The first juvenile dromaeosaurid (Dinosauria: Theropoda) from Arctic Alaska

Abstract

Compared to the osteological record of herbivorous dinosaurs from the Late Cretaceous Prince Creek Formation of northern Alaska, there are relatively fewer remains of theropods. The theropod record from this unit is mostly comprised of isolated teeth, and the only nondental remains known can be attributed to the troodontid cf. Troodon and the tyrannosaurid Nanuqsaurus. Thus far, the presence of members of Dromaeosauridae has been limited to isolated teeth. Here we describe a symphyseal portion of a small dentary with two ziphodont teeth. Based on tooth shape, denticle morphology, and the position of the Meckelian groove, we attribute this partial dentary to a saurornitholestine dromaeosaurid. The fibrous bone surface, small size, and higher number of mesial denticles compared to distal ones point to a juvenile growth stage for this individual. Multivariate comparison of theropod teeth morphospace by means of principal component analysis reveals an overlap between this dentary and Saurornitholestinae dromaeosaurid morphospace, a result supported by phylogenetic analyses. This is the first confirmed non-dental fossil specimen from a member of Dromaeosauridae in the Arctic, expanding on the role of Beringia as a dispersal route for this clade between Asia and North America. Furthermore, the juvenile nature of this individual adds to a growing body of data that suggests Cretaceous Arctic dinosaurs of Alaska did not undergo long-distance migration, but rather they were year-round residents of these paleopolar latitudes.

Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North America1

“…Interdental plates are fused in dromaeosaurids (Currie 1987(Currie , 1995 dromaeosaurids. This includes a deep Meckelian groove, contrasting with the shallow groove in dromaeosaurids and other maniraptorans; an enlarged, rounded oral mandibular foramen, rather than slit-like; and box-like alveoli, in contrast to the lenticular alveoli of dromaeosaurids (Chiarenza et al 2020). However, this latter character is more pronounced in the posterior alveoli than the anterior alveoli of MOR 268, and thus is probably variable along the dentary in tyrannosaurids-the same variation is described for Nanuqsaurus (Fiorillo and Tykoski 2014).…”

“…6E). This morphology was described by Chiarenza et al (2020) as distinguishing tyrannosauroids from dromaeosaurids. Borders between the alveoli are demarcated by ridges projecting medially from the lateral wall of the dentary, and these are met by separate ossifications on the medial side of the alveoli, representing the interdental plates ( Fig.…”

“…Two other characters mentioned by Chiarenza et al (2020), specifically the presence of dorsoventral furrows in the interdental plates and the well-developed interdenticular sulci of the teeth, cannot be scored in MOR 268: the interdental plates are composed of porous bone without a finished surface that could exhibit furrows, and the teeth do not yet have denticles.…”

Nesting at extreme polar latitudes by non-avian dinosaurs

“…One of these sites, Pediomys Point, has been long known to produce small bones and teeth of fish, dinosaurs and mammals. 22,61 Three new microvertebrate sites provided much of the perinatal material described in this study. One, OJsaurus is located approximately 9 km downstream of Pediomys Point.…”

“…3,18 The migratory hypothesis, like the year-round scenario, envisions smaller taxa overwintering in the Arctic owing to their small sizes. Although sexually immature, postnestling-stage individuals (sensu Horner and Currie 19 ) of large Please cite this article in press as: Druckenmiller et al, Nesting at extreme polar latitudes by non-avian dinosaurs, Current Biology (2021), https:// doi.org/10.1016/j.cub.2021.05.041 hadrosaurids, 20 ceratopsids, 21 and dromaeosaurid taxa 22 have been described from the PCF, direct evidence of their reproduction in the paleo-Arctic in the form of eggs, perinatal remains, or brooding individuals has not been reported.…”