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Ceratopsids are among the most ubiquitous dinosaur taxa from the Late Cretaceous terrestrial formations of the Western Interior of North America, comprising two subfamilies, Chasmosaurinae and Centrosaurinae. The Two Medicine Formation of northwestern Montana has produced numerous remains of centrosaurine dinosaurs, which represent three taxa previously considered valid:  Rubeosaurus ovatus ,  Einiosaurus procurvicornis  and  Achelousaurus horneri . Here, we reassess the previous referral of specimens to  Rubeousaurus ovatus  and demonstrate that this taxon is represented solely by its holotype specimen, which was first diagnosed as  Styracosaurus ovatus . One of the specimens previously referred to ‘ Rubeosaurus ’  ovatus  instead represents a new eucentrosauran centrosaurine taxon diagnosed here,  Stellasaurus ancellae  gen. et sp. nov.  Stellasaurus  expresses a unique combination of eucentrosauran centrosaurine characters, including an elongate nasal horncore, diminutive supraorbital horncores, and a parietal bearing straight, elongate P3 processes, semi-elongate P4 processes and non-elongate P5, P6 and P7 processes. Within the stratigraphic succession of Eucentrosaura,  Stellasaurus  occurs intermediate to  Styracosaurus albertensis  and  Einiosaurus , and likewise reflects intermediate morphology. Assessed within the stratigraphic, geographical, taphonomic, ontogenetic and phylogenetic framework of Unified Frames of Reference, we fail to reject the hypothesis that  Stellasaurus ancellae  represents a transitional taxon within an anagenetic lineage of eucentrosauran centrosaurines.

royal society open science

A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the ‘ <i>Styracosaurus</i> -line' dinosaurs

A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the ‘ Styracosaurus -line' dinosaurs