Apatosaurus is a genus of sauropod dinosaur that lived in North America during the Late Jurassic period, about 152 to 151 million years ago.

The name Apatosaurus ajax was coined in 1877 by Othniel Charles Marsh based on a nearly complete skeleton (holotype, YPM 1860) discovered in the eastern foothills of the Rocky Mountains in Gunnison County, Colorado.

Considered one of the largest land animals of all time, Apatosaurus weighed as much as 80 tons (160,000 lbs)--the equivalent of 11 African elephants!

The name is derived from the Greek words apate (meaning "deceptive") and sauro (meaning "lizard"). It was given this name because some of its bones were dissimilar to those of other dinosaurs, more closely resembling the bones of unrelated mosasaurs, large aquatic reptiles that ruled the world's oceans during the later Cretaceous period.

Like modern birds, sauropods had an extensive system of air sacs to keep their bones light without sacrificing strength.

Its jaws were lined with spatulate (chisel-like) teeth suited to an herbivorous diet. Apatosaurus' main food source was probably conifers, which were the dominant plant when the large sauropods lived. Secondary food sources may have included gingkos, seed ferns, cycads, bennettitaleans, ferns, club mosses, and horsetails.

As in other diplodocids, the tail transformed into a whip-like structure towards the end. Apatosaurus and other sauropod dinosaurs may have supersonically whipped their tails for purposes of defense, communication, same-species rivalry, or courtship.

Apatosaurus laid some of the largest eggs of any dinosaur--about the size of a basketball. This is tiny compared to the full-grown animal, which means the hatchlings must have grown very fast.

Trackways of sauropods like Apatosaurus show that they may have had a range of around 25-40 km (16-25 mi) per day, and that they could potentially have reached a top speed of 30 km per hour (19 mph). A trackway of a juvenile has led some paleontologists to speculate that they were capable of bipedalism, though this is disputed.

It was confused with that of Camarasaurus and Brachiosaurus until 1909, when the holotype of A. louisae was found, and a complete skull just a few meters away from the front of the neck. Henry Fairfield Osborn disagreed with this association, and went on to mount a skeleton of Apatosaurus with a Camarasaurus skull cast. Apatosaurus skeletons were mounted with speculative skull casts until 1970, when John Stanton McIntosh showed that more robust skulls assigned to Diplodocus were more likely from Apatosaurus.