Mounted specimen, Carnegie Museum of Natural History

Apatosaurus (“deceptive lizard”) is a genus of extinct sauropod dinosaurs that lived in North America during the Late Jurassic period. Fossils from the Morrison Formation of Colorado, Oklahoma, and Utah indicate an average length of 21–22.8 m (69–75 ft) and an average mass of 16.4–22.4 t (18.1–24.7 short tons), but a few specimens are 11% to 30% longer and 32.7–72.6 t (36.0–80.0 short tons). The cervical vertebrae 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. It had a single claw on each forelimb and three on each hindlimb. The skull is similar to that of Diplodocus. It browsed foliage and likely held its head elevated. Air sacs permeated and lightened its vertebrae. In North America during the late Jurassic, Apatosaurus lived alongside dinosaurs such as Diplodocus, Allosaurus, Camarasaurus, and Stegosaurus.

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