Brachiosaurus

Brachiosaurus, meaning "arm lizard", is a genus of sauropod dinosaur that lived during the Late Jurassic Period and possibly the Early Cretaceous Period. It was named thus because its forelimbs were longer than its hind limbs. One of the largest animals known to have walked the earth, it has become one of the most famous of all dinosaurs and is recognized worldwide.

Size
For many decades, Brachiosaurus was the largest dinosaur known. It has since been discovered that a number of giant titanosaurians (Argentinosaurus, for example) surpassed Brachiosaurus in terms of sheer mass. More recently, another brachiosaurid, Sauroposeidon, has also been discovered; based on incomplete fossil evidence, it too is likely to have outweighed Brachiosaurus.

Brachiosaurus is the largest dinosaur known from a relatively complete fossilized skeleton. The most complete specimens, including the Brachiosaurus in the Humboldt Museum of Berlin (excavated in Africa)—the tallest mounted skeleton in the world—are members of the species B. brancai. The holotype material of the type species, B. altithorax. includes a sequence of seven posterior dorsal vertebrae, sacrum, proximal caudal vertebra, coracoid, humerus, femur and ribs: enough from which to estimate size.

Based on a complete composite skeleton, Brachiosaurus attained 25 meters (82 ft) in length and was probably able to raise its head about 13 meters (43 ft) above ground level. Fragmentary material from larger specimens indicates that it could grow 15% longer than this. Such material includes an isolated fibula HMN XV2 1.34 meters (4.40 ft) in length and the brachiosaurid scapulocoracoid referred to Ultrasauros.

Historically, Brachiosaurus has been estimated to have weighed as little as 15 tonnes (17 short tons) (Russell et al., 1980) and as much as 78 tonnes (86 short tons). However these extreme estimates are now considered unlikely; that of Russell et al.. was based on limb-bone allometry rather than a body model, and Colbert's was based on an outdated and overweight model. More recent estimates based on models reconstructed from osteology and inferred musculature are in the range 32 tonnes (35 short tons) to 37 tonnes (41 short tons) (Christiansen 1997). The 15% longer specimens hinted at above would have massed 48 to 56 tonnes (53 to 62 short tons).

Discovery and species
The first Brachiosaurus was discovered in 1900 by Elmer S. Riggs, in the Grand River Canyon of western Colorado, in the United States.

Brachiosaurus species
Brachiosaurus includes at least two known species, and possibly a third:
 * B. altithorax Riggs, 1903: The type species is known from two partial skeletons recovered in Colorado and Utah in the United States. It lived from 145 to 150 million years ago, during the Kimmeridgian to Tithonian ages.
 * ?B. nougaredi de Lapparent, 1960: While it may not be a distinct species (nomen dubium?) it is known from set of fused bones over the hip (sacrum) and parts of a forelimb, which were recovered in Wargla, Algeria in Africa. It lived 112 million years ago, during the early Albian age of the early Cretaceous period.
 * B. brancai Janensch, 1914 is known from five partial skeletons, including at least three skulls and some limb bones, which were recovered near Lindi, Tanzania, in Africa in the early 1900s. It lived from 145 to 150 million years ago, during the Kimmeridgian to Tithonian ages of the Late Jurassic period.

Giraffatitan
In 1988, Gregory S. Paul noted that the African form (on which most popular depictions of Brachiosaurus are based) showed significant differences from the North American form (B. altithorax), especially in the proportions of its trunk vertebrae and in its more gracile build. Paul used these differences to create a subgenus he named Brachiosaurus (Giraffatitan) brancai. In 1991, George Olshevsky asserted that these differences are enough to place the African brachiosaurid in its own genus, simply Giraffatitan. This renaming has not achieved widespread acceptance. B. brancai has withers over its shoulder and a rounded crest over its nostrils.

Possibly adding further differences between the two species was the description in 1998 of a North American brachiosaurid skull (Carpenter & Tidwell, 1998). This skull, which had been found nearly a century earlier (it is the skull Marsh used on his early reconstructions of Brontosaurus), is identified as "Brachiosaurus sp." and may well belong to B. altithorax. The skull is more camarasaur-like than the distinctive high-crested skull of B. brancai.

Removed species
B. alataiensis, described by de Lapparent and Zbyszewski in 1957 has been referred to the new genus Lusotitan (Antunes and Mateus 2003). It is known from back bones (vertebrae), and parts of the hip and limbs, which were recovered in Estremadura, Portugal. It lived about 150 million years ago, during the Kimmeridgian age of the Late Jurassic period.

Description and environment
Brachiosaurus was a sauropod, one of a group of four-legged, plant-eating dinosaurs with long necks and tails and relatively small brains. Unlike other families of sauropods, it had a giraffe-like build, with long forelimbs and a very long neck. Brachiosaurus had spatulate teeth (resembling chisels), well-suited to its herbivorous diet. Its skull featured a number of holes, probably aiding weight-reduction. The first toe on its front foot and the first three toes on its hind feet were clawed.

Skull
Brachiosaurus has traditionally been characterised by its distinctive high-crested skull, though this may have been unique to B. brancai. Another complete Brachiosaurus skull is known, which Marsh used for his early reconstructions of Brontosaurus. Carpenter and Tidwell studied this skull in 1998 and found that it belonged to one of the North American Brachiosaurus species. The skull of this North American Brachiosaurus is more camarasaur-like than the distinctive high-crested skull of B. brancai.

Metabolism
If the Brachiosaurus was endothermic (warm-blooded), it would have taken an estimated ten years to reach full size, if it were instead poikilothermic (cold-blooded), then it would have required over 100 years to reach full size. As a warm-blooded animal, the daily energy demands of Brachiosaurus would have been enormous; it would probably have needed to eat more than ~182 kg (400 lb) of food per day. If Brachiosaurus was fully cold-blooded or was a passive bulk endotherm, it would have needed far less food to meet its daily energy needs. Some scientists have proposed that large dinosaurs like Brachiosaurus were gigantotherms.

Environment and behavior
Brachiosaurus was one of the largest dinosaurs of the Jurassic era; it lived on prairies filled with ferns, bennettites and horsetails, and it moved through vast conifer forests and groves of cycads, seed ferns and ginkgos. Its contemporary genera included Stegosaurus, Dryosaurus, Apatosaurus and Diplodocus. While it is speculated that groups of Brachiosaurus moved in herds, fully grown individuals had little to fear from even the largest predators of the time, Allosaurus and Torvosaurus, on account of their sheer size.

Brachiosaurus nostrils, like the huge corresponding nasal openings in its skull, were long thought to be located on the top of the head. In past decades, scientists theorized that the animal used its nostrils like a snorkel, spending most of its time submerged in water in order to support its great mass. The current consensus view, however, is that Brachiosaurus was a fully terrestrial animal. Studies have demonstrated that water pressure would have prevented the animal from breathing effectively while submerged and that its feet were too narrow for efficient aquatic use. Furthermore, new studies by Larry Witmer (2001) show that, while the nasal openings in the skull were placed high above the eyes, the nostrils would still have been close to the tip of the snout (a study which also lends support to the idea that the tall "crests" of brachiosaurs supported some sort of fleshy resonating chamber).