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.

洛基美杜莎角龙（Medusaceratops lokii）是一类谜样的角龙科（Ceratopsidae）类群，其化石标本来自美国蒙大拿州坎潘期朱迪思河组（Judith River Formation）曼斯菲尔德骨层（Mansfield bonebed）出土的部分顶骨。最初，该骨层中采集到的所有角龙科化石材料均被归入尖角龙亚科（Centrosaurinae）的阿尔伯塔角龙（Albertaceratops），但后续研究将其中两件顶骨指定为开角龙亚科（Chasmosaurinae）洛基美杜莎角龙的模式标本，部分依据是其被认为具有双侧对称的3根顶骨棘（epiparietal）。
本文描述了该骨层出土的新化石材料，借此可对该类群开展系统修订。基于新发现的顶骨所构建的修订版颈盾复原图显示，洛基美杜莎角龙拥有宽阔的中线支突，且颈盾边缘至少具有5根顶骨棘（ep），这两个特征均为尖角龙亚科的典型特征。从内侧至外侧，顶骨棘装饰结构依次为：1根小型、形态多变的前弯顶骨棘（ep1）、1根向前外侧弯曲的厚骨性钩状结构（ep2）、1根更小的厚骨性突起（ep3），以及2根小型三角形顶骨棘（ep4与ep5）。
本次首次将洛基美杜莎角龙纳入角龙科的系统发育分析，结果显示其是一类独特的早期尖角龙亚科角龙类，相较于短角龙族（Nasutoceratopsini），其与尖角龙族（Centrosaurini）和厚鼻龙族（Pachyrhinosaurini）的亲缘关系更近。曼斯菲尔德骨层中未发现明确的开角龙亚科骨骼或其他角龙科的诊断性化石材料，这表明该骨层是目前已知最古老的单优尖角龙亚科角龙类堆积记录之一。