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A Subadult Individual of Styracosaurus Albertensis

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A Subadult Individual of Styracosaurus Albertensis Vertebrate Anatomy Morphology Palaeontology 8:67–95 67 ISSN 2292-1389 A subadult individual of Styracosaurus albertensis (Ornithischia: Ceratopsidae) with comments on ontogeny and intraspecific variation inStyracosaurus and Centrosaurus Caleb M. Brown1,*, Robert B. Holmes2, Philip J. Currie2 1Royal Tyrrell Museum of Palaeontology, Box 7500, Drumheller, AB, T0J 0Y0, Canada; [email protected] 2Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada; [email protected]; [email protected] 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 dis- coveries were rare until 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; short, 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. In early ontogeny, the nasal horncores of Styracosaurus and Centrosaurus are thin, recurved, and unfused, but in the former the recurved morphology is retained into large adult size and the horncore never develops the procurved morphology common in Centrosaurus. The postorbital horncores of Styracosaurus are shorter and more rounded than those of Centrosaurus throughout ontogeny, and show great- er resorption later in ontogeny. 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 orna- mentations, does not appear to correlate with stratigraphy. INTRODUCTION close to, Dinosaur Provincial Park and Manyberries in southern Alberta. Although first described over 100 years Styracosaurus is an iconic centrosaurine ceratopsid ago (Lambe 1913), few representative specimens were dinosaur characterized by large, posteriorly and laterally known until relatively recently (Ryan et al. 2007; Holmes projecting spike-like epiossifications on the posterolat- et al. 2020). Juvenile skulls of well-represented ceratopsid eral margins of the parietosquamosal frill (Lambe 1913; taxa are relatively rare, and with one notable exception Ryan et al. 2007). It is stratigraphically restricted to (Currie et al. 2016) are either incomplete (Goodwin et al. the Campanian-aged upper Dinosaur Park Formation, 2006; Mallon et al. 2015) or are represented by isolated 29–50 m above the contact with the underlying Oldman elements and specimens from monodominant bonebeds Formation (Ryan et al. 2007; Brown 2013). With the ex- (Dodson and Currie 1988; Tokaryk 1997; Ryan 2007). ception of three fragmentary skulls from the Two Medicine Other putative juvenile ceratopsid material has been de- Formation of Montana (Gilmore 1930; McDonald 2011), scribed (Gilmore 1917; Gilmore 1922; Dodson 1989), but it is also restricted geographically to the area within, or these specimens have proven difficult to link to diagnostic Published May 11, 2020 *corresponding author. © 2020 by the authors; submitted February 18, 2020; revisions received April 22, 2020; accepted April 28, 2020. Handling editor: Jordan Mallon. DOI 10.18435/vamp29361 Vertebrate Anatomy Morphology Palaeontology is an open access journal http://ejournals.library.ualberta.ca/index.php/VAMP Article copyright by the author(s). This open access work is distributed under a Creative Commons Attribution 4.0 International (CC By 4.0) License, meaning you must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits. Vertebrate Anatomy Morphology Palaeontology 8:67-95 adult material, reducing their utility for studies of ontogeny examined at the TMP. A full list of specimens is provided (but see Penkalski and Dodson 1999; McDonald 2011). in Appendix 1. To quantify variability in the position and Sampson et al. (1997) and Frederickson and Tumarkin- orientation of the serially homologous parietal spines, the Deratzian (2014) reviewed the craniofacial ontogeny of position and orientation of each spike was measured in each Centrosaurinae and Centrosaurus, respectively, but most of reasonably complete parietal of Styracosaurus. The radial the available data for diagnostic material pertains to larger position of the base of each process along the margin of the subadults and adults. As such, little is still known about the parietal was measured by projecting a circular coordinate early ontogeny of the Ceratopsidae, including Styracosaurus. (compass) onto the centre of the fenestra (with 0˚ orien- One recently collected Styracosaurus specimen (TMP tated medially, 90˚ oriented posteriorly, and 180˚ oriented 2009.080.0001) comprises a nearly complete skull and laterally; Appendix 2). A radial coordinate system was chosen partial postcranial skeleton. Comparison with other known because this metric is independent of specimen size and specimens of Styracosaurus indicates that this specimen is robust to inconsistencies in the shape of the periphery of the the smallest, and likely youngest, essentially complete skull parietal. The centre of the fenestra was approximated as the known for the genus. As such, it provides the opportunity intersection point of the transverse line bisecting the fenes- to explore ontogeny and individual variation in this taxon, tra at the anteroposterior midpoint, and a parasagittal line specifically the development of the cranial ornamentation, bisecting the fenestra at the transverse midpoint. Similarly, and to contrast these patterns with the better sampled close the orientation of the long axis (base to apex) of each spike relative Centrosaurus apertus. was recorded using the same orientations (0˚ = medial, 90˚ = posterior, and 180˚ = lateral). All measurements were taken MATERIALS AND METHODS to the nearest degree using ImageJ (V 1.44) from dorsal im- ages. Resulting data were plotted using R studio (V 3.4.3). The specimen TMP 2009.080.0001 was examined at the In addition to material of Styracosaurus, a large sample of Royal Tyrrell Museum of Palaeontology. The specimen the closely related (Ryan et al. 2012) species Centrosaurus consists of a nearly complete skull, a posterior cervical ver- apertus was also examined to provide a comparison with the tebra, two dorsal vertebrae, sacrum, two ribs and a scapu- cranial ontogeny in Styracosaurus. Specifically, an exhaust- locoracoid. Measurements were taken using digital and ive sample of nasal horncores and postorbital horncores dial calipers (under 150 mm) and fiberglass measuring tape of Centrosaurus apertus from both articulated skulls and (over 150 mm; Tab. 1). Unless otherwise stated, measure- bonebed material, was examined (Appendix 3). A size series ments follow those of Ryan et al (2007). Photographs were (based on basal size measurements) of Centrosaurus and taken using a Canon EOS 6D digital SLR camera with 50 Styracosaurus horncores in lateral views was created used mm [1:1.4] and 24–105 mm [1:4] lenses. Scientific line Adobe Illustrator (CS5). drawings were prepared from photographs. Dimensions The assumed homologies of epiparietal follow those of were checked against the specimen, and then the final re- Ryan (1992) and Sampson (1993; 1995) with epiossifica- constructions were inked in using Koh-i-noor Rapidograph tions being numbered from medial to lateral. Under such a pens. Photographs were prepared into figures, i.e., remov- scheme a process projecting from the dorsal margin of the ing backgrounds, assembling composite images, using parietal, posterior to the fenestra, and curving anteriorly Adobe Photoshop and Illustrator (CS5). (if present) is designated P1, a process projecting from the As part of a review of ontogeny and variation in the genus, posterior surface of the parietal and curving medial is desig- all available diagnostic Styracosaurus albertensis material, par- nated P2, the medial most posterior spike is designated ticularly articulated skulls, parietosquamosal frills, and cra- P3, and so on. For the squamosal, the epiossifications are nial material from monodominant bonebeds was examined. numbered from anterior to posterior. These materials were spread across five museums: Canadian Museum of Nature, Ottawa (CMN), Royal Alberta Museum, Edmonton (RAM), Royal Ontario Museum, GEOLOGICAL AND GEOGRAPHICAL Toronto (ROM), Royal Tyrrell Museum of Palaeontology, CONTEXT Drumheller and its Field Station in Dinosaur Provincial The Styracosaurus skull TMP 2009.080.0001 was found Park (TMP), and the University of Alberta Laboratory for in August of 2008 and
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