קובץ:Apatosaurus by Papo.jpg

Apatosaurus was a genus of herbivorous dinosaur of the family Diplodocidae of long giant sauropods.

Apatosaurus was discovered and named by Othniel Charles Marsh in the 19th century (1877) and was one of the largest dinosaurs known at that time. It was rediscovered by Marsh and named Brontosaurus, but later it turned out that "Brontosaurus" is just a species of Apatosaurus and thus the name was cancelled. However, a study published in 2015 claims that the Brontosaurus is a distinct genus and hence a valid taxon. Even if it is true, Apatosaurus was bigger than Brontosaurus.

During the 20th century bigger sauropods were discovered such as Argentinosaurus, Alamosaurus, Supersaurus and Sauroposeidon. Traditional estimates of Apatosaurus size were length of 21-26 meters and mass (weight) of 18-35 metric tons. However, recent finds suggest it was considerably bigger than thought. According to isolated giant vertebrae and a giant femur paleontologists estimates that Apatosaurus was 26-30 meters long and may has weighed 32-72 metric tons. According to paleontologist Scott Hartman Apatosaurus was at the same size as Supersaurus and Alamosaurus, reestablishing its place as one of the largest dinosaurs that ever lived.