Abstract

The history of Monoclonius crassus Cope 1876 is reviewed. The type is based on a composite of disarticulated and unassociated individuals from the Judith River Formation exposed along the Missouri River in Montana. The parietal (AMNH 3998) is well preserved and diagnostic, and is herein designated as the neotype of Monoclonius crassus. Cope named three further species in 1889, all based on fragmentary associations, none of which can be shown to be valid. Lambe named Centrosaurus apertus in 1904 on the basis of an isolated parietal from Judith River sediments in southern Alberta. Barnum Brown discovered complete skulls in Alberta, and described them in 1914 and 1917 as new species that he referred to Monoclonius, believing Centrosaurus to be a synonym. Confusion has persisted to the present day.

In this study, the types of all Judithian centrosaurine ceratopsids are subjected to biometric analysis. It is shown that Monoclonius and Centrosaurus are distinct from each other, and that Styracosaurus is closer to the latter than to the former. Sexual dimorphism is inferred for Centrosaurus and Styracosaurus. Morphological differences previously regarded as indicative of species-level differentiation are regarded as sexual dimorphism. It is necessary to recognize only single species of Centrosaurus, Styracosaurus, and Monoclonius. Brachyceratops is not a juvenile of Monoclonius, but is a valid taxon.

Introduction

Horned dinosaurs of the subfamily Centrosaurinae reached the peak of their diversity in the Late Cretaceous Campanian stage, and their remains are well preserved in the Judith River Formation of Montana and particularly of Alberta.