Abstract
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.
Highlights
Styracosaurus is an iconic centrosaurine ceratopsid dinosaur characterized by large, posteriorly and laterally projecting spike-like epiossifications on the posterolateral margins of the parietosquamosal frill (Lambe 1913; Ryan et al 2007)
Skull ontogeny of Styracosaurus resembles that of Centrosaurus and centrosaurines in general, but the timing of some events appears to be different, with some being accelerated, others delayed, or truncated
Parietal process 2 is totally absent in a few skulls, but in others, it is expressed as a large, medially curving hook approaching the size and morphology exhibited by Centrosaurus
Summary
Styracosaurus is an iconic centrosaurine ceratopsid dinosaur characterized by large, posteriorly and laterally projecting spike-like epiossifications on the posterolateral margins of the parietosquamosal frill (Lambe 1913; Ryan et al 2007). With the exception of three fragmentary skulls from the Two Medicine Formation of Montana (Gilmore 1930; McDonald 2011), it is restricted geographically to the area within, or close to, Dinosaur Provincial Park and Manyberries in southern Alberta. First described over 100 years ago (Lambe 1913), few representative specimens were known until relatively recently (Ryan et al 2007; Holmes et al 2020). Juvenile skulls of well-represented ceratopsid taxa are relatively rare, and with one notable exception (Currie et al 2016) are either incomplete (Goodwin et al 2006; Mallon et al 2015) or are represented by isolated elements and specimens from monodominant bonebeds (Dodson and Currie 1988; Tokaryk 1997; Ryan 2007). Other putative juvenile ceratopsid material has been described (Gilmore 1917; Gilmore 1922; Dodson 1989), but these specimens have proven difficult to link to diagnostic
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