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Journal of Paleontology 92(2):272–288
forming the HSZ and the channelized deposits in which the Medusaceratops bonebed occurs, the reciprocal occurrence of these taxa may be slightly inaccurate.
Materials and methods
The Mansfield bonebed has been commercially excavated by Canada Fossils Ltd since 1994. The bonebed specimens pre- viously described in Ryan (2007) and Ryan et al. (2010) are housed in the Royal Tyrrell Museum of Palaeontology (TMP) and the Wyoming Dinosaur Center (WDC). The new material described here was collected in the summer of 2011 and 2012 by a crewled by D. Trexler of the TwoMedicine Dinosaur Center in Bynum, Montana, and subsequently acquired by the Royal Ontario Museum. The quarrywas visited byDCE and FF on July 16, 2016, where a detailed stratigraphic section was measured and sedimentological observations were made at the quarry site. Photogrammetric 3D models of selected specimens
(Figs. 3, 6) were created using Agisoft PhotoScan Standard Edition, ver. 1.1 and 1.2 to help visualize certain morphologies that can be obscured by the dark color of the specimens. The models were exported as STL files and scaled to the original size in mm using MeshLab, ver. 1.3.3 (SourceForge). The models were then oriented and screen-captured on Avizo, ver. 6.1.1. (FEI). The 3D models were uploaded to Dryad (
http://doi.org/ 10.5061/dryad.8h067). For the phylogenetic analysis, Medusaceratops was coded
includes 97 characters used first in Farke et al. (2011) and sub- sequently used in Sampson et al. (2013), and four characters added by Evans and Ryan (2015). All characters were equally weighted, but Character 20 was treated as an ordered (additive) character following previous analyses (e.g., Farke et al., 2011; Sampson et al., 2013; Evans and Ryan, 2015). Codings of two characters aremodified from Ryan et al. (2017). Reassignment of theMansfield bonebed material to Medusaceratops in this paper results in the coding change for Character 30 (curvature of supraorbital horncore in rostral view) for Albertaceratops from ‘2’ (lateral) to ‘?’. Character 100 (shape of epiparietal 1) was modified for two taxa as follows: from 1 to 2 (elongate flattened process or spike, greater than twice as long as wide) in Coronosaurus brinkmani (Ryan and Russell, 2005) based on
into the data matrix of Ryan et al. (2017) based on the hypodigm listed in the supplemental material (Text S1 and Table S1). Based on our interpretation of the available material, there is no indication of any other ceratopsid species in the Mansfield bonebed collections, and it is therefore assumed to be a mono- dominant bonebed, which is common for centrosaurines (Ryan et al., 2001; Eberth and Getty, 2005; Ralrick and Tanke, 2008; Chiba et al., 2015; Eberth, 2015). Hence, all ceratopsid material from the bonebed is interpreted to represent a single species, and coded as a single operational taxonomic unit, following com- mon practice in centrosaurine studies (e.g., Sampson et al., 1995; Ryan and Russell, 2005; Currie et al., 2008; Farke et al., 2011; Fiorillo and Tykoski, 2012; Ryan et al., 2012; Evans and Ryan, 2015). We added Medusaceratops lokii and the recently described centrosaurine, Machairoceratops cronusi Lund et al., 2016a into the original data matrix, which includes 28 cera- topsian operational taxonomic units. The final matrix is composed of 101 characters, which
TMP 2002.068.0001; from 1 to (0 and 1) in Styracosaurus albertensis Lambe, 1913 reflecting polymorphism based on CMN 344 and ROM 1436 (Ryan et al., 2007). We follow the epiparietal homology argument of Evans and Ryan (2015), which counts the most medially positioned paired epiparietals as ep 1 in non-eucentrosauran (see following paragraph) cen- trosaurines. The phylogenetic analysis was performed using Traditional Search with the Tree Bisection Reconnection algo- rithm in TNT ver. 1.5 (Goloboff and Catalano, 2016) with Leptoceratops gracilis Brown, 1914b designated as the outgroup taxon, following the previous analysis (Sampson et al., 2013). The analysis was run with 1,000 replicates, and up to 1,000 trees were saved in each replication. Branches are collapsed if there is no possible support (“Rule 3”). To assess the robustness of the attained topology, standard bootstrap resampling (sampling with replacement) was conducted with 10,000 replications of tradi- tional tree search, and Bremer support was computed with retaining trees suboptimal by ten steps. In order to further test the subfamilial phylogenetic position of Medusaceratops lokii as a centrosaurine rather than a chasmosaurine, a constraint analysis was performed in which the position ofM. lokii was constrained as the sister taxon of Chasmosaurus and Pentaceratops in a monophyletic Chasmosaurinae, using this option in TNT. We define the new node-based centrosaurine clade
Eucentrosaura as the least inclusive clade containing Centrosaurus apertus and Pachyrhinosaurus canadensis in order to facilitate the morphological comparisons of the Medu- saceratops material with other centrosaurines in this study. Eucentrosaura includes the members of Centrosaurini and Pachyrhinosaurini recovered in the strict consensus tree topol- ogy described in the phylogenetic analysis section of this paper, which is consistent with other recent phylogenetic analyses of centrosaurines (e.g., Farke et al., 2011; Evans and Ryan, 2015; Ryan et al., 2017). A histological thin section of the diaphysis of a large tibia
(ROM67873)wasmade using standard techniques (Lamm, 2013) in the palaeohistology lab at the Royal Ontario Museum. The original thin section imagesmade using plane and cross-polarized light are available at
http://doi.org/10.5061/dryad.8h067.
Repositories and institutional abbreviations.—CanadianMuseum of Nature (CMN), Ottawa, Canada; Fukui Prefectural Dinosaur Museum (FPDM), Fukui, Japan;Museumof the Rockies (MOR), Bozeman,Montana, USA; Royal Ontario Museum (ROM), Tor- onto, Ontario, Canada; Royal Tyrrell Museum of Palaeontology (TMP), Drumheller, Alberta, Canada;Wyoming Dinosaur Center (WDC), Thermopolis,Wyoming, USA.
Systematic paleontology
Dinosauria Owen, 1842 Ornithischia Seeley, 1887 Ceratopsia Marsh, 1890 Neoceratopsia Sereno, 1986 Ceratopsidae Marsh, 1888 Centrosaurinae Lambe, 1915
Medusaceratops Ryan, Russell, and Hartman, 2010 Medusaceratops lokii Ryan, Russell, and Hartman, 2010
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