Abstract
ABSTRACT Two new ceratopsid dinosaurs, Einiosaurus procurvicornis and Achelousaurus horneri, are described from the Two Medicine Formation (Upper Cretaceous) of Montana. E. procurvicornis is known from three skulls and numerous cranial and postcranial elements from two bonebed assemblages. A. horneri is based on three skulls, one with associated postcranial elements. A phylogenetic review of the subfamily Centrosaurinae reveals two clades, one containing Centrosaurus and Styracosaurus and the other Pachyrhinosaurus plus the two new taxa from Montana. Diagnostic traits for resolving within-group relationships are found only in the skull roof in association with what appear to be secondary sexual characters, probably the result of sexual selection. In addition to illuminating the pattern of ceratopsian evolution, these taxa suggest an increased rate of evolution that may correlate with the late Campanian transgression of the Bearpaw Sea.
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