Chiba et al.—Systematic re-evaluation of Medusaceratops
traits, including a broad midline ramus on ROM 73832, imbri- cated epiparietals on ROM 73836 (also seen on the previously described specimens, WDC-DJR-001 and TMP 2002.069. 0005), and a convex squamosal contact on ROM 73836 (also seen on WDC-DJR-001). The constrained analysis, in which Medusaceratops lokii is inferred to be a chasmosaurine, resulted in a hypothesis of eight extra steps compared to the original analysis, strongly supporting Medusaceratops as a member of Centrosaurinae. Our new reconstruction of the parietal ornamentation of
Medusaceratops allows for more detailed comparison to Albertaceratops and other ceratopsids. Medusaceratops is most similar to Xenoceratops in terms of having a small dorsally projecting ep 1 and a large pachyostotic and laterally projecting ep 2; however, the ep 2 of Xenoceratops is straight in the two known specimens, unlike the strongly curved ep 2 of Medusaceratops. Although we assigned epiparietal numbers from the midline laterally, following the character coding methodology of Evans and Ryan (2015), the laterally oriented ep 2 of Medusaceratops is morphologically similar to the massive ep 1 of Albertaceratops. Therefore, it is possible that these two epiparietals could be homologous (i.e., the ep 1 is not developed in Albertaceratops as has been inferred for Pachyrhinosaurus), which is congruent with the interpretation of Albertaceratops parietal ornamentation in Farke et al. (2011), who assigned the large pachyostotic epiparietal of this taxon to ep 2. More work on the homology of ceratopsid frill ornamenta- tion is needed to resolve these issues (e.g., Farke et al., 2011). We also note that the ep 1 is expressed differently onROM
73832, where it occurs on the dorsal surface of the parietal ramus medially and extends to the posterior surface laterally, than on the holotype, where it is so much smaller that it was not recognized as an epiparietal by Ryan et al. (2010). The basal centrosaurines Wendiceratops and Xenoceratops also exhibit a high degree of intraspecific plasticity in epiparietal size and morphology. For example, in the holotype of Xenoceratops, the morphology of ep 1 is asymmetrical. On the left side, it is dorsally curved and larger than on the right side, where it does not appear to curve dorsally. In Wendiceratops, the size of the lateral epiparietals varies considerably from the subadult speci- men (TMP 2011.051.0019) compared to the holotype (TMP 2011.051.0009). Similarly, eucentrosaurans can display a large degree of intraspecific variability in mature size, shape, and symmetry of their most-prominent epiparietals. For example, the ep 1 of Centrosaurus apertus (e.g., AMNH 5239, Brown, 1914a; CMN 971, Frederickson and Tumarkin-Deratzian, 2014), Styracosaurus albertensis (Ryan et al., 2007, fig. 14B), and ep 3 of Pachyrhinosaurus lakustai Currie, Langston, and Tanke, 2007 (Currie et al., 2007, figs. 32, 33) exhibits con- siderable variation, but this clade is generally very conservative within the other epiparietals. Thus, it is possible that a high degree of plasticity across all epiparietals may be plesiomorphic for Centrosaurinae, with this variability being primarily limited to only the prominently modified, more medially positioned epiparietals in eucentrosaurans. A future quantitative study of intraspecific frill variability may lead to refinement of ceratopsid systematics and phylogenetics studies. The large size of the thin-sectioned tibia and the postorbital horncores of Medusaceratops are notable for an early ceratopsid.
285
300
260
220 humerus tibia
Figure 10. Comparison of humerus and tibia circumference of ceratopsids from the Belly River Group and the Judith River Formation. In the box plots, mean values are represented by lines in the boxes, lower and upper bounds of the boxes represent the first and third quartiles, and the ends of the dashed lines indicate minimum and maximum values of the data. Data used for this plot are provided in Table S3. Open circles and triangles represent Medusaceratops and Wendiceratops, respectively.
The size range of Medusaceratops limb elements, as well as the humeral size of the penecontemporaneous centrosaurine Wendiceratops (Evans and Ryan, 2015), are comparable to the late Campanian centrosaurines such as Centrosaurus and Styracosaurus, as well as the non-Triceratopsini chasmosaurines from the Belly River Group and correlative strata of the Judith River Formation (Fig. 10; Table S3). Centrosaurus apertus Lambe, 1904 and Styracosaurus albertensis have adult basal skull lengths that range in size from 666mm to 786mm (Table S3), with associated estimated body masses between 2,500 and 4,800 kg (body mass estimation based on sum of humerus and femur circumference using an interspecificlimb scaling equation provided in Campione and Evans, 2012; Table S3). Earlier ceratopsids, notably the middle Campanian (~79Ma)Diabloceratops (basal skull length=620mm; Kirkland and DeBlieux, 2010) and Machairoceratops, the holotype of which is interpreted as being approximately the same size as Diabloceratops (Lund et al., 2016a), are smaller than Centrosaurus and Styracosaurus. The large body size of Medusaceratops and Wendiceratops extends the fossil record of large-bodied ceratopsids into the middle Campanian and may have implications for the paleobiology of these taxa. After examination of all available material, no unequivocal
chasmosaurine bones, or diagnostic material from any other ceratopsid, could be identified in the Mansfield bonebed collections, suggesting that it represents a monodominant accu- mulation of a single centrosaurine taxon,
Medusaceratops.The stratigraphic positions of theMansfield Medusaceratops bonebed within the lower half of the Judith River Formation at Kennedy Coulee, Montana, the chronostratigraphically equivalent Wendi- ceratops bonebed from the lower Oldman Formation of Alberta (Evans and Ryan, 2015), and a low-density bonebed of Xeno- ceratops fromthe slightly chronostratigraphically older Foremost Formation (Ryan et al., 2012) mark some of the oldest occur- rences of centrosaurine bonebeds. Monodominant centrosaurine bonebeds become more abundant in slightly higher stratigraphic units (e.g., Coronosaurus brinkmani from younger strata of the Oldman Formation;Ryan and Russell, 2005), and are common in the Dinosaur Park Formation and younger strata in Alberta,
circumference (mm)
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