnamed saurornitholestine, an unnamed ornithomimosaur, and multiple avialans.[8] 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.[14]

• Timeline of tyrannosaur research