Apatosaurus is a massive Late Jurassic sauropod whose remains were found in 1877 in the Morrison Geological Formation, located in the western states of the United States. The name of the genus means "deceptive lizard", and, according to scientists, the skeleton of this herbivorous giant reached up to 15 meters in length: it was a large four-legged animal with a long neck and a long whip-shaped tail. Its forelimbs were slightly shorter than its hind limbs. The cervical vertebrae of Apatosaurus were of medium length and quite strong, and limb bones were strong and stocky. The tail when moving the sauropod kept above the ground. Like other diplodocids, its tail could be used as a whip to create loud sounds or, as suggested recently, as a sense organ. Apatosaurus had one claw on each forelimb and three on each hind limb. To lighten the vertebrae, animal's large neck was filled with an extensive system of air sacs. This allowed Apatosaurus to keep its head high when it fed on the foliage of prehistoric trees.