Apatosaurus (meaning “deceptive reptile” ) is an extinct genus of sauropod dinosaur that lived in North America during the Late Jurassic period. The genus was first described in 1877 when Othniel Charles Marsh named A. ajax, and many species have since been described. They range in age from about 154 to 150 million years ago throughout the Kimmeridgian and early Tithonian ages of the Morrison Formation. Among the species is A. excelsus, long considered to be separate under the genus Brontosaurus, which is now considered a junior synonym of Apatosaurus. It had an average length of and a mass of at least , although a few specimens indicate a length up to 30% greater and a weight of . Apatosaurus was one of the more common sauropods in the Morrison, along with Diplodocus and Camarasaurus. The cervical vertebrae were less elongated and more heavily constructed than those of Diplodocus and the bones of the leg were much stockier despite being longer, implying that Apatosaurus was a more robust animal. The tail was held above the ground during normal locomotion. Like all sauropods, Apatosaurus had only one claw on the forelimbs and three on its hindlimbs. The skull of Apatosaurus, long thought to be similar to Camarasaurus, is much more similar to that of Diplodocus. Apatosaurus is a genus in the family Diplodocidae. It is more basal than other genera like Diplodocus and Barosaurus, and is sometimes included in its own subfamily, Apatosaurinae. Apatosaurus is one of the more common sauropods in the Morrison Formation, and is often found associated with skeletons of Allosaurus, Camarasaurus, Stegosaurus and Diplodocus. Apatosaurus is also one of the longer genera, although Supersaurus exceeds it.

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