Apatosaurus is a genus of extinct sauropod dinosaurs that lived in North America during the Late Jurassic period. It was originally described in 1877, when Othniel Charles Marsh named the first-known species, Apatosaurus ajax. A second species, Apatosaurus louisae, was discovered in 1916. They range in age from 152 to 151 million years ago (mya), during the early Tithonianages of the Morrison Formation in modern-day Colorado, Oklahoma, and Utah, in the United States. Apatosaurus had an average length of 22.8 m (75 ft), and an average mass of at least 16.4 metric tons (18.1 short tons). A few specimens indicate a maximum length up to 30% greater than average and a mass of 33–73 t (32–72 long tons; 36–80 short tons).




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