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Freedman Fowler and Horner, 2015 and Fowler, 2016 for discussion).
Emended diagnosis.—Centrosaurine ceratopsid with five epi- parietals on posterolateral parietal ramus, from medial to lateral: ep 1, a small, sometimes procurved epiparietal; ep 2, compara- tively massive, broad-based pachyostotic process that is strongly curved and projects anterolaterally; ep 3, small pachyostotic process curved anterolaterally, with a similar shape to ep 2; ep 4 and ep 5, unmodified, small triangular-shaped epiparietals. Medusaceratops differs from Albertaceratops in possession of a low, elongate ep 1 as the medialmost epiparietal, and differs from Xenoceratops in the strong anterior curvature and projection of ep 2.
Remarks.—The specimen figured as the paratype in the original description of Medusaceratops (Fig. 12.2 in Ryan et al., 2010) is notWDC-DJR-002, butTMP 2002.069.0005. Here, we retain WDC-DJR-002 as a paratype since this specimen pre- serves the diagnostic combination of parietal ornamentation, notably part of the small ep 1 and the pachyostotic ep 2, rather than TMP 2002.069.0005. Casts of the holotype and paratype specimens are reposited at the ROM and at the TMP. WDCB-MC-001, is a large, intact region of the skull roof
that includes nasal and postorbital ornamentation, that was reported to have been collected from the Mansfield bonebed by Ryan (2007). However, new information suggests that, although it was collected in Kennedy Coulee, it probably did not originate in the bonebed (D. Trexler, personal communication, 2017). For this reason, and the fact that the postorbital horncores are much shorter than any known from the bonebed, we do not include it in the hypodigm of Medusaceratops lokii at this time.
Description
The frill ornamentation, composed of co-ossified epiossifi- cations, and the facial horns are generally the most diagnostic suite of traits for ceratopsid dinosaurs, especially for cen- trosaurines (Dodson et al., 2004), thus we describe only the parietal, squamosal, postorbital, and nasal below, following previous work (e.g., Ryan and Russell, 2005). Due to the incomplete nature of the material, and to provide clarity with respect to the new interpretations presented here, detailed descriptions of individual specimens are provided in the appropriate sections, following a general description of each element. A comprehensive list of specimens comprising the hypodigm of Medusaceratops referred to in this study is pro- vided in the supplemental data (Table S1).
Parietal.—Re-examination of WDC-DJR-001 (holotype; Fig. 2.1–2.4), with reference to ROM 73832 (Fig. 3.1–3.5) and ROM73836 (Fig. 3.8–3.10) indicates that Medusaceratops lokii has five, not three, epiparietals (Fig. 4.3). These are the newly recognized diminutive epiparietal (ep 1) and the small
triangular-shaped epiparietal (ep 5) that are positioned medially and laterally, respectively, to the three epiparietals that were originally recognized by Ryan et al. (2010). From medial to lateral, these epiparietals have the following morphology: one small epiparietal (ep 1) sits on the posterior margin of the pos- terior ramus adjacent to the midline (i.e., ROM 73832, ROM 73837, WDC-DJR-001, and WDC-DJR-002); a large pachyostotic hook (ep 2) curves anterolaterally (i.e., WDC- DJR-001, WDC-DJR-002, and TMP 2002.069.0005), a smaller pachyostotic hook (ep 3) curves subtly anterolaterally (i.e., WDC-DJR-001, TMP 2002.069.0005), and two smaller, triangular-shaped epiparietals (ep 4 and 5; i.e., ROM 73836 and WDC-DJR-001). The number of parietal processes of Medusaceratops and
other morphological features seen on parietals from the bonebed do not reveal any definitive chasmosaurine traits. However, they exhibit several key centrosaurine characters, such as a broad midline ramus (ROM 73832), imbricated epiparietals (ROM 73836; also seen on the previously described holotype WDC-DJR-001, as well as TMP 2002.069.0005), and a convex, interdigitating squamosal contact (ROM 73836) (Longrich, 2013). Longrich (2013) noted that the preserved epiparietals are different between WDC-DJR-001 and TMP 2002.069.0005 due to the size discrepancies of ep 3 (P2 in Ryan et al., 2010). However, the ep 3 of TMP 2002.069.0005 is lobe-shaped rather than being an unmodified, triangular shape. Therefore, we agree with the interpretation of Ryan et al. (2010) that the preserved epiparietals on TMP 2002.069.0005 are ep 2–4. We interpret the size difference of ep 3 on TMP 2002.069.0005 and WDC-DJR-001 as an individual and/or ontogenetic variation. ROM 73832 (Fig. 3.1–3.5).—ROM 73832 preserves the
posterior half of a midline ramus and the medial part of the left posterior ramus with a distinct yet diminutive epiparietal (ep 1) adjacent to the midline. The ventral surface is relatively smooth, whereas the dorsal side is incised by numerous vascular grooves, especially on the tori on the midline ramus and the epiparietal on the posterior ramus, and has adult bone texture (sensu Sampson et al., 1997; Brown et al., 2009). Ep1 is well fused to the posterior ramus, and its contact boundary is indistinguishable (Fig. 3.3). Taken together, the bone surface texture, the epiparietal fusion, and the thickened posterior margin of the midline ramus (31.6 mm), suggest that this specimen is derived from a skeletally mature, adult individual (Sampson et al., 1997). In cross-section, the midline ramus tapers laterally on either side of the midline, making it transversely broad with a triangular cross-sectional shape (Fig. 3.4), which is a synapomorphy for Centrosaurinae (Farke et al., 2011; Sampson et al., 2013; Evans and Ryan, 2015). The midline ramus has two low tori (‘bumps’) on the dorsal side (Fig. 3.1 and 3.5). An embayment of the posterior ramus at the midline is present, but it is wide and relatively shallow (Fig. 3.3), as in Xenoceratops (Ryan et al., 2012). Ep 0 is definitively absent in this specimen. The base of the preserved epiparietal, ep 1, is unusual compared to other
Figure 3. Newly described parietals from the Mansfield bonebed: (1) oblique view (direction is figured by arrows above) of 3D model, (2) posterior view, (3) dorsal view, (4) anterior view, and (5) left lateral view of ROM 73832; (6) dorsal and (7) oblique view (direction figured by arrows above) of 3D model of ROM 73837; (8) lateral view, (9) dorsal view, and (10) close-up of ventral surface of ROM 73836. Abbreviations: ep, epiparietal; mb, bumps on midline ramus; mr, midline ramus; pr, posterior ramus; sqc, squamosal contact.
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