The ceratopsian subfamily Chasmosaurinae: sexual dimorphism and systematics

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Abstract
Advanced ceratopsians (family Ceratopsidae) are divisible into two groups, based primarily on the relative proportions and fenestration of their neck frills. These groups have been informally termed the Ceratops-Torosaurus and Monoclonius-Triceratops phyla, the “long-frilled” and “short-frilled” ceratopsids, or the “long-faced” and “short-faced” ceratopsids. Although Lawrence Lambe recognized three subfamilies within the Ceratopsidae, subsequent workers have preferred informal groups and have not adopted his classification. Lambe's subfamily Eoceratopsinae is abandoned because of the placement of Eoceratops in synonymy with Chasmosaurus, and the inclusion of Triceratops within the “longfrilled” ceratopsids. Lambe's subfamilies Chasmosaurinae and Centrosaurinae should be revived, however, for the “longfrilled” and “short-frilled” ceratopsids, respectively. Members of these subfamilies are distinguished by many features of the skull, and it is useful to formally recognize their separation. Although the placement of Triceratops has been problematical, its affinities are now believed to lie with the Chasmosaurinae. Pachyrhinosaurus represents an aberrant Early Maastrichtian survivor of the Centrosaurinae. Species-level taxonomy of ceratopsids is complicated by pronounced individual and ontogenetic variability, and sexual dimorphism, in most species. A population sample of Chasmosaurus from Texas suggests that orientation of the supraorbital horncores is a useful criterion for separating sexual morphs. Based on this criterion, all chasmosaurine genera contain a species or group of species exhibiting the supposed “female” morph, and one exhibiting the supposed “male” morph.

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