Data from: New material and systematic re-evaluation of Medusaceratops lokii (Dinosauria: Ceratopsidae) from the Judith River Formation (Campanian, Montana)

Medusaceratops lokii 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 subsequently two parietals were designated the types of the chasmosaurine, M. lokii, in part, because they were interpreted 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 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.

Medusaceratops lokii（Medusaceratops lokii）是一类神秘的角龙科（Ceratopsidae）类群，其化石记录以采自美国蒙大拿州坎潘期朱迪河组（Campanian Judith River Formation）曼斯菲尔德骨层（Mansfield bonebed）的部分顶骨（parietal）标本为代表。最初，该骨层出土的所有角龙科化石均被归入尖角龙亚科（Centrosaurinae）的艾伯塔角龙（Albertaceratops）；但后续有两块顶骨被指定为开角龙亚科（Chasmosaurinae）M. lokii的正模标本，部分依据是这些标本被认为具有双侧对称的3块顶骨缘角（epiparietal）。本文描述了该骨层中新发现的化石材料，借此可对该类群开展系统性分类修订。基于新发现的顶骨标本完成的颈盾（frill）重建修订结果显示，M. lokii拥有宽阔的中线支，且在颈盾边缘至少具有5块顶骨缘角（epiparietal，简称ep），这两项特征均为尖角龙亚科的典型鉴别特征。从内侧至外侧，顶骨缘角装饰依次为：一块小型、形态多变的前弯顶骨缘角（ep1）、一块向前外侧弯曲的厚骨化钩状结构（ep2）、一块更小的厚骨化突起（ep3），以及两块小型三角形顶骨缘角（ep4与ep5）。本次首次将Medusaceratops纳入角龙科的系统发育分析，结果显示M. lokii是一类独特的早期尖角龙亚科角龙类，相较于纳索角龙族（Nasutoceratopsini），其与尖角龙族（Centrosaurini）和厚鼻龙族（Pachyrhinosaurini）的亲缘关系更为紧密。曼斯菲尔德骨层中未发现明确的开角龙亚科骨骼化石，也未鉴定出其他角龙科的诊断性标本，这表明该骨层是目前已知最早的单优势类群尖角龙亚科化石堆积点之一。