Chiba et al.—Systematic re-evaluation of Medusaceratops
Coronosaurus brinkmani parietals from the Milk River Ridge Reservoir bonebed (Ryan and Russell, 2005). The lateral parietal ramus ofWDC-DJR-001 has a wide gap
(96.3mm) between the squamosal contact and its anteriormost fused epiparietal (ep 4), which is unusual in centrosaurines. However, ROM 73834, which lacks this gap, has an epiparietal very close to the squamosal contact. The corresponding area of the holotype shows an open, asymmetrical, interdigiting suture on the lateral surface (Fig. 2.4), indicative of the prior presence of an unfused epiparietal (ep 5) that was lost post-mortem. TMP2002.069.0005 (Fig. 2.7, 2.8).—TMP2002.069.0005
is composed of a right lateral ramus with a large pachyostotic ep 2, a lobe-like ep 3, and an undulating lateral surface representing the locus for unattached ep 4. The ep 3 and the ep 4 loci are imbricated. The ep 2 of this specimen differs in the lack of overgrowth at the base on the ventral side of the parietal ramus. The ep 3 is also smaller than that of WDC-DJR-001, but the morphology of this epiparietal is more similar to the anteriorly curved ep 3 on WDC-DJR-001 than to an unmodified epiparietal.
WDC-DJR-002 (paratype; Fig. 2.5, 2.6).—WDC-DJR-002
is a right half of the posterior ramus of the parietal. There is a large, laterally oriented, pachyostotic hook, which is compar- able to ep 2 on WDC-DJR-001 and TMP 2002.069.0005. The ventral overgrowth of the ep 2 does not appear on this specimen, similar to TMP 2002.069.0005, but unlike WDC-DJR-001 and ROM73837. Medial to the ep 2, this specimen has a lateral edge of the ep 1 on the posterior margin of the posterior ramus. Lateral to the ep 2, there is a partially preserved undulating surface representing a locus for ep 3, but the morphology of this ep 3 cannot be inferred due to the broken nature of the specimen. FPDM-V-10 (not figured).—This specimen is a composite
skeleton composed of the cranial and postcranial material from the Mansfield bonebed. Since the bonebed material was thought to represent a chasmosaurine, the frill reconstruction is incorrect. Three original frill specimens (not figured), including one partial midline ramus, one partial right ramus, and one partial left lateral ramus, are incorporated into the frill. The midline ramus has long-grained bone texture and a thin posterior margin (22.7 mm), suggesting this specimen is derived from a juvenile individual. There are three bumps along the dorsal midline, and the cross-section is triangular, as in centrosaurines. From posterior to anterior, the right lateral parietal ramus preserves a partial lobe-shaped epiparietal and two unmodified triangular epiparietals. The morphology and arrangement of the epiparietals on this specimen are congruent with the morpho- logy of the ep 3-5 of other Mansfield bonebed parietal specimens (WDC-DJR-001 and TMP 2002.069.0005). The less complete left ramus has two unmodified, triangular epiparietals that likely represent ep 4 and ep 5. ROM 77214 (not figured).—This specimen is a lateral bar
of a right parietal preserving the squamosal contact, three epiparietals, and another epimarginal locus. This parietal ramus exhibits long-grained bone textures on both ventral and dorsal surfaces, suggesting it represents a relatively young individual. The anterolateral half of the pachyostotic ep 2 epiparietal locus is preserved. It is thickened (28.5mm) compared to the lateral ramus (17.7mm, measured lateral to the base of the ep 2) and protruding by at least ~60mm from the lateral margin of the
279
ramus (the tip of the epiparietal is slightly damaged). As indicated by the bone surface texture, we refer this epiparietal to an undeveloped ep 2. The ep 3 is a low (32.3mm), broad-based (103.6mm), and rounded epiparietal. This ep 3 is not pachyostotic, and its tip is not projecting anterolaterally, unlike the ep 3 of WDC-DJR-001. The ep 4 is low and triangular- shaped, and fused to the ramus. Ep 3 and 4 are imbricated. Anterolateral to the ep 3, the lateral margin of the ramus has a rugose texture continuing to the squamosal contact, suggesting this area represents an open suture for either a distinct ep 5 or an epiparietosquamosal. Although the epiparietals are not fully developed, the configuration of the epiparietals on this specimen is congruent with other parietals described above.
Squamosal.—None of the available squamosals are complete,
but all show centrosaurine features. The preserved posterior flanges that form the lateral sides of frills are round rather than rectangular (ROM 73833, Fig. 5.4; TMP 2002.069.0003, Fig. 5.3), which is similar to taxa in the newly defined Eucentrosaura rather than basal centrosaurines (Maiorino et al., 2013). The maximum number of preserved episquamosals (or loci) on a single specimen is three (ROM 73833, Fig. 5.4; TMP 2002.069.0003, Fig 5.3). The profiles of the episquamosals are wide and low crescent-shaped (TMP 2002.069.0003), or sub- rectangular (ROM 73833) when viewed dorsally, except the anteriormost episquamosal on TMP 2002.069.0002, which is large, completely fused, and semicircular in
outline.Aridge or a series of dorsal tori are often developed on the lateral side of centrosaurine squamosal, which are particularly prominent in early centrosaurines (Penkalski and Dodson, 1999; Evans and Ryan, 2015; Rivera-Sylva et al., 2016). These bumps are only weakly developed on TMP 2002.069.0002 (Fig. 5.1, 5.2), except for a larger eminence preserved on the anterodorsal part of the squamosal near the broken postorbital contact. The shape of the parietal contact of the squamosal is uncertain because this region is only preserved on the pathological WDC-DJR-017 (Ryan, 2007), however, the convex squamosal contacts of the parietals (e.g., ROM 73834) indicate that the squamosal has the typical centrosaurine concave parietal contact. TMP2002.069.0002 (Fig. 5.1, 5.2).—TMP2002.069.0002
is a partial left squamosal. The anterior part is relatively intact and the jugal contact is almost completely preserved on the ventral side of the lateral surface, although the postorbital contact is damaged. Most of the posterior flange is missing except for the posteroventral corner, therefore the overall shape of the flange cannot be assessed. The corner is represented by a large, completely fused semicircular episquamosal. A ridge associated with a series of bumps is weakly developed on the dorsal side of the squamosal, except for a more prominent bump preserved on the anterodorsal part of the squamosal near the broken postorbital contact. The jugal notch is deeply embayed compared to the dorsal margin of lateral temporal fenestra. TMP2002.069.0003 (Fig. 5.3).—This specimen is a partial
right squamosal lacking the anterior portion and the dorsal margin of the posterior flange. Three crescentic episquamosals are preserved and two of them are intact. Both of them are wide (97.1 and 110mm for anteroventral and posterodorsal ones, respectively) and low when viewed dorsally.
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