He collaborated with Darla Zelenitsky and Jared Voris of the University of Calgary, as well as Kohei Tanaka of the University of Tsukuba in Japan. For the study, published this week in the Canadian Journal of Earth Sciences, the researchers examined the lower jaws from the Albertosaurus and Gorgosaurus, types of tyrannosaurs commonly found in Canada that predated the T. rex by millions of years. "Our fossil records for those two species of tyrannosaurs is excellent," Therrien said about the collection at the museum. "We have so many specimens of those ... that represent a full growth series from very young individuals that were probably three or four years of age all the way to fully grown adults that were over 20 years of age."