Amphicoelias amfih-SIːLEEUHS

Amphicoelias (, meaning “biconcave”, from the Greek ἀμφί, amphi: “on both sides”, and κοῖλος, koilos: “hollow, concave”) is a genus of herbivorous sauropod dinosaur that lived approximately 150 million years ago during the Tithonian (Late Jurassic Period) of what is now Colorado, United States. Amphicoelias was moderately sized at about 18 m in length and 15 MT in body mass, shorter than its close relative Diplodocus. Its hindlimbs were very long and thin, and its forelimbs were proportionally longer than in relatives.

The namesake fossil of the type species Amphicoelias altus, American Museum of Natural History (AMNH) 5764, is uncertain in included material. When described by Edward Drinker Cope shortly after its discovery in 1877, Amphicoelias was noted to include many back vertebrae, a single pubis, and a femur.