Apatosaurus ăpˌətəsôrˈəs, āˌpătˌə– [key], [Gr.,=deceptive lizard], quadruped saurischian dinosaur, estimated to be from 70 to 90 ft (21 to 27 m) in length and to weigh up to 30 tons (27 metric tons). The similar dinosaur formerly called Brontosaurus [Gr.,=thunder lizard] was reclassified as Apatosaurus after it was determined that the two animals were, in fact, the same. Paleontologist O. C. Marsh named a specimen he uncovered in 1877 Apatosaurus ajax. Marsh discovered parts of a second fossil skeleton in 1879 and, not recognizing it as the same animal, called it Brontosaurus excelsus. When the two animals were determined in the early 1900s to be same, the name Apatosaurus, which was published first, took priority as the valid name. Brontosaurus, however, continued to be used popularly, and it became one of the best known dinosaurs. A 2015 study argued that the animals should be divided into two genera due to differences in the surviving specimens' skeletons and that the name Brontosaurus should be restored.

Apatosaurus had a long, whiplike tail, stout legs, and a long neck. It was herbivorous and dwelled on land, possibly in forests, using its long neck to feed from trees. The eyes and nostrils were located toward the top of the skull. It flourished in the late Jurassic period. Apatosaurus bones and those of other sauropods have long been excavated in the Morrison formation of the late Jurassic and early Cretaceous strata in Colorado, Wyoming, and other Western states. No satisfactory skull specimen has been discovered attached to an Apatosaurus, and for many years an incorrect Camarasaurus-like head was mounted on the famous skeleton on display at the American Museum of Natural History in New York City. Apatosaurus is related to Diplodicus and Barosaurus.

See S. J. Gould, Bully for Brontosaurus (1991).

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