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.