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.