A subadult individual of Styracosaurus albertensis (Ornithischia: Ceratopsidae) with comments on ontogeny and intraspecific variation in Styracosaurus and Centrosaurus.

Styracosaurus albertensis is an iconic centrosaurine horned dinosaur from the Campanian of Alberta, Canada, known for its large spike-like parietal processes. Although described over 100 years ago, subsequent discoveries were rare the last few decades, during which time several new skulls, skeletons, and bonebeds were found. Here we described an immature individual, the smallest known for the species, represented by a complete skull and fragmentary skeleton.  Although ~80% maximum size, it possesses a suite of characters associated with immaturity, and is regarded as a subadult.  The ornamentation is characterized by a small, recurved, but fused nasal horncore; low, rounded postorbital horncores; and short, triangular, and flat parietal processes. Using this specimen, and additional skulls and bonebed material, the cranial ontogeny of Styracosaurus is described, and compared to Centrosaurus.  Styracosaurus shows a similar early ontogeny of the nasal horncore, starting thin, recurved, and unfused, but retains the recurved morphology into large adult size, and never develops the procurved morphology common in Centrosaurus. The postorbital horncores of Styracosaurus are lower and more rounded than those of Centrosaurus throughout ontogeny, and show greater resorption later in ontogeney. The length and thickness of the parietal processes increase drastically through ontogeny, but their position and orientation are static across the size series. Several diagnostic Styracosaurus albertensis specimens now preserve medially orientated P3 spikes, causing issues for the diagnosis of S. ovatus. Variability in parietal ornamentation, either expression of P1 and P2 parietal processes, or other cranial ornamentations, does not appear to correlate with stratigraphy.