Brachiosaurus is a genus of sauropod dinosaur from the Jurassic Morrison Formation of North America. It was first described by Elmer S. Riggs in 1903 from fossils found in the Grand River Canyon (now Colorado River) of western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax, declaring it “the largest known dinosaur”. Brachiosaurushad a disproportionately long neck, small skull, and large overall size, all of which are typical for sauropods. However, the proportions of Brachiosaurus are unlike most sauropods: the forelimbs were longer than the hindlimbs, which resulted in a steeply inclined trunk, and its tail was shorter in proportion to its neck than other sauropods of the Jurassic.
Brachiosaurus is the namesake genus of the family Brachiosauridae, which includes a handful of other similar sauropods. Much of what is known by laypeople about Brachiosaurus is in fact based on Giraffatitan brancai, a species of brachiosaurid dinosaur from the Tendaguru Formation of Tanzania that was originally described by German paleontologist Werner Janensch as a species of Brachiosaurus. Recent research shows that the differences between the type species of Brachiosaurusand the Tendaguru material are significant enough that the African material should be placed in a separate genus. Several other potential species of Brachiosaurus have been described from Africa and Europe, but none of them are thought to belong to Brachiosaurus at this time.
Brachiosaurus is one of the rarer sauropods of the Morrison Formation. The type specimen of B. altithorax is still the most complete specimen, and only a relative handful of other specimens are thought to belong to the genus. It is regarded as a high browser, probably cropping or nipping vegetation as high as possibly 9 metres (30 ft) off of the ground. Unlike other sauropods, and its depiction in the film Jurassic Park, it was unsuited for rearing on its hindlimbs. It has been used as an example of a dinosaur that was most likely ectothermic because of its large size and the corresponding need for forage, but more recent research finds it to have been warm-blooded.
FMNH P 25107, the holotype of both the genus Brachiosaurus and the species B. altithorax, consists of the right humerus (upper arm bone), the right femur (thigh bone), the right ilium (a hip bone), the right coracoid (a shoulder bone), the sacrum (fused vertebrae of the hip), the last seven thoracic (trunk) and two caudal (tail) vertebrae, and a number of ribs. Riggs described the coracoid as from the left side of the body, but restudy has shown it to be a right coracoid.
In 1969, during a study by Kingham, Brachiosaurus altithorax, along with species now assigned to other genera, were moved from the genus. Kingham found “B.” atalaiensis, “B.” brancai, and B. altithorax were referable to Astrodon creating many new species of Astrodon apart from the type. Kingham’s views of brachiosaurid taxonomy have not been accepted by many authors.
Other assigned species
- “B.” atalaiensis: Originally described by de Lapparent and Zbyszewski, this material’s reference to Brachiosaurus was doubted by Upchurch, Barret and Dodson, who listed it as an unnamed brachiosaurid, and placed in its own genus Lusotitan by Antunes and Mateus. De Lapparent and Zbyszewski described a series of remains but did not designate a type specimen. Antunes and Mateus selected a partial postcranial skeleton (MIGM 4978, 4798, 4801–4810, 4938, 4944, 4950, 4952, 4958, 4964–4966, 4981–4982, 4985, 8807, 8793–87934) as a lectotype; this specimen includes 28 vertebrae, chevrons, ribs, a possible shoulder blade, humeri, forearm bones, partial left pelvis, lower leg bones, and part of the right ankle. The low neural spines, the prominent deltopectoral crest of the humerus (a muscle attachment site on the upper arm bone), the elongated humerus (very long and slender), and the long axis of the ilium tilted upward indicate that Lusotitan is a brachiosaurid.
- “B.” brancai: Janensch based his description on “Skelett S” (skeleton S) from Tendaguru, but later realized that it comprised two partial individuals: S I and S II. He at first did not designate them as a syntype series, but in 1935 made S I (MB.R.2180) the lectotype. Taylor in 2009, unaware of this action, proposed the larger and more complete S II (MB.R.2181) as the lectotype. It includes, among other bones, several dorsal (trunk) vertebrae, the left scapula, both coracoids, both sternals (breastbones), both humeri, both ulna and radii (lower arm bones), a right hand, a partial left hand, both pubes (a hip bone) and the right femur, tibia and fibula (shank bones). Later Taylor realised that Janensch had in 1935 designated the smaller skeleton S I as the lectotype. A re-assessment of the relation between the African and American brachiosaur material indicates that a separate generic name is warranted for the Tendaguru material, so “B.” brancai has been moved to its own genus: Giraffatitan.
- “B.” fraasi: erected by Janensch in 1914, but later synonymized with “B.” brancai; this material now belongs to Giraffatitan.
- “B.” nougaredi: This species is known from fragmentary remains discovered in eastern Algeria, in the Sahara Desert. The present type material consists of a sacrum and some of the left metacarpals and phalanges. Found at the discovery site but not collected were partial bones of the left forearm, wrist bones, a right shin bone, and fragments that may have come from metatarsals. de Lapparent, who described and named the material in 1960, reported the discovery locality as being in the Late Jurassic–age Taouratine Series (he assigned the rocks this age in part because of the presumed presence of Brachiosaurus). This material was found disjointed over an area of several hundred meters, and probably does not represent a single species.
Brachiosaurus is the namesake genus of Brachiosauridae. Over the years, a number of sauropods have been assigned to Brachiosauridae, such as Astrodon, Bothriospondylus, Dinodocus, Pelorosaurus, Pleurocoelus, and Ultrasaurus, but most of these are currently regarded as dubious or of uncertain placement. A phylogenetic analysis of sauropods published in the description of Abydosaurus found that genus to form a clade with Brachiosaurus and Giraffatitan (included in Brachiosaurus). A more recent analysis focused on possible Asian brachiosaurid material found a clade including Abydosaurus, Brachiosaurus, Cedarosaurus, Giraffatitan, and Paluxysaurus, but not Qiaowanlong, the putative Asian brachiosaurid. Related genera include Lusotitan and Sauroposeidon. Brachiosauridae is situated at the base of Titanosauriformes, a group of sauropods that also includes the titanosaurs.
According to the revised diagnosis by Taylor, Brachiosaurus altithorax is diagnosed by a plethora of characters, many to be found on the dorsal (back) vertebrae. Among the characters placing it in the family Brachiosauridae are a ratio of humerus length to femur length of at least 0.9 (i.e. the upper arm bone is at least nearly as long as the thigh bone), and a very flattened femur shaft (ratio ≥1.85).