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Background  The validity of  Nedoceratops hatcheri , a chasmosaurine ceratopsid dinosaur known from a single skull recovered in the Lance Formation of eastern Wyoming, U.S.A., has been debated for over a century. Some have argued that the taxon is an aberrant Triceratops, and most recently it was proposed that  N. hatcheri  represents an intermediate ontogenetic stage between “young adult” and “old adult” forms of a single taxon previously split into  Triceratops  and  Torosaurus .    Methodology/Principal Findings  The holotype skull of  Nedoceratops hatcheri  was reexamined in order to map reconstructed areas and compare the specimen with other ceratopsids. Although squamosal fenestrae are almost certainly not of taxonomic significance, some other features are unique to  N. hatcheri . These include a nasal lacking a recognizable horn, nearly vertical postorbital horncores, and relatively small parietal fenestrae. Thus,  N. hatcheri  is tentatively considered valid, and closely related to  Triceratops  spp. The holotype of  N. hatcheri  probably represents an “old adult,” based upon bone surface texture and the shape of the horns and epiossifications on the frill. In this study,  Torosaurus  is maintained as a genus distinct from  Triceratops  and  Nedoceratops . Synonymy of the three genera as ontogenetic stages of a single taxon would require cranial changes otherwise unknown in ceratopsids, including additions of ossifications to the frill and repeated alternation of bone surface texture between juvenile and adult morphotypes.    Conclusions/Significance    Triceratops ,  Torosaurus , and likely  Nedoceratops , are all distinct taxa, indicating that species richness for chasmosaurine ceratopsids in the Lance Formation just prior to the Cretaceous-Paleocene extinction was roughly equivalent to that earlier in the Cretaceous.

Anatomy and Taxonomic Status of the Chasmosaurine Ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A