The first partial skeleton of Brachiosaurus was discovered and excavated during expeditions from the Field Columbian Museum to Colorado, USA, in 1900. It was described and named by American paleontologist and expedition leader, Elmer Riggs, in 1903. Riggs gave it the full name Brachiosaurus altithorax, and after the fossil material was prepared, he proceeded to describe it in more detail in 1904. Brachiosaurus is one of the rarest dinosaurs in the Morrison Formation of America and only one other partial skeleton is known for certain, found in Utah. This means that the head and neck of Brachiosaurus altithorax are not known with certainty, and so estimates of the total length and height of the animal are based on its close relatives.
A second species of Brachiosaurus was named in 1914 from Tanzania, Africa. This species, named Brachiosaurus brancai at the time, was represented by much more material and shaped the popular image of Brachiosaurus. However, this African species differed significantly from the American one. The American B. altothorax had a longer and deeper body, more widely splayed arms, and a longer tail. This led to Brachiosaurus brancai being given a new name, Giraffatitan, in 1988. This classification gained widespread acceptance following a thorough comparison of Brachiosaurus specimens in 2009, and today there is only one valid species of Brachiosaurus, the American B. altithorax.
Brachiosaurus lived in the Late Jurassic about 153 million years ago. It shared its habitat with several other famous North American dinosaurs including the plated Stegosaurus, long-necked Diplodocus, and predatory Allosaurus. However, adult Brachiosaurus were so large they were probably immune to attacks from predators.