Journal of Paleontology, 92(2), 2018, p. 272–288 Copyright © 2017, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.62
New material and systematic re-evaluation of Medusaceratops lokii (Dinosauria, Ceratopsidae) from the Judith River Formation (Campanian, Montana)
Kentaro Chiba,1 Michael J. Ryan,2 Federico Fanti,3,4 Mark A. Loewen,5,6 and David C. Evans1,7
1Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada ⟨
kentaro.chiba@
mail.utoronto.ca⟩ 2Department of Vertebrate Paleontology, Cleveland Museum of Natural History, 1 Wade Oval Dr., University Circle, Cleveland, Ohio,
44106, USA ⟨
mryan@cmnh.org⟩ 3Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Alma Mater Studiorum, Università di Bologna, Via Zamboni, 67 - 40126
Bologna, Italy ⟨
federico.fanti@
unibo.it⟩ 4Museo Geologico Giovanni Capellini, Via Zamboni, 63 - 40126 Bologna, Italy 5Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, 84112-0102, USA 6Natural History Museum of Utah, Salt Lake City, Utah, 84108, USA ⟨
mloewen@umnh.utah.edu⟩ 7Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, M5S 2C6, Canada ⟨
d.evans@utoronto.ca⟩
Abstract.—Medusaceratops lokii Ryan, Russell, and Hartman, 2010 is an enigmatic taxon of ceratopsid represented by partial parietals from the Mansfield bonebed in the Campanian Judith River Formation, Montana. Originally, all ceratopsid material collected from this bonebed was referred to the centrosaurine ceratopsid Albertaceratops, but sub- sequently two parietals were designated the types of the chasmosaurine, M. lokii, in part, because they were inter- preted to have three epiparietals bilaterally. Here we describe new material from the bonebed that allows a systematic revision of the taxon. A revised reconstruction of the frill, informed by newly discovered parietals, reveals that M. lokii had a broad midline ramus and at least five epiparietals (ep) around the margin of the frill, both traits that are characteristic of Centrosaurinae. From medial to lateral, the epiparietal ornamentation consists of a small, variably procurving epiparietal (ep 1), an anterolaterally curving pachyostotic hook (ep 2), a smaller pachyostoic process (ep 3), and two small triangular epiparietals (ep 4 and 5). A phylogenetic analysis of ceratopsids, which is the first to include Medusaceratops, indicates that M. lokii is a unique, early centrosaurine ceratopsid taxon that is more closely related to Centrosaurini and Pachyrhinosaurini than Nasutoceratopsini. No unequivocal chasmosaurine bones or diagnostic material from any other ceratopsid could be identified from the Mansfield bonebed, suggesting that it represents one of the oldest occurrences of a monodominant accumulation of a centrosaurine ceratopsid on record.
Introduction
Ceratopsids are a clade of large-bodied herbivorous dinosaurs that rapidly diversified in the latest Cretaceous (Campanian– Maastrichtian) of North America, and have a well-sampled fossil record (Dodson et al., 2004). Recent discoveries from southern Alberta (Ryan, 2007; Ryan et al., 2010, 2012; Evans and Ryan, 2015), Montana (Longrich, 2013; Ryan et al., 2014), and Utah (Loewen et al., 2013; Sampson et al., 2013; Lund et al., 2016a) have revealed a diversity of early ceratopsids that have significantly increased our knowledge of the plesiomorphic anatomy of the group. However, the fossil record of the early radiation of cera- topsids remains poor, and in light of new anatomical information that obscures the morphological distinction between the two subfamilies, Centrosaurinae and Chasmosaurinae, conventional wisdom about the systematic position of taxa known from frag- mentary material may require re-evaluation (Mallon et al., 2016). Medusaceratops lokii Ryan, Russell, and Hartman, 2010 from the lowermost strata of the Judith River Formation is one
such ceratopsid taxon that has a complicated taxonomic history, with the hypodigm having been previously assigned to both subfamilies. The material of the taxon was collected from a middle Campanian bonebed, called the Mansfield bonebed (ca. 79 Ma; Roberts et al., 2013), making it one of the oldest known members of Ceratopsidae (Kirkland and DeBlieux, 2010; Longrich, 2013; Evans and Ryan, 2015; Campbell et al., 2016). The Mansfield bonebed ceratopsid material was initially assigned to Albertaceratops nesmoi Ryan, 2007, which is unequivocally a basal member of Centrosaurinae, based on superficial resemblances of some morphologies to the A. nesmoi holotype, such as the large supraorbital horncores and the large pachyostotic parietal hook (Ryan, 2007). Subsequently, the two most complete parietals from the bonebed were described as a new chasmosaurine species, Medusaceratops lokii, based on an interpretation of the specimens exhibiting only three epiparietals on each side of the midline, which is typical of Chasmosaurinae (Ryan et al., 2010). The remaining material from the bonebed was left as indeterminate Centrosaurinae (Ryan et al., 2010).
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