Brachiosaurus (/ˌbrækiəˈsɔːrəs/) is a genus of sauropod dinosaur that lived in North America during the Late Jurassic, about 154 to 150 million years ago. It was first described by American paleontologist Elmer S. Riggs in 1903 from fossils found in the Colorado River valley in western Colorado, United States. Riggs named the dinosaur Brachiosaurus altithorax; the generic name is Greek for "arm lizard", in reference to its proportionately long arms, and the specific name means "deep chest". Br