Brachiosaurus

Brachiosaurus, meaning "arm lizard", due to its long forelimbs, 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 the general public 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 Janenschas 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 relative Giraffatitan 's 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 due to 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, herbivorous 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. altithoraxhas 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], 23.3 metric tons (25.7 short tons) for Taylor [2009], or 34 metric tons (37 short tons) for Benson et al 2010). The length of Brachiosaurus has been estimated at 26 metres (85 ft).

Separation of Giraffatitan
When describing the brachiosaurid material from Tendaguru in 1914, Janensch listed a number of differences and commonalities between them and B. altithorax. In three further publications in 1929, 1950  and 1961Janensch compared the two species in more detail, listing 13 putative shared characters.[5] Of these, however, only four appear to be valid, while six pertain to more inclusive groups than Brachiosauridae, and the rest are either difficult to assess or refer to material that is not Brachiosaurus.

In 1988, Gregory Paul published a new reconstruction of the skeleton of ''"B." brancai, highlighting a number of differences in proportion between it and B. altithorax''. Chief among them is a difference in the way the trunk vertebrae vary: they are fairly uniform in B. altithorax, but vary widely in the African material. Paul believed that the limb and girdle elements of both species were very similar, and therefore suggested to separate them not at genus, but only atsubgenus level.

Giraffatitan was raised to genus level by Olshevsky without comment. A detailed study of all material, including the limb and girdle bones, by Michael Taylor in 2009 found that there are significant differences between Brachiosaurus altithorax and the Tendaguru material in all elements known from both species. Taylor found 26 distinct osteological (bone-based) characters, a larger difference than that between, e.g., Diplodocus and Barosaurus, and therefore argued that the African material should be placed in its own genus, Giraffatitan, as G. brancai. An important difference between the two genera is the overall body shape, with Brachiosaurus having a 23% longer dorsal (trunk) vertebrate series and a 20 to 25% longer and also taller tail.

Neck position
In contrast to most other sauropods, brachiosaurids had an inclined back, due to their long forelimbs. Therefore, if the neck exited the body in a straight line, it already pointed upwards. The exact angle is influenced by how the pectoral girdle is reconstructed, that is how the shoulder blades are placed on the ribcage. The mobility of the neck was reconstructed as quite low by Stevens and Parrish, while other researchers like Paul and Christian and Dzemski argued for more flexible necks.

Feeding ecology
Brachiosaurus is thought to have been a high browser, feeding on foliage well above the ground. Even if it did not hold its neck near vertical, and instead had a straighter neck, its head height may still have been over 9 metres (30 ft) above the ground. It probably fed mostly on foliage above 5 metres (16 ft). This does not preclude the possibility that it also fed lower at times, between 3 to 5 metres (9.8 to 16.4 ft) up. Its diet likely consisted of ginkgos, conifers, tree ferns, and large cycads, with intake estimated at 200 to 400 kilograms (440 to 880 lb) of plant matter daily. However, more recent studies estimate that ~240 kilograms (530 lb) of plant matter would have been sufficient to feed a 70 metric tons (77 short tons) sauropod, so Brachiosaurus may have required only about 120 kilograms (260 lb) of fodder a day. Brachiosaur feeding involved simple up–and–down jaw motion. The teeth were arranged to shear material as they closed, and were probably used to crop and/or nip vegetation.

It has repeatedly been suggested, e.g. in the movie Jurassic Park, that Brachiosaurus could rear into a bipedal or tripodal (with tail support) pose to feed. However, a detailed physical modelling-based analysis of sauropod rearing capabilities by Heinrich Mallison showed that while many sauropods could rear, the unusual brachiosaurid body shape and limb length ratio made them exceptionally ill suited for rearing. The forward position of the center of mass would have led to problems with stability, and required unreasonably large forces in the hips to obtain an upright posture. Brachiosaurus would also have gained relatively little from rearing (only 33% more feeding height), compared to other sauropods, for which a bipedal pose may have tripled the feeding height.

In popular culture
Brachiosaurus is one of the best-known dinosaurs among both paleontologists and the general public. A main belt asteroid, 1991 GX7, has been named 9954 Brachiosaurus in honor of the genus. The genus has been featured in many films and television programs, most notably the Jurassic Park and Walking with Dinosaurs series (however, in Jurassic Park, the "Brachiosaurus" was in fact a Giraffatitan).