T he Williams Fork Formation preserves a diverse dinosaur fauna containing at least nine saurischian and five ornithischian species. Among the dinosaur specimens recovered is a partial ceratopsian skull (SDMNH 43470) referable to the ceratopsid subfamily Chasmosaurinae (Diem, 1999). Chasmosaurines, or long-frilled ceratopsians, existed throughout western North America during the Campanian and Maastrichtian (Judithian, Edmontonian, and Lancian). Recent discoveries have renewed debate over the phylogenetic relationships and biogeographic history among the chasmosaurines (Forster et al., 1993; Lehman, 1996; Holmes et al., 2001).

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There are some latitudinal differences in occurrences of taxa. Northern chasmosaurines include...

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