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Apatosaurus is a genus of extinct sauropod dinosaurs that lived in North America during the Late Jurassic period. Othniel Charles Marsh described and named the first-known species, A. ajax in 1877, and a second species, A. louisae, was discovered and named by William H. Holland in 1916. Fossils are known from the Morrison Formation of modern-day Colorado, Oklahoma, and Utah, in the United States. Apatosaurus had an average length of 22.8 m, and an average mass of at least 16.4 MT.

The cervical vertebrae of Apatosaurus are less elongated and more heavily constructed than those of Diplodocus, and the bones of the leg are much stockier despite being longer, implying that Apatosaurus was a more robust animal. The tail was held above the ground during normal locomotion. Apatosaurus had a single claw on each forelimb and three on each hindlimb. Apatosaurus was a generalized browser that likely held its head elevated. To lighten its vertebrae, Apatosaurus had air sacs that made the bones internally full of holes. Like that of other diplodocids, its tail may have been used as a whip to create loud noises.

Apatosaurus is a genus in the family Diplodocidae. It is one of the more basal genera. Brontosaurus has long been considered a junior synonym of Apatosaurus; its only species was reclassified as A. excelsus in 1903. However, the 2015 study concluded that Brontosaurus was a valid genus of sauropod distinct from Apatosaurus. (see more...)