A reinterpretation of Nanuqsaurus hoglundi (Tyrannosauridae) from the late cretaceous Prince Creek formation, northern Alaska

Abstract

The Late Cretaceous (late Campanian, ca. 72.8 Ma) Prince Creek Formation on the North Slope of Alaska is well-known for preserving the highest latitude dinosaur fauna in either hemisphere. Within this diverse dinosaurian fauna, a single tyrannosaurid theropod has been described: Nanuqsaurus hoglundi. Little work has been devoted to the taxon since 2014, when it was initially described based on three fragmentary cranial bones. Notably, it was characterized as a "diminutive" taxon, thought to have been substantially smaller than related Late Cretaceous tyrannosaurid species. New cranial and postcranial material attributable to the taxon collected by and housed at the University of Alaska Museum, challenges many aspects of its anatomy, size, and paleobiology. Here, I incorporate new specimens and critically reanalyze the holotype material to address the taxonomic validity of Nanuqsaurus. Further, I conduct the first quantitative analysis to assess body size and test the hypothesis that the Alaskan tyrannosaurid is a diminutive taxon. New material (such as the proximal condyle of a metatarsal and a complete dorsal rib) allowed for the first histological analysis of the Alaskan taxon to be performed to better understand growth dynamics for the taxon and the ontogenetic status of these key specimens. Both specimens are revealed to have been at least 14 years of age at the time of their death and lack an external fundamental system, suggesting that growth had not stopped. New data also facilitates a critical taxonomic re-evaluation of N. hoglundi, which results in a more robust phylogenetic analysis and designation of the taxon as a nomen dubium. Direct proportional scaling of new material suggests a body size comparable to other Late Cretaceous tyrannosaurid taxa, such as Albertosaurus, Gorgosaurus, and Daspletosaurus (all 9 - 10 m body length). Application of theropod regression equations suggests a body size approaching these taxa (at approximately 8 m in length), and a much larger body mass than originally hypothesized (1615 - 1900 kg). The updated size warrants an examination of previously drawn paleobiological conclusions, such as a decrease in body size to reach an "optimal predator size" or as a response to a lack of resources. Regardless of the validity of the taxon, these data collectively provide new insights into the ecology and life history strategies of the northernmost large-bodied theropod known.