In 1970, Dale Russell described Daspletosaurus torosus. For decades, this specimen was regarded as the “prototypical” Campanian tyrannosaurine. It was discovered in 1921 by Charles Mortram Sternberg, who thought it was a new species of Gorgosaurus. But Daspletosaurus was heavier and more powerfully built than Gorgosaurus, and specilized in feeding on ceratopsians. A new study by a team of scientists from Canada and Argentina provides the first detailed study of the braincase of Daspletosaurus. The holotype of Daspletosaurus torosus (CMN 8506) from the Oldman Formation of Dinosaur Provincial Park, southern Alberta, includes a complete braincase. TMP 2001.36.1 (Daspletosaurus sp.) is a nearly complete skull and skeleton of an ontogenetically mature individual from the Oldman Formation. Both specimens were used to digitally reconstruct the brain, inner ear, and the braincase of Daspletosaurus and revealed extensive morphological variations.
Our knowledge of dinosaurian braincases and the structure of their endocranial cavities has a surprisingly long history. The first well-preserved braincase (NHMUK R2501) was found almost 150 years ago in the Isle of Wight and was described as probably belonging to Iguanodon. In 1897, Charles William Andrews – using the same specimen – suggested that dinosaurian brains, and in particular their lobes and surface convolutions, were not closely pressed against the cranial wall. Almost sixty years later, John Ostrom published a study on the anatomy of the hadrosaurian dinosaurs of North America and reinforced the general opinion that they had brains that were not packed tightly within the braincase. In 1970, Dale Russell was the first to recognize taxonomic significance in the variation of the braincase morphology among tyrannosaurids.
The new study reveals a set of characters useful to distinguish the genus Daspletosaurus from other tyrannosaurids: a deep midbrain flexure, the presence of an anterodorsal chamber of the basisphenoid recess, and the presence of a prohypophyseal sinus of the anterior tympanic recess at large body size Among the variations that potentially distinguish TMP 2001.36.1 from Daspletosaurus torosus are the short, vertical lagena, and the paired sinuses on the lateral wall of the basisphenoid recess (unique among tyrannosaurids), the entirely pneumatized paroccipital process (shared with Gorgosaurus), the well-developed crista tuberalis, and the curved ala basisphenoidalis. Additionally, the bottlenecked olfactory tract in TMP 2001.36.1 occurs mainly in smaller tyrannosaurids.
Ariana Paulina Carabajal, Philip J. Currie, Thomas W. Dudgeon, Hans C.E. Larsson, and Tetsuto Miyashita. Two braincases of Daspletosaurus (Theropoda: Tyrannosauridae): anatomy and comparison. Canadian Journal of Earth Sciences. DOI: 10.1139/cjes-2020-0185
Russell, D.A. 1970. Tyrannosaurs from the Late Cretaceous of Western Canada. National Museum of Natural Sciences Publications in Palaeontology, 1: 1–32. DOI: 10.5281/zenodo.1040973