June 30 , 2016

We document previously undescribed isolated cranial, axial, dental, and appendicular skeletal elements of tyrannosaurid theropods, which were collected in the middle Campanian deposits of the Lower Judith River Formation in north-central Montana. Examination of these bones and some of the teeth indicates that they belong to members of the Albertosaurinae clade. The cranial material described here is a cast of a well-preserved and nearly complete isolated right dentary that is referred to Gorgosaurus sp. The referral of the dentary to this genus is based on the proportions of the minimum dorsoventral depth of the main body of the dentary and the anteroposterior length of the tooth row, which is similar to that of subadult examples of Gorgosaurus libratus from the Dinosaur Park Formation of Alberta, Canada. Additionally, the paleogeographic and paleostratigraphic position of the specimen further supports the generic identification. The dentary of Gorgosaurus sp. exhibits a combination of morphological characters that differ from G. libratus and from other North American tyrannosaurid taxa, supporting the previous assumptions that the genus Gorgosaurus was morphologically diverse. The morphological characters include a sinuous “chin” that lacks the characteristic strong projection at its anteroventral margin, large and deep vascular foramina located at the alveolar margin on the anterolateral surface of the bone, and a strongly convex anteroventral articular facet for the splenial. The axial and appendicular skeletal elements described here are also referred to Gorgosaurus sp. and several isolated teeth to cf. Gorgosaurus sp., respectively. The referral of these skeletal fossil elements to Gorgosaurus is based on the paleogeographic and paleostratigraphic position and the overall morphology similar to that of G. libratus. Additionally, several small unserrated premaxillary teeth are identified here as cf. Daspletosaurus sp. Given our current knowledge, the unserrated D-shape in cross-section premaxillary teeth referred to tyrannosaurids represent an ontogenetic dimorphism that is known, thus far, only for Daspletosaurus and Tyrannosaurus. The paleogeographic and paleostratigraphic position of the teeth further supports their referral to cf. Daspletosaurus sp. and not to another North American tyrannosaurid.

Sebastian G. Dalman & Spencer G. Lucas (2015)

New data on the tyrannosaurid theropods from the middle Campanian Lower Judith River Formation of north-central Montana.

Bulletin of the New Mexico Museum of Natural History and Science 68: 77-89.