A genus of sauropod dinosaurs that lived during the Jurassic period, from about 155.7 to 150.8 Ma, mainly in what is now North America. It was a herbivore with a very long neck and tail, the tail possibly serving to counterbalance the weight of the neck, and it could swing its tail so fast that the tip moved at supersonic speed, making a loud boom. The largest species was 33 m long. Its neck was flexible, so that the animal could browse, and could stand on its hind legs to browse high.