Probable Juvenile Frontal of Daspletosaurus Horneri (Dinosauria: Theropoda) From the Two Medicine Formation of Montana, With Implications for Tyrannosaurid Ontogeny

Abstract

A partial left frontal (UCM 55499) of a small tyrannosaurid theropod from the Campanian Two Medicine Formation of Montana is described, and is considered to probably represent a juvenile of Daspletosaurus horneri. UCM 55499 compares favorably with the frontals of other young juvenile tyrannosaurid specimens, and its diminutive size suggests that the element comes from a small juvenile individual. Differences (including width of the nasal process, and degree of expansion of the postorbital buttress and the caudal shelf) between it and frontals of the larger individuals of Daspletosaurus horneri are similar to those between juveniles and adults of other tyrannosaurids such as Tyrannosaurus rex, suggesting that overall ontogenetic trends in the frontal bone of Daspletosaurus horneri are largely consistent with the patterns seen in other tyrannosaurids.

Two braincases of Daspletosaurus (Theropoda: Tyrannosauridae): anatomy and comparison1

“…FMNH PR308 was identified as Gorgosaurus (or Albertosaurus libratus at the time) by Russell (1970), but later revised as Daspletosaurus (Carr 1999;Currie 2003a). An isolated frontal from the Dinosaur Park Formation (SDNHM 32701) was assigned to Daspletosaurus torosus (Yun 2020), but suites of characters present in this specimen only suggest tyrannosaurine affinity. Yun (2020) assigned SDNHM 32701 to Daspletosaurus based on the longitudinal ridge on the nasal process.…”

“…An isolated frontal from the Dinosaur Park Formation (SDNHM 32701) was assigned to Daspletosaurus torosus (Yun 2020), but suites of characters present in this specimen only suggest tyrannosaurine affinity. Yun (2020) assigned SDNHM 32701 to Daspletosaurus based on the longitudinal ridge on the nasal process. Although sometimes used as an autapomorphy of Daspletosaurus (Carr et al 2017), this character is a correlate of the frontal-nasal suture, is influenced by body size, and has a broader distribution.…”

“…Possibly the most recent member on this list, tyrannosaurid material collected from the Foremost Formation of Alberta has been proposed as a distinct taxon, Thanatotheristes degrootorum (Voris et al 2020). Characters of the frontal used to diagnose this taxon have been critically reviewed (Yun 2020).…”

“…Furthermore, the communication with T.D. Carr cited by Yun (2020) to suggest the holotype of Daspletosaurus torosus (CMN 8506) as collected from the Dinosaur Park Formation is erroneous, because the locality for CMN 8506 (Quarry 72) clearly sits below the Oldman -Dinosaur Park boundary (Currie 2005).…”