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Protoceratidae An American offshoot from the main traguloid stem of Artiodactyla, ranging from the Oligocene to the Pliocene, that were deer-sized mammals with horn-like structures in the nasal region, above the orbits, and on the upper part of the brain case. The Pliocene Synthetoceras had two horn-pairs, the nasal pair being fused into a Y shape.