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Ceratopsians and Pachycephalosaurs Ceratopsians and Pachycephalosaurs Ceratopsians and Pachycephalosaurs Around 144 million years ago, began the emergence of the Ornithischian dinosaurs during the cretaceous period and diversified into North America and Asia. Ornithischians were classified as having a hip structure similar to that of birds, although they are not the descendants of birds. Marginocephalians, meaning.