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156 10 New Material of ‘‘Styracosaurus’’ ovatus from the Two Medicine Formation of Montana ANDREW T. MCDONALD AND JOHN R. HORNER a partial skull, MOR 492, is attributed to Styracosaurus ovatus Gilmore 1930, hitherto known from only the holotype, USNM 11869, a partial parietal. With the addition of MOR 492, the cranial ornamentation of S. ovatus now includes a tall, erect, long-based nasal horncore and low, rounded supraorbital horncores. Newly recognized morphological features and a phylogenetic analysis indicate that S. ovatus is most closely related to Einiosaurus procurvicornis and is not congeneric with S. albertensis. A new generic name, Rubeosaurus, is created for ‘‘S.’’ ovatus, bringing to three the number of valid centrosaurine species in the Two Medicine Formation and making Styracosaurus a monotypic taxon confined to the Dinosaur Park Formation of Alberta. Hypotheses regarding centrosaurine evolution are reviewed in light of this analysis. Introduction The Two Medicine Formation of Montana has yielded material of more than 20 species of dinosaur over the last century (Trexler 2001; Weishampel et al. 2004), including four taxa of centrosaurine ceratopsids: ‘‘Brachyceratops montanensis’’ (Gilmore 1914; nomen dubium), Styracosaurus ovatus (Gilmore 1930), Einiosaurus procurvicornis (Sampson 1995), and Achelousaurus horneri (Sampson 1995). The latter two species are well established upon two paucispecific bonebeds and three isolated partial specimens, respectively. The material of ‘‘Brachyceratops ’’ includes the holotype partial juvenile skull, USNM 7951, and the partial remains of four other juvenile individuals excavated from the same quarry (Gilmore 1917). ‘‘Brachyceratops montanensis’’ was designated a nomen dubium by Sampson et al. (1997) due to the lack of diagnostic adult features , a designation followed herein. A partial subadult skull, USNM 14765, was also referred to this taxon by Gilmore (1939), but it too lacks diagnostic adult features and cannot be conclusively shown to pertain to the same species as the original ‘‘Brachyceratops.’’ Styracosaurus ovatus has been problematic since its discovery by George F. Sternberg in 1928, and has variously been considered dubious (Dodson et al. 2004) or valid (Ryan et al. 2007). MOR 492 (Figs. 10.1–10.4) was discovered in 1986 and comprises an associated partial adult skull, including a partial left premaxilla, nearly complete co-ossified nasals with horncore, a partial left postorbital, and most of the right half of the parietal lacking the median bar. Comparison of the parietal of MOR 492 with other centrosaurines reveals that it is referable to Styracosaurus ovatus due to the medially inclined P3 spikes, a feature shared with only USNM 11869. With the addition of the more complete MOR 492, this taxon may now be coded with fewer missing data, allowing a more robust phylogenetic analysis of Centrosaurinae. Institutional Abbreviations. AMNH: American Museum of Natural History, New York; CMN: Canadian Museum of Nature , Ottawa; MOR: Museum of the Rockies, Bozeman; ROM: New Material of ‘‘Styracosaurus’’ ovatus 157 Royal Ontario Museum, Toronto; TMP: Royal Tyrrell Museum of Palaeontology, Drumheller; USNM: United States National Museum, Washington, DC. Anatomical Abbreviations. BNH: base of nasal horncore; BNO: base of nasal ornamentation; BP3: base of parietal process 3; BP4: base of parietal process 4; CS: cornual sinuses; LM: lateral margin; NR: narial rim; OM: oral margin; OR: orbit; P1– P7: parietal processes 1–7; RM: rugose rostral margin; SOH: supraorbital horncore; SPR: sutural surfaces for premaxillae; SRO: sutural surface for rostral; TP: tab-like process; VT: vascular traces. Systematic Paleontology Ornithischia Seeley 1888 Ceratopsia Marsh 1890 Ceratopsidae Marsh 1888 Centrosaurinae Lambe 1915 Rubeosaurus ovatus (Gilmore 1930) gen. et comb. nov. Holotype. USNM 11869, a partial adult parietal consisting of most of the transverse bar together with three pairs of fused, spike-like processes at the P3, P4, and P5 positions. Referred Specimen. MOR 492, an adult partial skull including partial left premaxilla, co-ossified left and right nasals with horncore, partial left postorbital with horncore, and nearly complete right parietal with two spikes. Generic Etymology. Rubeus, Latin adjective for thornbush or bramble, plus saurus, Latinized Greek noun for lizard; ‘‘thornbush lizard’’ in reference to the array of spikes adorning the parietals. Generic Diagnosis. As for type and only species. Specific Diagnosis. Centrosaurine ceratopsid diagnosed by a single autapomorphy: caudomedially inclined elongate spikes at the P3 positions. Differential Diagnosis. Elongate, long-based nasal horncore as in Einiosaurus; nasal horncore erect and slightly recurved as in adult specimens of Centrosaurus and Styracosaurus; small postorbital horncore with rounded apex, as in unmodified adult specimens of Styracosaurus and Einiosaurus; straight P3 spikes as Einiosaurus; P4 process variably developed as elongate spike, as...

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