Chiba et al.—Systematic re-evaluation of Medusaceratops 1 2 cm 2 3


283


med. lat.


ant. post.


Figure 8. Thin-section photographs of mid diaphyseal cross-section of a tibia from the Mansfield bonebed (ROM 67873). Whole cross section images under (1) plane polarized and (2) cross-polarized light; (3) close-up images under plane polarized (left half) and cross-polarized light (right half).


unequivocally ceratopsid, and, given that no other ceratopsids can be confirmed from the locality, we infer that this specimen represents the tibia of Medusaceratops. The mid-diaphyseal thin section of ROM 67873 is mainly


composed of densely vascularized secondary bone (Fig. 8). A lenticular medullary cavity (~3cm × 2.5 cm) is decentered posteriorly. A 0.5–1 cm layer of trabecular bone consistently surrounds the medullary cavity, except on the lateral side where the trabecular bone spans 3 cm. The cortical bone is distributed relatively consistently on the posterior half of the shaft (2 cm), but gradually thickens towards the anteriormost part where cortical bone thickness reaches ~3 cm. The cortical bone is composed of heavily remodeled Haversian bone with occa- sional Volkmann’s canals. Therefore, primary bone tissue is severely obliterated, except in a thin peripheral region ~5mm from the outer periosteal margin. The primary bone tissue along the outer periphery of the bone is comprised of poorly vascu- larized, parallel-fibered bone tissue with multiple growth lines that likely represent closely packed lines of arrested growth (LAGs), although any given growth mark cannot be traced around the entire periphery of the bone. Spacing between these growth lines rapidly narrows towards the outer periosteal mar- gin, and forms an external fundamental system (EFS; Fig. 8.3). Extensive remodeling, parallel-fibered bone, and the presence of an EFS indicate that this individual had reached its asymptotic body size. This indicates that the largest individuals in the Mansfield bonebed were likely mature individuals, and allows comparisons of body size with other later ceratopsids (see Discussion).


Phylogenetic analysis


Medusaceratops could be coded for 64 out of 101 characters in the phylogenetic analysis (Supplemental data). The analysis recovered 630 most parsimonious trees, each with 181 steps. The Consistency Index and Retention Index of each most par- simonious tree is 0.624 and 0.791, respectively. In the strict consensus tree (Fig. 9), Medusaceratops lokii is supported by three autapomorphies (character 59[2], elongate flattened ep 2; character 60[1], laterally curved ep 2; and, character 62[1] laterally curved ep 3). Medusaceratops is nested within


Centrosaurinae with four unambiguous synapomorphies (char- acter 1[0], a rostral with short dorsal and ventral processes; character 2[1], a semicircular premaxillary septum; character 8[1], caudoventral expansion of a premaxilla, and character 51[1], a relatively wide parietal midline bar) and one ambiguous character (character 3[1], the septum composed of premaxilla and nasal). Medusaceratops is recovered in a polytomy with


Albertaceratops, Wendiceratops + Sinoceratops, and the newly defined clade Eucentrosaura. In our analysis, this new clade includes Centrosaurini (sensu Maiorino et al., 2015 and Ryan et al., 2017; Rubeosaurus, Styracosaurus, Spinops, Centrosaurus, Coronosaurus), Einiosaurus, and members of Pachyrostora with nasal bosses (sensu Fiorillo and Tykoski, 2012). Eucentrosaura does not include Xenoceratops, Wendiceratops,or Sinoceratops, unlike Sampson et al. (2013) and Lund et al. (2016a, 2016b), where these taxa were recovered in Pachyrhinosaurini. The members of Eucentrosaurua here have five synapomorphies: 17[1], maxillary tooth row at the same level as the rostral edentulous portion of the maxilla; 20 [1], presence of a distinct nasal horncore; 28[0], short post- orbital horncore; 66[1], presence of ep 6; 68[1], presence of ep 7. Xenoceratops is recovered as the sister taxon to the least inclusive clade containing Medusaceratops and Pachyrostra in the strict consensus tree. The overall morphology of the strict consensus tree is similar to that of Evans and Ryan (2015) and Ryan et al. (Ryan et al., 2017), except for the loss of resolution in the strict consensus tree within the Centrosaurini. The strict consensus topology recovered in this analysis is


generally weakly supported, as is typical of most recent centrosaurine phylogenetic analyses (e.g., Evans and Ryan, 2015). Within Ceratopsidae, the Bremer Decay Index is only high (>2) at the bases of Chasmosaurinae, Centrosaurinae, Pachyrhinosaurini, and the genus Pachyrhinosaurus,with most nodes within Centrosaurinae having a Bremer Decay value of 1. Bootstrap values shows similar patterns of relative support (Fig. 9). The constrained analysis, in which Medusaceratops lokii is


inferred to be a chasmosaurine, resulted in eight extra steps in the most parsimonious trees compared to the original analysis where it is recovered nested within Centrosaurinae.


1 cm


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