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Amphicoelias (, meaning "biconcave", from the Greek αμφι, amphi: "on both sides", and κοιλος, koilos: "hollow, concave") is a genus of herbivorous sauropod dinosaur that includes what may be the largest dinosaur ever discovered, A. fragillimus. Based on surviving descriptions of a single fossil bone, A. fragillimus may have been the longest known vertebrate at 40 to 60 m in length, and may have had a mass of up to 122 tonnes (135 short tons). However, because the only fossil remains were lost at some point after being studied and described in the 1870s, evidence survives only in drawings and field notes. The fossil was found in the Morrison Formation. (see more...)