Supporting CT Data for: Two exceptionally preserved juvenile specimens of Gorgosaurus libratus (Tyrannosauridae, Albertosaurinae) provide new insight into the timing of ontogenetic changes in tyrannosaurids

<p>Known from dozens of specimens discovered since the early 20^th century, <em>Gorgosaurus libratus</em> has arguably contributed more than any other taxon to our understanding of the life history of tyrannosaurids. However, juvenile material for this taxon is rare. Here, we describe two small, articulated <em>Gorgosaurus</em> specimens (skull lengths of ~500 mm) that help advance our knowledge of the anatomy and ontogeny of this taxon and of tyrannosaurids in general. The new specimens exhibit hallmark juvenile tyrannosaurid features, including long, low, and narrow skulls, large circular orbits, absent or incipient cranial ornamentation, ziphodont dentition, and an overall gracile skull frame. Comparison with other <em>Gorgosaurus</em> specimens of various ontogenetic stages allows for an examination of the timing of morphological changes that occurred through ontogeny in this taxon relative to other tyrannosaurids. Of particular note, <em>Gorgosaurus</em> and the larger <em>Tyrannosaurus rex</em> are found to have experienced similar ontogenetic transformations at similar percent skull length relative to the large known individuals for each respective taxon but at different absolute body sizes and biological ages, occurring at a larger size and older age in <em>T. rex</em> than in <em>Gorgosaurus</em>. These results suggest a dissociation between the timing of cranial development and body size in tyrannosaurids. Finally, the recognition of ontogenetically invariant characters in <em>Gorgosaurus</em> makes it possible to determine the taxonomic identity of previously misidentified specimens.</p>