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189 13 Description of a Complete and Fully Articulated Chasmosaurine Postcranium Previously Assigned to Anchiceratops (Dinosauria: Ceratopsia) JORDAN C. MALLON AND ROBERT HOLMES a nearly complete ceratopsid postcranial skeleton with associated skull fragments, long attributed to Anchiceratops , is described. The skeleton is identified as a chasmosaurine , but the paucity of diagnostic cranial material prevents more precise identification. Based on its provenance from unit 2 of the Horseshoe Canyon Formation of Alberta, the skeleton probably pertains to either Anchiceratops or Arrhinoceratops. The skeleton exhibits several unique features, including a humerus with an enlarged deltopectoral crest and underdeveloped medial tubercle, thickened ribs, and a unique vertebral formula (10 cervicals, 13 thoracics, 12 sacrals, and 39 caudals). The skeleton is also unusually stout and robust for its size, lending credence to the idea that it may have been adapted to a semi-aquatic lifestyle much like the common hippo (Hippopotamus amphibious). This interpretation is supported by the estuarine depositional environment in which the skeleton was found. Introduction In the late summer of 1925, Charles M. Sternberg and his team collected the skeleton of a ceratopsid dinosaur from the upper Horseshoe Canyon Formation (Edmonton Group) of the Red Deer River valley, near Rumsey, Alberta (Fig. 13.1). Most of the skull was missing, but the postcranium was complete and fully articulated. The skeleton was attributed to Anchiceratops by Sternberg, although he did not give a reason for his identi- fication. The specimen was subsequently prepared as a panel mount in 1929 and displayed in the National Museum of Canada (now the Canadian Museum of Nature) with the cast skull of the type of A. ‘‘longirostris’’ (Sternberg 1929) in place. Given the rarity of complete, fully articulated ceratopsid postcrania (only one other specimen of comparable quality is known; Brown 1917), it is surprising that, except for a brief description by Lull (1933), this skeleton has remained undescribed for so long. Here we describe the specimen in more detail, paying particular attention to those defining features of the skeleton. The fragmentary skull material is also described for the first time in an effort to confirm the taxonomic identity of this specimen. Institutional Abbreviations. AMNH: American Museum of Natural History, New York; CM: Carnegie Museum of Natural History, Pittsburgh; CMN: Canadian Museum of Nature, Ottawa ; NSM-PV: National Science Museum, Division of Vertebrate Paleontology, Tokyo; PMU: University of Uppsala, Sweden; ROM: Royal Ontario Museum, Toronto; TMP: Royal Tyrrell Museum of Palaeontology, Drumheller. 190 mallon & holmes FIGURE 13.1. Locality map for CMN 8547 modified from Eberth (unpublished). The specimen locality is indicated by an asterisk SW of Rumsey. More detailed locality information is available from the TMP Collections Section. Upper right inset shows field area in relation to the Province of Alberta. Systematic Paleontology Ornithischia Seeley 1888 Ceratopsia Marsh 1890 Ceratopsidae Marsh 1888 Chasmosaurinae indet. Lambe 1915 Material. CMN 8547, a nearly complete, articulated postcranial skeleton with associated frill fragments. The ‘‘anterior portion of the beak’’ reported by Sternberg in his field notes (1925, CMN) could not be located. The unguals of the first and third digits of the left manus depicted in both old field photographs and photographs of the original panel mount have since been lost. Locality and Horizon. TMP locality L1508. East side of Red Deer River, 10.3 km SW of Rumsey, AB, in NW1/4 of S6-T33R21 -W4, 725 masl. Uppermost unit 2 of Horseshoe Canyon Formation (Fig. 13.2), approximately 26 m below coal seam 11 (Carbon coal zone), 16 m below an extensive oyster bed in the area (part of the Drumheller Marine Tongue). Description of CMN 8547 SKULL Four flattened frill fragments are associated with the skeleton of CMN 8547 (Fig. 13.3). According to Sternberg’s field notes (1925, CMN), these belonged to the left side of the frill, but based on their curvature, probably pertain to the right side. At their thickest points, usually located along the frill margin, the fragments measure approximately 28–30 mm. At their thinnest points, they measure 7–10 mm. Two of the four frill fragments (Fig. 13.3A, B) form part of the scalloped frill margin. The largest of these, a partial right squamosal 704 mm long, bears two episquamosals (sensu Horner and Goodwin 2008) on its proximalmost edge immediately adjacent to the jugal notch. These are followed by a series of four low, blunt scallops lacking sutures at their bases that indicate where the more distal episquamosals were positioned. One of the frill fragments...

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