ARRHINOCERATOPS BRACHYOPS PARKS, 1925 Holotype specimen: ROM 796, complete skull lacking lower jaws. Holotype locality and horizon: Five kilometres upstream of Bleriot Ferry, along the Red Deer River, Alberta. ROM 796 is from between the no. 8 and no. 10 coal seams, below the DMT, and within a narrow stratigraphic range spanning the upper half of the Horsethief Member to the lower half of the Morrin Member of the Horseshoe Canyon Formation (Mallon et al., 2014). Referred specimens: CMN 8882, partial skull of a juvenile, including dentaries with prominent lateral ridge; ROM 1439, an almost complete skull with lower jaws, syncervical and left forelimb elements (scapula, humerus, radius, metacarpals and several phalanges); CMN 56500 (field #WL 123), nasal horncore with triangular transverse cross section. Tentatively referred specimens: TMP 1981.048.0006, partial frill with low, blunt epi-ossifications. Distribution: From the upper Horsethief Member through the Morrin Member up to the DMT of the Horseshoe Canyon Formation, Alberta, Canada (sensu Eberth & Braman, 2012). TMP 1981.048.0006 is from the Horseshoe Canyon Formation (unknown horizon) near Devon, Alberta (Tanke, 1984). Age: The base of the Horseshoe Canyon Formation near Drumheller has been assigned an age of approximately 72.5 Ma (Eberth & Braman, 2012; Eberth et al., 2013). The younger DMT in the middle of the formation lies just above the inferred position of the Campanian– Maastrichtian boundary (Eberth & Braman, 2012, fig. 4), with an accepted age of 70.6 Ma (see discussion in Eberth & Braman, 2012; Eberth et al., 2013). The stratigraphic interval that extends upwards from the base of the formation to the DMT is approximately 165 m thick (Eberth & Braman, 2012: fig. 4), and suggests a rate of sedimentation of 8.7 cm kyr –1. Identifiable Arrhinoceratops remains are known to occur through a stratigraphic interval of 75 m that extends down section from the base of the DMT (Eberth et al., 2013). This suggests a temporal range of approximately 860 kyr for that taxon in the region, between 71.5 and 70.6 Ma. DESCRIPTION OF CMN 8882 As reconstructed (Fig. 1), the skull has a basal length (rostral to occipital condyle) of approximately 465 mm, and is 900 mm in total length. Other relevant measurements are provided in Table 1.

Published as part of Mallon, Jordan C., Ryan, Michael J. & Campbell, James A., 2015, Skull ontogeny in Arrhinoceratops brachyops (Ornithischia: Ceratopsidae) and other horned dinosaurs, pp. 910-929 in Zoological Journal of the Linnean Society 175 (4) on page 911, DOI: 10.1111/zoj.12294, http://zenodo.org/record/5339511