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

Known from dozens of specimens discovered since the early 20th century,
Gorgosaurus libratus 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
Gorgosaurus 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 Gorgosaurus 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, Gorgosaurus and the
larger Tyrannosaurus rex 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 T.
rex than in Gorgosaurus. These results suggest a dissociation between the
timing of cranial development and body size in tyrannosaurids. Finally,
the recognition of ontogenetically invariant characters in Gorgosaurus
makes it possible to determine the taxonomic identity of previously
misidentified specimens.