A genus of very large, long-necked, herbivorous, sauropod dinosaurs with long, whip-like tails, that lived from about 155–152 million years ago. The largest species was up to 22 m long and weighed up to 15 tonnes. The name was found to be a junior synonym of Apatosaurus and in 1903 it was changed; some authorities now believe there were two similar genera, Apatosaurus and Brontosaurus, and both names are valid.