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203 14 A New, Small Ceratopsian Dinosaur from the Latest Cretaceous Hell Creek Formation, Northwest South Dakota, United States: A Preliminary Description CHRISTOPHER J. OTT AND PETER L. LARSON a fragmentary skull and skeleton of a small ceratopsian dinosaur from the Hell Creek Formation of South Dakota represents a new taxon of horned dinosaur . This specimen is estimated to have a skull length of approximately 1 m and an approximate total length of 3 m. All preserved elements of the skeleton show characteristic adult features such as rugose bone surface texturing , well-developed dendritic veination patterns, and fusion of sutural contacts to the point of obscurity. The adult status, diminutive size, and significant morphological differences between the new animal and contemporaneous ceratopsians support the designation of a new taxon for this specimen. Introduction The neoceratopsian dinosaur fauna of the Hell Creek Formation is currently represented by four taxa: Triceratops horridus (Marsh 1889; Forster 1996b), Triceratops prorsus (Marsh 1890; Forster 1996b), Torosaurus latus (Marsh 1891), and the basal neoceratopsian Leptoceratops gracilis (Brown 1914; Ott 2006). The Hell Creek Formation is one of the most intensively collected and studied dinosaur-bearing formations in the world, and the neoceratopsian fauna within it has been studied for over 100 years, though significant work remains. Recent work on the advanced neoceratopsian taxa includes description of very young individuals and discussion of the ontogenetic variation of Triceratops (Goodwin et al. 2006). Other work on neoceratopsian ontogeny has demonstrated that certain characteristics, such as development of ornamentation, adult bone surface texture, fusion of sutures in cranial and postcranial elements, and horncore orientation, are consistent indicators of adult status (Lehman 1990; Sampson et al. 1997; Goodwin et al. 2006). A small, fragmented, partial skeleton of a ceratopsid (BHI 6226), consisting of at least 47 identifiable skeletal elements and over 2,000 fragments, was collected from exposures of the Hell Creek Formation near Buffalo, South Dakota, beginning in 1997. BHI 6226 is similar in size to, and shares some characteristics with, the enigmatic Judithian centrosaurine Avaceratops, but is clearly separated temporally and taxonomically . This specimen has an estimated skull length of approximately 1 m, but shows fully developed adult characteristics. With an approximate skull length of 1 m, the estimated total length of BHI-6226 would have been 3–3.5 m, based upon the proportions of Triceratops horridus (adult skull length approximately 2 m). Based on the adult features of BHI-6226 and morphological differences from other Hell Creek ceratopsians, a new taxon, Tatankaceratops sacrisonorum, is proposed for this animal (Fig. 14.1). Institutional Abbreviations. ANSP: Academy of Natural Sciences , Philadelphia; BHI: Black Hills Institute of Geologic Research , Hill City; LACM: Los Angeles County Museum, Los Angeles; MOR: Museum of the Rockies, Bozeman; ROM: Royal 204 ott & larson FIGURE 14.1. Holotype skull of Tatankaceratops sacrisonorum (BI-6226). (A) Known cranial material; (B) reconstruction of the skull of Tatankaceratops sacrisonorum (BHI-6226) superimposed on the reconstructed skull of Triceratops horridus (TCM 2001.93.1; in grey). Scale bar is 10 cm. Ontario Museum, Toronto; SMNH: Royal Saskatchewan Museum , Regina; TCM: Children’s Museum, Indianapolis; UCMP: University of California Museum of Paleontology, Berkeley. Systematic Paleontology Ornithischia Seeley 1887 Neoceratopsia Sereno 1986 Ceratopsidae Marsh 1888 Chasmosaurinae Lambe 1915 Tatankaceratops gen. nov. Type species. Tatankaceratops sacrisonorum n. sp. by monotypy . Etymology. ‘‘Tatanka’’: Lakota Sioux word for the American Bison (Bison bison), in reference to this specimen being roughly the size of an American Bison, its locality being very close to the town of Buffalo, South Dakota, and to recognize and honor the Lakota Sioux Tribe, who are the prior inhabitants of the area. ‘‘ceratops’’: (Latin) horned face. Diagnosis. As for type species, by monotypy. Distribution. As for type species, by monotypy. Tatankaceratops sacrisonorum gen. et. sp. nov. Holotype. BHI-6226, a skull and partial skeleton from the Hell Creek Formation in Harding County, South Dakota. Diagnosis. The new species is distinguishable from other late Maastrichtian ceratopsids based on its small adult size, lack of development of cornual sinuses, proportionally massive nasal horncore, nasal horn the same length as post-orbital horns, nasal horncore recurved, proportionally large orbit compared to skull size, origination of postorbital horncores caudal to the orbit, no overlap of orbit by postorbital horncore, and no evidence of epijugal ossifications. Type Locality. BHI-6226 was collected on the Niemi Ranch in northwestern South Dakota, approximately 10 miles northnorthwest of the town of Buffalo, Harding County. Precise [3.140.185.170] Project MUSE (2024-04-16...

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