Brachiosaurus

***Brachiosaurus*** /ˌbrækiəˈsɔːrəs/ 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". *Brachiosaurus* had 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 *Brachiosaurus* and 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.

## Description

Like all sauropod dinosaurs, *Brachiosaurus* was a quadrupedal animal with a small skull, a long neck, a large trunk with a high-ellipsoid cross section, a long, muscular tail and slender, columnar limbs. The skull had a robust, wide muzzle and thick jaw bones, with spoon–shaped teeth. As in *Giraffatitan*, there was an arch of bone over the snout and in front of the eyes that encircled the nasal opening, although this arch was not as large as in its relative. Large air sacs connected to the lung system were present in the neck and trunk, invading the vertebrae and ribs, greatly reducing the overall density. Unusually for a sauropod, the forelimbs were longer than the hind limbs. The humerus (upper arm bone) of *Brachiosaurus* was relatively lightly built for its size, measuring 2.04 metres (6.7 ft) in length in the type specimen. The femur(thigh bone) of the type specimen was only 2.03 metres (6.7 ft) long. Unlike other sauropods, *Brachiosaurus* appears to have been slightly sprawled at the shoulder joint, and the ribcage was unusually deep. This led to the trunk being inclined, with the front much higher than the hips, and the neck exiting the trunk at a steep angle. Overall, this shape resembles a giraffe more than any other living animal.

### **Size**

Because "Brachiosaurus" *brancai* (*Giraffatitan*) is known from much more complete material than *B. altithorax*, most size estimates for *Brachiosaurus* are actually for the African form. There is an additional element of uncertainty for North American *Brachiosaurus* because the most complete skeleton appears to have come from a subadult. Over the years, the mass of *B. altithorax* has been estimated as 35.0 metric tons (38.6 short tons), 43.9 metric tons (48.4 short tons), 28.7 metric tons (31.6 short tons) and, most recently, 56.3 metric tons (62.1 short tons). In cases when the authors also provided estimates for *Giraffatitan*, and found that genus to be somewhat lighter (31.5 metric tons (34.7 short tons) for Paul [1988], 39.5 metric tons (43.5 short tons) for Mazzetta et al [2004], 23.3 metric tons (25.7 short tons) for Taylor [2009], and 34.0 metric tons (37.5 short tons) for Benson et al [2014]). The length of *Brachiosaurus* has been estimated at 26 metres (85 ft).

## Classification

*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).