A reassessment of the horned dinosaur Judiceratops tigris (Ornithischia: Ceratopsidae) from the Upper Cretaceous (Campanian) of Montana, USA

Abstract

Judiceratops tigris is a recently described ceratopsid, collected from sediments of the Upper Cretaceous (middle Campanian) Judith River Formation (JRF) of northern Montana, USA. The current diagnosis for this taxon is tentative, as it is based on four fragmentary cranial specimens that have limited anatomical overlap. Although these four specimens were not found associated, they were all collected from the same localized area (Kennedy Coulee). New observations, interpretations, and reconstructions of J. tigris are presented in this study, based on the original assumption that these specimens represent the same taxon. Based on these findings, J. tigris had elongate postorbital horncores with an autapomorphic ovate (egg-shaped) cross section; squamosals with non-imbricated episquamosals, and an anterolaterally oriented anteriormost episquamosal; a wide medial parietal bar with an autapomorphic meniscus-shaped cross section; an anteroposteriorly wide posterior parietal bar, and, consequently, reduced parietal fenestrae; and at least four small, low-lying epiparietals on each side of the frill. A phylogenetic analysis incorporating the revised diagnosis of J. tigris supports the previous referral of this taxon to Chasmosaurinae, but missing anatomy prevents an understanding of how J. tigris is related to other chasmosaurines. Judiceratops tigris is retained here as a distinct taxon, and, hence, the oldest known chasmosaurine, but it has more in common with other chasmosaurines than was previously thought. Further fossil collecting in the JRF of Montana may produce more complete specimens of J. tigris and provide a means of testing the new reconstructions presented here.