Brachiosaurus

Not to be confused with Branchiosaurus.

For the asteroid, see 9954 Brachiosaurus.

Brachiosaurus Temporal range: Late Jurassic,
Reconstructed replica of the holotype skeleton outside the Field Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	†Sauropodomorpha
Clade:	†Sauropoda
Clade:	†Macronaria
Family:	†Brachiosauridae
Genus:	†Brachiosaurus Riggs, 1903
Species:	†Brachiosaurus
Binomial name
†Brachiosaurus Riggs, 1903

Brachiosaurus /ˌbrækiəˈsɔːrəs/ is a genus of sauropod dinosaur that lived in North America during the Late Jurassic, about 154–153 million years ago. It was first described by American paleontologist Elmer S. Riggs in 1903 from fossils found in the Colorado River valley in western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax; the generic name is Greek for "arm lizard", in reference to its proportionately long arms, and the specific name means "deep chest". Brachiosaurus is estimated to have weighed about 56.3 metric tons (62.1 short tons) and to have been 26 meters (85 ft) long. It 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 proportionally shorter than in most other sauropods.

Brachiosaurus is the namesake genus of the family Brachiosauridae, which includes a handful of other similar sauropods. Most popular depictions of Brachiosaurus are in fact based on Giraffatitan, a genus of brachiosaurid dinosaur from the Tendaguru Formation of Tanzania that was originally described by German paleontologist Werner Janensch in 1914 as a species of Brachiosaurus, B. brancai, but moved to its own genus in 2009. Several other potential species of Brachiosaurus have been described from Africa and Europe, but none of them are currently thought to be vaild.

Brachiosaurus is one of the rarer sauropods of the Morrison Formation. The type specimen of B. altithorax that was originally described by Riggs in 1903 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 possibly as high as 9 meters (30 ft) off the ground. Unlike other sauropods, 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 sufficient forage, but more recent research finds it to have been warm-blooded. Among the most iconic dinosaurs, Brachiosaurus has appeared in popular culture, notably in the 1993 film Jurassic Park.

Description

Size

Size compared to a human

Most size estimates that have been made for Brachiosaurus are actually based on the related African brachiosaurid Giraffatitan (formerly known as Brachiosaurus brancai), since it is known from much more complete material than Brachiosaurus. There is an additional element of uncertainty for the North American Brachiosaurus because the type (and most complete) specimen appears to represent a subadult, as indicated by the unfused suture between the coracoid, a bone of the shoulder girdle that forms part of the shoulder joint, and the scapula (shoulder blade). Over the years, the mass of B. altithorax has been estimated at 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) and even 58 metric tons (64 short tons). The length of Brachiosaurus has been estimated at 26 meters (85 ft).

While the limb bones of the most complete Giraffatitan skeleton (MB.R.2181) were very similar in size to those of the Brachiosaurus type specimen, the former specimen was found to be somewhat lighter than the Brachiosaurus specimen given its proportional differences. In studies including estimates for both Brachiosaurus and Giraffatitan, the latter was estimated at 31.5 metric tons (34.7 short tons) by Gregory S. Paul in 1988, 39.5 metric tons (43.5 short tons) by Gerardo Mazzetta and colleagues in 2004, 23.3 metric tons (25.7 short tons) by Michael P. Taylor in 2009, and 34.0 metric tons (37.5 short tons) by Roger Benson and colleagues in 2014. As is the case with the Brachiosaurus type specimen, Giraffatitan specimen MB.R.2181 does likely not reflect the maximum size of the genus, as a fibula (specimen HM XV2) is 13% longer than that of MB.R.2181.

General build

Life restoration.

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. 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. While the neck is not preserved in the holotype specimen, it was very long even for sauropod standards in the closely related Giraffatitan, consisting of thirteen elongated cervical (neck) vertebrae. The neck would have been held in a slight S-curve, with bended lower and upper sections and a straight middle section. Brachiosaurus likely shared the very elongated neck ribs with Giraffatitan, which run down the underside of the neck, overlapping several preceding vertebrae. These bony rods were attached to neck muscles at their ends, allowing these muscles to operate distal portions of the neck while themselves being located closer to the body, thus lightening the neck. Brachiosaurus and Giraffatitan probably possessed a small shoulder hump between the third and fifth dorsal (back) vertebra, where the sidewards- and upwards-directed vertebral processes were longer, providing additional surface for neck muscle attachment. The ribcage was unusually deep. Although the humerus (upper arm bone) and femur (thigh bone) were roughly equal in length, the entire forelimb would have been longer than the hindlimb, as can be inferred from the elongated forearm and metacarpus known from other brachiosaurids. This lead 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. In contrast, most other sauropods show a shorter forelimb than hindlimb; the forelimb is especially short in diplodocoids.

Brachiosaurus differed in its overall body proportions from the closely related Giraffatitan. The trunk was about 25–30% longer than in the latter genus given its more elongated dorsal vertebrae, resulting in a dorsal vertebral column longer than the humerus. Only a single complete caudal (tail) vertebra has been discovered, but its great height indicates that the tail was taller than in Giraffatitan. Furthermore, this vertebra showed a much greater area for ligament attachment due to the broadened neural spine, indicating that the tail was also longer than in Giraffatitan, possibly by 20–25%. Although Paul, in 1988, suggested that the neck was shorter in Brachiosaurus than in Giraffatitan, two cervical vertebrae likely belonging to Brachiosaurus suggest identical proportions. Unlike Giraffatitan and other sauropods, which showed vertically oriented forelimbs, the arm of Brachiosaurus appears to have been slightly sprawled at the shoulder joint, as indicated by the sidewards orientation of the joint surface of the coracoid. The humerus was less slender than that of Giraffatitan, while the femur showed similar proportions. This might indicate that the forelimbs of Brachiosaurus supported a greater fraction of the body weight than is the case for Giraffatitan.

Postcranial skeleton

Vertebral anatomy of the holotype skeleton. Top: Sixth dorsal vertebra in back (A) and right side view (B). Bottom: Second caudal vertebra in back (C) and side view (D).

Although the vertebral column of the trunk or torso is incompletely known, the back of Brachiosaurus most likely possessed twelve dorsal vertebrae, as can be inferred from the complete dorsal vertebral column preserved by an unnamed brachiosaurid specimen, BMNH R5937. Vertebrae of the front part of the dorsal column were slightly taller but much longer than those of the back part. This is in contrast to Giraffatitan, where the vertebrae at the front part were much taller but only slightly longer. The centra (vertebral bodies), which form the lower part of the vertebrae, were more elongated and roughly circular in cross-section, while those of Giraffatitan were broader than tall in cross-section. The foramina (small openings) on the sides of the centra, which allowed for the intrusion of air sacs, were larger than in Giraffatitan. The diapophyses (large projections extending sideways from the neural arch of the vertebrae) were horizontal, while those of Giraffatitan were inclined upwards. At their ends, these projections articulated with the ribs; the articular surface was not distinctly triangular as in Giraffatitan. The upwards projecting neural spines, when seen in side view, stood vertically and were twice as wide at the base than at the top, while those of Giraffatitan were tilted backwards and were not broadened at their base. When seen in front or back view, the neural spines widened towards their tops. In Brachiosaurus, this widening occurred gradually, resulting in a paddle-like shape, while in Giraffatitan the widening occurred abruptly and only in the uppermost portion. At both their front and the back side, the neural spine showed large, triangular and rugose surfaces, which were semicircular and much smaller in Giraffatitan. The various vertebral processes were connected by thin sheets or ridges of bone, which are called laminae. Brachiosaurus lacked postspinal laminae, which were present in Giraffatitan, running down the back side of the neural spines. The spinodiapophyseal laminae, which stretched from the neural spines to the diapophyses, were conflated with the spinopostzygapophyseal laminae, which stretched between the neural spines and the articular process at the back of the vertebra, and therefore terminated at mid-height of the neural spine. In Giraffatitan, both laminae were not conflated, and the spinodiapophyseal laminae reached up to the top of the neural spine. Brachiosaurus is further distinguished from Giraffatitan in lacking three details of the laminae of the back vertebrae that are unique to the latter genus.

Anatomy of the sacrum, ilium, and coracoid. Top: Sacrum in bottom (A) and right side view (B). Bottom: Right ilium in side view (C) and left coracoid in side view (D).

Air sacs did not only invade the vertebrae, but also the ribs. In Brachiosaurus, the air sacs invaded through a small opening on the front side of the rib shafts, while in Giraffatitan openings were present on both the front and back sides of the tuberculum, a bony projection articulating with the diapophyses of the vertebrae. Paul, in 1988, stated that the ribs of Brachiosaurus were longer than in Giraffatitan, which was, however, questioned by Taylor in 2009. Behind the dorsal vertebral column, the sacrum consisted of five co-ossified sacral vertebrae. As in Giraffatitan, the sacrum was proportionally broad and featured very short neural spines. Poor preservation of the sacral material in Giraffatitan however precludes detailed comparisons between both genera. Of the tail, only the second caudal vertebra is well preserved. As in Giraffatitan, this vertebra was slightly amphicoelous (concave on both ends), lacked openings on the sides, and possessed a short neural spine that was rectangular and tilted backwards. In contrast to the second caudal vertebra of Giraffatitan, that of Brachiosaurus showed a proportionally taller neural arch, making the vertebra around 30% taller. The centrum showed no depressions at its sides, in contrast to Giraffatitan. In front or back view, the neural spine broadened towards its tip to approximately three times its minimum width. In Giraffatitan, in contrast, no widening is apparent. The neural spines were also inclined backwards by about 30°, more than in Giraffatitan (20°). The caudal ribs projected laterally and were not tilted backwards as in Giraffatitan. The articular facets of the articular processes at the back of the vertebra were directed more downwards, while those of Giraffatitan faced more towards the sides. Besides the articular processes, the hyposphene-hypantrum articulation formed an additional articulation between vertebrae, making the vertebral column more rigid; in Brachiosaurus, the hyposphene was much more pronounced than in Giraffatitan.

Femur (left) and humerus of the holotype

The coracoid was semicircular and taller than broad. Differences to Giraffatitan are related to its shape in side view, including the straighter suture between the coracoid and the scapula. Moreover, the articular surface forming the shoulder joint was thicker and directed more sidewards than in Giraffatitan and other sauropods, possibly indicating a more sprawled forelimb. The humerus, as preserved, measures 204 centimeters (80 in) in total length, although part of its lower end are lost to erosion; its original length is estimated to have been 216 centimeters (85 in). This bone was more slender in Brachiosaurus than in most other sauropods, measuring only 28.5 centimeters (11.2 in) in width at its narrowest part. It was, however, more robust than that of Giraffatitan, being around 10% broader at the upper and lower ends. At its upper end, it featured a low bulge visible in side view, which is absent in Giraffatitan. Distinguishing features can also be found in the ilium of the pelvis. In Brachiosaurus, the ischiadic peduncle, a downwards projecting extension connecting to the ischium, reaches farther downwards than in Giraffatitan. While the latter genus shows a sharp notch between the ischiadic peduncle and the back portion of the ilium, this notch is more rounded in Brachiosaurus. On the upper surface of the hind part of the bone, Brachiosaurus showed a pronounced tubercle that is absent in other sauropods. Of the hind limb, the femur was very similar to that of Giraffatitan. As in the latter genus, it was strongly elliptical in cross-section, being more than twice as wide in front or back view than in side view. The femur of Brachiosaurus, however, 203 centimeters (80 in) long, was slightly more robust than that of Giraffatitan. Further differences include the more prominent and further downwards located fourth trochanter, a bulge on the femur serving as the anchor point for the most important locomotory muscle, the caudofemoralis connecting the hindlimb to the tail. Furthermore, the condyles at the lower end were not extending backwards as strongly as in Giraffatitan; both condyles were similar in width in Brachiosaurus but unequal in Giraffatitan.

Skull

Reconstruction of the Felch Quarry Brachiosaurus sp. skull, Denver Museum of Nature & Science. The fleshy external nostril would have been placed at the front of the nasal fossa, here demarcated by a dashed line

Though no skull remains were discovered with the original Brachiosaurus skeleton, one partial skull from a different location, referred to as the Felch Quarry skull (specimen USNM 5730), may belong to Brachiosaurus. Since there is no overlapping material between the two specimens, the skull has only been assigned to Brachiosaurus sp. (of uncertain species). As reconstructed, the skull was about 81 centimeters (2.66 ft) long from the occipital condyle at the back of the skull to the front of the premaxillae (the front bones of the upper jaw), making it the largest sauropod skull known from the Morrison Formation. It appears to have been most similar to and intermediate between that of Giraffatitan and Camarasaurus. Overall, the skull was tall as in Giraffatitan, with a snout that was long (about 36% of the skull length) in front of the nasal bar between the nostrils, which is typical of brachiosaurids. The snout was set at an angle relative to the rest of the skull, which gave the impression that the snout pointed downwards. The frontal bones on top of the skull were short and wide (similar to Giraffatitan), fused together, and connected by a suture to the parietal bones, which were also fused together. The surface of the parietals between the supratemporal fenestrae (openings at the rear skull roof) was wider than that of Giraffatitan, but narrower than that of Camarasaurus. The skull differed from that of Giraffatitan in having a U-shaped (instead of W-shaped) suture between the frontal and nasal bones, enhanced by the frontal bones extending forwards over the orbits (eye sockets).

Similar to Giraffatitan, the neck of the occipital condyle was very long. The premaxilla appears to have been longer than that of Camarasaurus, sloping more gradually towards the nasal bar, which created the very long snout. Brachiosaurus had a long and deep maxilla (the main bone of the upper jaw), which was thick along the margin where the alveoli (tooth sockets) were placed, thinning upwards. The interdental plates of the maxilla were thin, fused, porous, and triangular. There were triangular nutrient foramina between the plates, each containing the tip of an erupting tooth. The narial fossa (depression) in front of the bony nostril was long and contained a subnarial fenestra, which was much larger than those of Giraffatitan and Camarasaurus. The dentaries (the bones of the lower jaws that contained the teeth) were robust, though less than in Camarasaurus. The upper margin of the dentary was arched in profile, but not as much as in Camarasaurus. The interdental plates of the dentary were somewhat oval, with diamond shaped openings between them. The dentary had a Meckelian groove that was open until below the ninth alveolus, continuing thereafter as a shallow through.

Each maxilla had space for about 14 or 15 teeth, whereas Giraffatitan had 11 and Camarasaurus 8 to 10. The maxillae contained replacement teeth which showed rugose enamel, similar to Camarasaurus, but lacked the small denticles (serrations) along the edges. Since the maxilla was wider than that of Camarasaurus, Brachiosaurus would have possessed larger teeth. The replacement teeth in the premaxilla had crinkled enamel, and the most complete of these teeth did not have denticles. Each dentary had space for about 14 teeth. The only well preserved tooth of this skull is large, spoon-shaped, and may be from the front part of the left dentary. It differs from those of Giraffatitan in that the crown is much wider than the root, similar to Camarasaurus. That the tooth is not worn implies that it had erupted around the time the animal died. The outer and inner sides of the tooth were crenelated (had indented vertical grooves); the crenelations of one side met with those of the other side at the top of the tooth, where they formed denticles. The maxillary tooth rows of Brachiosaurus and Giraffatitan ended well in front of the antorbital fenestra (the opening in front of the orbit), whereas they ended just before and below the fenestra in Camarasaurus and Shunosaurus.

The bony nasal openings of neosauropods like Brachiosaurus were large and placed on the top of their skulls. Traditionally, the fleshy nostrils of sauropods were thought to have been placed likewise on top of the head, roughly at the rear of the bony nostril opening, because these animals were inaccurately thought to have been amphibious, using their large nasal openings as snorkels when submerged. The American paleontologist Lawrence M. Witmer rejected this reconstruction in 2001, pointing out that all living vertebrate land animals have their external fleshy nostrils placed at the front of the bony nostril. The fleshy nostrils of such sauropods would have been placed in an even more forward position, at the front of the narial fossa, the depression which extended far in front of the bony nostril towards the snout tip.

Air sacs

The respiration system of sauropods, like that of birds, made use of air sacs. There was not a bidirectional airflow as with mammals, in which the lungs function as bellows, first inhaling and then exhaling air. Instead the air was from the trachea sucked into an abdominal air sac in the belly which then pumped it forwards through the parabranchi, air loops, of the stiff lung. Valves prevented the air from flowing backwards when the abdominal air sac filled itself again; at the meantime a cervical air sac at the neck base sucked out the spent air from the lung. Both air sacs contracted simultaneously to pump the used air out of the trachea. This procedure guaranteed an unidirectional airflow, the air always moving in a single forward direction in the lung itself. This significantly improved the oxygen intake and the release of carbon dioxide. Not only was dead air removed quickly but also the blood flow in the lung was counterdirectional in relation to the airflow, leading to a far more effective gas exchange.

In sauropods, the air sacs did not simply function as an aid for respiration. By means of air channels they were connected to much of the skeleton. These branches, the diverticula, via pneumatic openings invaded many bones and strongly hollowed them out. It is not entirely clear what the evolutionary benefit of this phenomenon was but in any case it considerably lightened the skeleton. They might also have removed excess heat to aid thermoregulation.

In 2016, Mark Hallett and Mathew Wedel for the first time reconstructed the entire air sac system of a sauropod, using Brachiosaurus altithorax as an example of how such a structure might have been formed. In their reconstruction a large abdominal air sac was located between the pelvis and the outer lung side. As with birds, three smaller sacs assisted the pumping process from the underside of the breast cavity: at the rear the posterior thoracic air sac, in the middle the anterior thoracic air sac and in front the clavicular air sac, in that order gradually diminishing in size. The cervical air sac was positioned under the shoulder blade, on top of the front lung. The air sacs were via tubes connected with the vertebrae. Diverticula filled the various fossae and pleurocoels that formed depressions in the vertebral bone walls. These were again connected with inflexible air cells inside the bones.

History of discovery

Holotype specimen

Holotype material during excavation

The genus Brachiosaurus, and its type species B. altithorax, are based on a partial postcranial skeleton from Fruita, in the valley of the Colorado River of western Colorado. This holotype specimen was collected from rocks of the Brushy Basin Member of the Morrison Formation (late Kimmeridgian) in 1900 by American paleontologist Elmer S. Riggs and his crew from the Field Columbian Museum (now the Field Museum of Natural History) of Chicago. It is currently cataloged as FMNH P 25107.

Riggs and company were working in the area as a result of favorable correspondence between Riggs and Stanton Merill Bradbury, a dentist in nearby Grand Junction. In the spring of 1899, Riggs had sent letters to mayors in western Colorado, inquiring after possible trails leading from railway heads into northeastern Utah, where he hoped to find fossils of Eocene mammals. To his surprise, he was informed by Bradbury, an amateur collector himself and president of the Western Colorado Academy of Science, that dinosaur bones had been collected near Grand Junction since 1885. Riggs was sceptical of this claim, but his superior, curator of geology Oliver Cummings Farrington, was very eager to add a large sauropod skeleton to the collection, to outdo other institutions, and convinced the museum management to invest five hundred dollars in an expedition. Arriving on 20 June, they set camp at the abandoned Goat Ranch. During a prospecting trip on horse-back, Riggs' field assistant Harold William Menke found the humerus of FMNH P 25107, on 4 July 1900, exclaiming it was "the biggest thing yet!". Riggs at first took the find for a badly preserved Brontosaurus specimen and gave priority to excavating Quarry 12, which held a more promising Morosaurus skeleton. Having secured that, on 26 July he returned to the humerus in Quarry 13, which soon proved to be of enormous size, convincing a puzzled Riggs that he had discovered the largest land animal ever.

Elmer S. Riggs’ preparator, H. W. Menke, lying by the humerus during the excavation in 1900

The site, Riggs Quarry 13, was found on a small hill later known as Riggs Hill; it is today marked by a plaque. Additional Brachiosaurus fossils are reported on Riggs Hill, but other fossil finds on the hill have been vandalized. During excavation of the specimen, Riggs misidentified the humerus as a deformed femur due to its great length, and found himself confirmed when an equally sized, well-preserved femur of the same skeleton was discovered. In 1904, Riggs noted: "Had it not been for the unusual size of the ribs found associated with it, the specimen would have been discarded as an Apatosaur, too poorly preserved to be of value." It was only after preparation of the fossil material in the laboratory that the bone was recognized as a humerus. The excavation attracted large numbers of visitors, delaying the work and forcing Menke to guard the site to prevent bones from being looted. On 17 August, the last bone was jacketed in plaster. After a concluding ten-day prospecting trip, the expedition returned to Grand Junction and hired a team and wagon to transport all fossils to the railway station, during five days; another week was spent to pack them in thirty-eight crates with a weight of 12,500 pounds (5,700 kg). On 10 September, Riggs left for Chicago by train, arriving on the 15th; the railroad companies let both passengers and cargo travel for free, as a public relations gesture.

The holotype skeleton 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. At the time of discovery, the lower end of the humerus, the underside of the sacrum, the ilium and the preserved caudal vertebrae were exposed to the air and thus partly damaged by weathering. The vertebrae were only slightly shifted out of their original anatomical position; they were found with their top sides directed downwards. The ribs, humerus, and coracoid, however, were displaced to the left side of the vertebral column, indicating transportation by a water current. This is further evidenced by an isolated ilium of Diplodocus that apparently had drifted against the vertebral column, as well as by a change in composition of the surrounding rocks. While the specimen itself was embedded in fine-grained clay, indicating low-energy conditions at the time of deposition, it was cut off at the seventh vertebra by a thick layer of much coarser sediments consisting of pebbles at its base and sandstone further up, indicating deposition under stronger currents. Based on this evidence, Riggs in 1904 suggested that the missing front part of the skeleton was washed away by a water current, while the hind part was already covered by sediment and thus got preserved.

Riggs, on the right, and J. B. Abbott working on the holotype bones; the still jacketed thighbone can be seen on the left

Riggs published a short report of the new find in 1901, noting the unusual length of the humerus compared to the femur and the extreme overall size and the resulting giraffe-like proportions, as well as the lesser development of the tail, but did not publish a name for the new dinosaur. The titles of Riggs' 1901 and 1903 articles emphasized that the specimen was the "largest known dinosaur". Riggs derived the genus name from the Greek brachion/βραχίων meaning "arm" and sauros/σαυρος meaning "lizard", because he realized that the length of the arms was unusual for a sauropod. The specific epithet was chosen because of the unusually deep and wide chest cavity, from Latin altus "deep" and Greek thorax/θώραξ, "breastplate, cuirass, corslet". Latin thorax was derived from the Greek and had become a usual scientific designation for the chest of the body. Riggs followed his 1903 publication that named Brachiosaurus altithorax with a more detailed description in a monograph in 1904.

Preparation of the holotype began in the fall of 1900 shortly after it was collected by Riggs for the Field Museum. First the limb elements were processed. In the winter of 1904, the badly weathered vertebrae of the back and hip were prepared by James B. Abbott and C.T. Kline. As the preparation of each bone was finished, it was put on display in a glass case in Hall 35 of the Fine Arts Palace of the Worlds Columbian Exposition, the Field Museum's first location. All the bones were, solitarily, still on display by 1908 in Hall 35 when the Field Museum's newly mounted Apatosaurus was unveiled, the very specimen Riggs had found in Quarry 12, today catalogued as FMNH P25112 and identified as a Brontosaurus exemplar. However, no mount of Brachiosaurus was attempted because only 20% of the skeleton had been recovered. In 1993, the holotype bones were molded and cast, and the missing bones were sculpted based on Giraffatitan material in Berlin. This plastic skeleton was mounted and, in 1994, put on display at the north end of Stanley Field Hall, the main exhibit hall of the Field Museum's current building. The real bones of the holotype were put on exhibit in two large glass cases at either end of the mounted cast. The mount stood until 1999, when it was moved to the B Concourse of United Airlines' Terminal One in O'Hare International Airport to make room for the museum's newly acquired Tyrannosaurus skeleton, "Sue". At the same time, the Field Museum mounted a second plastic cast of the skeleton (designed for outside use) and it has been on display outside the museum on the NW terrace ever since.

In 1969, in a study by R.F. Kingham, Brachiosaurus altithorax, "B." brancai and "B." atalaiensis, along with many species now assigned to other genera, were placed in the genus Astrodon, creating an Astrodon altithorax. Kingham's views of brachiosaurid taxonomy have, however, not been accepted by many other authors.

Assigned material

O. C. Marsh's outdated 1891 skeletal reconstruction of Brontosaurus, with skull inaccurately based on that of the Felch Quarry B. sp.

In 1883, farmer Marshall Parker Felch, a fossil collector for the American paleontologist Othniel Charles Marsh, reported the discovery of a sauropod skull in Felch Quarry 1, near Garden Park, Colorado. The skull was found in yellowish white sandstone, near a 1 meter (3.3 ft) long cervical vertebra, which was destroyed during an attempt to collect it. The skull was catalogued as YPM 1986, and sent to Marsh at the Peabody Museum of Natural History, who incorporated it into his 1891 skeletal restoration of Brontosaurus (perhaps because Felch had identified it as belonging to that dinosaur). The Felch Quarry skull consists of the cranium, the maxillae, the right postorbital, part of the left maxilla, the left squamosal, the right quadrate, the dentaries, a possible partial pterygoid, and a front tooth from the dentary. The bones were roughly prepared for Marsh, which lead to some damage. Most of the specimens collected by Felch were sent to the National Museum of Natural History in 1899 after Marsh's death, including the skull, which was then catalogued as USNM 5730.

In 1975, the American paleontologists Jack McIntosh and David Berman investigated the historical issue of whether Marsh had assigned an incorrect skull to Brontosaurus (at the time thought to be a junior synonym of Apatosaurus), and found the Felch Quarry skull to be of "the general Camarasaurus type", while suggesting that the vertebra found near it belonged to Brachiosaurus. They concluded that if Marsh had not arbitrarily assigned the Felch quarry skull and another Camarasaurus-like skull to Brontosaurus, it would have been recognized earlier that the actual skull of Brontosaurus and Apatosaurus was more similar to that of Diplodocus. McIntosh later tentatively recognized the Felch Quarry skull as belonging to Brachiosaurus, and brought it to the attention of the American paleontologists Kenneth Carpenter and Virginia Tidwell, while urging them to describe it. They brought the skull to the Denver Museum of Natural History, where they further prepared it and made a reconstruction of it based on casts of the individual bones, with the skulls of Giraffatitan and Camarasaurus acting as templates for the missing bones. In 1998, Carpenter and Tidwell described the Felch Quarry skull, and formally assigned it to B. sp., since it is impossible to determine whether it belonged to the species B. altithorax itself. They based the skull's assignment to Brachiosaurus on its similarity to that of B. brancai, later known as Giraffatitan.

Scapulocoracoid BYU 9462 now assigned to Brachiosaurus, which was originally assigned to Ultrasauros (now a junior synonym of Supersaurus), Museum of Ancient Life

Additional discoveries of Brachiosaurus material in North America have been uncommon and consist of a handful of bones. Material has been described from Colorado, Oklahoma, Utah, and Wyoming, and undescribed material has been mentioned from several other sites. One of these specimens, a shoulder blade from Dry Mesa Quarry, Colorado, is one of the specimens at the center of the Supersaurus/Ultrasauros issue of the 1980s and 1990s. In 1985, James A. Jensen described disarticulated sauropod remains from the quarry as belonging to several exceptionally large taxa, including the new genera Supersaurus and Ultrasaurus, the latter renamed Ultrasauros shortly thereafter because another sauropod already received the name. Later study showed that the "ultrasaur" material mostly belonged to Supersaurus, although the shoulder blade did not. Because the holotype of Ultrasauros, a back vertebra, was one of the specimens that was actually from Supersaurus, the name Ultrasauros is a synonym of Supersaurus. The shoulder blade, specimen BYU 9462 (previously BYU 5001), is now assigned to Brachiosaurus, but the species is uncertain. In addition, the Dry Mesa "ultrasaur" was not as large as had been thought; the dimensions of the shoulder's coracoid bone indicate that the animal was smaller than Riggs' original specimen of Brachiosaurus.

Referred forelimb bone (humerus) from Potter Creek, USNM 21903

Taylor in his 2009 study listed a number of specimens that had been referred to Brachiosaurus. These include some Dry Mesa material (the latter partly described as Ultrasauros by Jensen), that clearly is not brachiosaurid in origin. From the Potter Creek a humerus (specimen USNM 21903) was described that is not clearly referable to Brachiosaurus. From the same site the disarticulated specimen BYU 4744 has been reported, containing a mid-dorsal vertebra, an incomplete left ilium, a left radius and a right metacarpal, that according to Taylor can confidently be referred to B. altithorax, as far as it is overlapping with its type specimen. Additionally, a cervical vertebra and the skull mentioned above may belong to either B. altithorax or an as-yet unknown brachiosaurid from North America. The cervical was found near Jensen, Utah, by Jensen, and – if it belongs to Brachiosaurus – is one of a handful of neck vertebrae known for American brachiosaurids. Because of a lack of articulated skeletons overlapping with the holotype, there is no unambiguous material of the skull, neck, anterior dorsal region, or distal limbs or feet. In 2012, José Carballido and colleagues reported on a nearly complete postcranial skeleton of a juvenile sauropod (approximately 2 meters (6.6 ft) long) from the Morrison Formation of the Bighorn Basin, north-central Wyoming. This specimen, SMA 0009 nicknamed "Toni", was originally thought to belong to a diplodocid, but the authors reinterpreted it as representing a brachiosaurid, probably Brachiosaurus altithorax. In 2018, the largest sauropod foot ever found was reported from the Morrison Formation in Wyoming, and though possibly belonging to Brachiosaurus (the femur of the specimen would have been about 2% longer than that of the B. altithorax holotype), the authors cautiously classify it as an indeterminate brachiosaurid.

Formerly assigned species

B. brancai and B. fraasi

Skeleton of Giraffatitan, formerly B. brancai, Natural History Museum, Berlin

Between 1909 and 1912, large-scale paleontological expeditions in German East Africa unearthed a considerable amount of brachiosaurid material from the Tendaguru Formation. In 1914, German paleontologist Werner Janensch listed a number of differences and commonalities between these fossils and B. altithorax, concluding they could be referred to the genus Brachiosaurus. From this material Janensch named two species: Brachiosaurus brancai for the larger and more complete taxon, and Brachiosaurus fraasi for the smaller and more poorly known species. In three further publications in 1929, 1950 and 1961, Janensch compared the species in more detail, listing thirteen putative shared characters between Brachiosaurus brancai (which he now considered to include B. fraasi) and Brachiosaurus altithorax. Of these, however, only four appear to be valid, while six pertain to more inclusive groups than the Brachiosauridae, and the rest are either difficult to assess or refer to material that is not Brachiosaurus.

There was ample material referred to B. brancai in the collections of the Museum für Naturkunde Berlin, some of which was destroyed during World War II. Other material was transferred to other institutions throughout Germany, some of which was also destroyed. Additional specimens are likely among the material collected by the British Museum of Natural History's Tendaguru expedition. Much or all of this material probably belongs to Giraffatitan, although some may represent a new brachiosaurid.

Janensch based his description of B. brancai 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 (presently 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 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 in 2011 Taylor realized that Janensch had designated the smaller skeleton S I as the lectotype in 1935.

Diagram incorporating bones of both Brachiosaurus and Giraffatitan, by William Diller Matthew, 1915

In 1988, 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 was a difference in the way the trunk vertebrae vary: they are fairly uniform in length in the African material, but differ widely in B. altithorax. 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 at subgenus level as Brachiosaurus (Brachiosaurus) altithorax and Brachiosaurus (Giraffatitan) brancai respectively. Giraffatitan was raised to full genus level by George Olshevsky in 1991, while referring to the vertebral variation. Between 1991 and 2009, the name Giraffatitan was almost completely disregarded by other researchers.

A detailed 2009 study by Taylor of all material, including the limb and girdle bones, found that there are significant differences between B. altithorax and the Tendaguru material in all elements known from both species. Taylor found twenty-six distinct osteological (bone-based) characters, a larger difference than that between e.g. Diplodocus and Barosaurus, and therefore argued that the African material should indeed be placed in its own genus—Giraffatitan—as Giraffatitan brancai. An important difference between the two genera is the overall body shape, with Brachiosaurus having a 23% longer dorsal vertebral series and a 20 to 25% longer and also taller tail. The split was in 2010 rejected by Daniel Chure, but from 2012 onwards most studies recognized the name Giraffatitan.

B. atalaiensis

In 1947, at Atalaia in Portugal, brachiosaurid remains were found in layers dating from the Tithonian. Albert-Félix de Lapparent and Georges Zbyszewski named them as the species Brachiosaurus atalaiensis in 1957. Its referral to Brachiosaurus was doubted in 2004 by Paul Upchurch, Barret, and Peter Dodson who listed it as an as yet unnamed brachiosaurid genus. Shortly before the publication of the 2004 book, the species had been placed in its own genus Lusotitan by Miguel Telles Antunes and Octávio Mateus in 2003. De Lapparent and Zbyszewski had 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 the lectotype; this specimen includes twenty-eight 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, which was confirmed by an analysis in 2013.

B. nougaredi

Diagram showing preserved parts of the "B." nougaredi sacrum in blue

In 1958, the French petroleum geologist F. Nougarède reported to have discovered fragmentary brachiosaurid remains in eastern Algeria, in the Sahara Desert. Based on these, Albert-Félix de Lapparent described and named the species Brachiosaurus nougaredi in 1960. He indicated the discovery locality as being in the Late Jurassic–age Taouratine Series. He assigned the rocks to this age in part because of the presumed presence of Brachiosaurus. However, a more recent review placed it in the "Continental intercalaire," which is considered to belong to the Albian age of the late Early Cretaceous, significantly younger.

The type material moved to Paris consisted of a sacrum, weathered out at the desert surface, 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.

"B." nougaredi was in 2004 considered to represent a distinct, unnamed brachiosaurid genus, but a 2013 analysis by Philip D. Mannion and colleagues found that the remains possibly belong to more than one species, as they were collected far apart. The metacarpals were concluded to belong to some indeterminate titanosauriform. The sacrum was reported lost in 2013. It was not analyzed and provisionally considered to represent an indeterminate sauropod, until such time that it could be relocated in the collections of the Muséum national d'histoire naturelle. Only four out of the five sacral vertebrae are preserved. The total original length was in 1960 estimated at 1.3 meters (4.3 ft), compared to 0.91 meters (3.0 ft) with Brachiosaurus altithorax. This would make it larger than any other sauropod sacrum ever found, except those of Argentinosaurus and Apatosaurus.

Classification

Replica skeleton outside the FMNH

Riggs, in 1903, considered the newly-named Brachiosaurus to be an obvious member of the Sauropoda. To determine the validity of the genus, he compared it to four other genera, the validity of which he questioned: Camarasaurus, Apatosaurus, Atlantosaurus, and Amphicoelias. He concluded that a lack of overlapping material prevented a meaningful comparison. In his opinion not enough fossils had been found to determine the relationships between the four genera. As a result little could be said about the relationships of Brachiosaurus itself until its vertebrae had been better studied. In 1904, Riggs described the holotype material of Brachiosaurus in more detail, especially the vertebrae. He admitted that he originally had assumed a close affinity with Camarasaurus but now decided that Brachiosaurus was more closely related to Haplocanthosaurus. Both genera shared a single line of spines on the back and had wide hips. Riggs considered the differences from other taxa significant enough to name a separate family Brachiosauridae, of which Brachiosaurus is the namesake genus. According to Riggs, Haplocanthosaurus was the more primitive genus of the family while Brachiosaurus was a specialized form.

When describing Brachiosaurus brancai and B. fraasi in 1914, Janensch observed that the unique elongation of the humerus was shared by all three Brachiosaurus species and the British Pelorosaurus. Cetiosaurus was also noted by him to have a more slender humerus, but not as much as in Brachiosaurus or Pelorosaurus. All four genera were considered to have been closely related. In 1929, Janensch assigned such genera to a subfamily Brachiosaurinae that was part of the family Bothrosauropodidae. Janensch strongly diverged from modern insights about sauropod phylogeny. Cetiosaurus is now understood to have been a much earlier branch of the sauropod evolutionary tree. Brachiosaurids are today seen as closely related to Titanosauridae, but Janensch allied the latter with the Diplodocidae based on similarities in their tooth shape.

During the twentieth century, a number of sauropods were assigned to either Brachiosauridae or Brachiosaurinae, such as Astrodon, Bothriospondylus, Dinodocus, Pelorosaurus, Pleurocoelus, and Ultrasaurus, but most of these are currently regarded as dubious or of uncertain placement. These assignments were largely based on the possession of some "key character" such as long forelimbs. In the 1990s, advances in computer power allowed to exactly determine the relationships between species by calculating those trees that implied the least number of evolutionary changes and thus were the most likely to be correct. Such cladistic analyses until 2009 had the disadvantage that they without exception treated Brachiosaurus altithorax and B. brancai as a single OTU or data subset into which all their traits were combined, making it impossible to test whether they were really sister species.

Fifth dorsal vertebra in front of the pelvis of the holotype, compared to a human back vertebral column

Cladistic analyses have also cast doubt on the validity of the concept Brachiosauridae. In 1993, Leonardo Salgado suggested that they were an unnatural group into which all kinds of unrelated sauropods had been combined. In 1997, he published an analysis in which species traditionally considered brachiosaurids were subsequent off-shoots of the stem of the Titanosauriformes, not a separate branch of their own. This study also pointed out that Brachiosaurus altithorax and B. brancai did not show any synapomorphies, shared new traits, so that there was no evidence to assume they were particularly closely related.

New fossil finds and an increasing understanding of sauropod anatomy allow for evermore reliable cladistic analyses which, however, are not necessarily in agreement with each other. Many studies have found that at least some species are part of a natural group—some branch at the base of Titanosauriformes, a group of sauropods that also includes the titanosaurs—that includes Brachiosaurus while other seemingly brachiosaurid sauropods are in fact not directly related basal titanosauriforms. The exact status of each sauropod varies from study to study. A phylogenetic analysis of sauropods published in 2010 e.g., by Chure and colleagues, found that Abydosaurus formed a clade, natural group, with Brachiosaurus. In this study, Giraffatitan was included in Brachiosaurus. This prevents determining the relationship between these two species. The study in 2009 by Taylor that split the genera also for the first time included them separately in a cladistic analysis, which found them to be sister species. Another 2010 analysis focused on possible Asian brachiosaurid material found a clade including Abydosaurus, Brachiosaurus, Cedarosaurus, Giraffatitan (as a separate OTU), and Paluxysaurus, but not Qiaowanlong, the putative Asian brachiosaurid. Several subsequent analyses have found Brachiosaurus and Giraffatitan not to be sister species, but instead located at different positions on the evolutionary tree. A study by Michael D. D'Emic placed Giraffatitan in a more basal position, as an earlier branch, than Brachiosaurus, while a paper by Philip Mannion and colleagues in 2013 had it the other way around.

The cladogram of the Brachiosauridae below follows that published by Michael D. D'Emic in 2012:

Brachiosauridae	Europasaurus Giraffatitan Brachiosaurus Abydosaurus Cedarosaurus Venenosaurus

Cladistic analyses also allow to determine which new traits the members of a group have in common, their synapomorphies. According to the 2009 study by Taylor, Brachiosaurus altithorax shares with other Brachiosauridae the classic trait of having an upper arm bone that is at least nearly as long as the thighbone (ratio of humerus length to femur length of at least 0.9). Another shared character is the very flattened femur shaft, its transverse width being at least 1.85 times the width measured from front to rear.

Paleobiology

Restoration

Habits

It was believed throughout the nineteenth and early twentieth centuries that sauropods like Brachiosaurus were too massive to support their own weight on dry land, and instead lived partly submerged in water. Riggs however, affirming observations by John Bell Hatcher, was the first to defend in length that most sauropods were fully terrestrial animals in his 1904 account on Brachiosaurus, pointing out that their hollow vertebrae have no analogue in living aquatic or semiaquatic animals, and their long limbs and compact feet indicate specialization for terrestrial locomotion. Brachiosaurus would have been better adapted than other sauropods to a fully terrestrial lifestyle through its slender limbs, high chest, wide hips, high ilia and short tail. In its back vertebrae the zygapophyses were very reduced while the hyposphene-hypanthrum complex was extremely developed, resulting in a stiff torso incapable of bending sideways. The body was only fit for quadrupedal movement on land. The more primitive Haplocanthosaurus however, might have been partly aquatic. Although Riggs' ideas were gradually forgotten during the first half of the twentieth century, the notion of sauropods as terrestrial animals has gained support since the 1950s, and is now universally accepted among paleontologists.

Neck posture

Further information: Sauropod neck posture

Assigned neck vertebra BYU 12866, BYU Museum of Paleontology

Ongoing debate revolves around the neck posture of brachiosaurids, with estimates ranging from near-vertical to horizontal orientations. The idea of near-vertical postures in sauropods in general was popular for until 1999, when Stevens and Parrish argued that the sauropod neck was not flexible enough to be held in an upright, S-curved pose, and instead was held horizontally. A reaction to this research, various newspapers ran stories criticizing the Field Museum Brachiosaurus mount for having an upwards curving neck. The paleontologists Olivier Rieppel and Christopher Brochu from the museum defended the posture in 1999, stating that the long forelimbs, upwards sloping backbone, and that the most developed neural spines for muscle attachment being positioned in the region of the shoulder girdle would have permitted the neck to be raised in a giraffe-like posture. They also stated that such a pose would have required less amount of energy than lowering its neck, and that the inter-vertebral discs would not have been able to counter the pressure caused by a lowered head for extended periods of time (though lowering its neck to drink must have been possible). Some recent studies also advocated a more upwards directed neck. Christian and Dzemski (2007) estimated that the middle part of the neck in Giraffatitan was inclined by 60–70 degrees; a horizontal posture could be maintained only for short periods of time.

With their heads held high above the heart, brachiosaurids would have had stressed cardiovascular systems. It is estimated that the heart of Brachiosaurus would have to pump double the blood pressure of a giraffe to reach the brain, and possibly weighed 400 kg (880 lb). The distance between head and heart would have been reduced by the S-curvature of the neck by more than 2 meters (6 ft 7 in) in comparison to a totally vertical posture. In addition, the neck may have been lowered during locomotion by 20 degrees. In studying the inner ear of Giraffatitan, Gunga & Kirsch (2001) concluded that brachiosaurids would have moved their necks in lateral directions more often than in dorsal-ventral directions.

Feeding and diet

Replica skeleton showing the neck pointing upwards, at O'Hare International Airport (formerly housed in the Field Museum)

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 less inclined neck, its head height may still have been over 9 meters (30 ft) above the ground. It probably fed mostly on foliage above 5 meters (16 ft). This does not preclude the possibility that it also fed lower at times, between 3 to 5 meters (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 in a 2007 study. Brachiosaurid feeding involved simple up-and-down jaw motion. As in other sauropods, animals would have swallowed plant matter without further oral processing, and relied on hindgut fermentation for food processing. As the teeth were not spoon-shaped as with earlier sauropods but of the compressed cone-chisel type, a precision-shear bite was employed. Such teeth are optimized for non-selective nipping, and the relatively broad jaws could crop large amounts of plant material. Even if a Brachiosaurus of forty tonnes would have needed half a tonne of fodder, its dietary needs could have been met by a normal cropping action of the head. If it ate during sixteen hours per day, biting off between a tenth and two-thirds of a kilogram, taking between one and six bites per minute, its daily food intake would have roughly equaled 1.5% of its body mass, which is comparable to the requirement of a modern elephant.

As Brachiosaurus shared its habitat, the Morrison, with many other sauropod species, its specialization for feeding at greater heights would have been part of a system of niche partitioning, the various taxa thus avoiding direct competition with each other. A typical food tree might have resembled Sequoiadendron. The fact that such tall conifers were relatively rare in the Morrison might explain why Brachiosaurus was much less common in its ecosystem than the related Giraffatitan, which seems to have been one of the most abundant sauropods in the Tendaguru. Brachiosaurus, with its shorter arm and lower shoulder, was not as well-adapted to high-browsing as Giraffatitan.

It has been suggested that Brachiosaurus could rear on its hind legs to feed, using its tail for additional ground support. However, a detailed physical modelling-based analysis of sauropod rearing capabilities by Heinrich Mallison showed that while many sauropods could rear, the unusual body shape and limb length ratio of brachiosaurids 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 rearing may have tripled the feeding height. A bipedal stance might have been adopted by Brachiosaurus in exceptional situations, like male dominance fights.

The downward mobility of the neck of Brachiosaurus would have allowed it to reach open water at the level of its feet, while standing upright. Modern giraffes spread their forelimbs to lower the mouth in a relatively horizontal position, in order to more easily gulp down the water. It is unlikely that Brachiosaurus could have attained a stable posture this way, forcing the animal to plunge the snout almost vertically into the surface of a lake or stream. This would have submerged its fleshy nostrils if they were located at the tip of the snout as Witmer hypothesized. Hallett and Wedel therefore in 2016 rejected his interpretation and suggested that they were in fact placed at the top of the head, above the bony nostrils, as traditionally thought. The nostrils might have evolved their retracted position to allow the animal to breath while drinking.

It has been proposed that sauropods, including Brachiosaurus, may have had proboscises (trunks) based on the position of the bony narial orifice, to increase their upwards reach. Fabien Knoll and colleagues disputed this for Diplodocus and Camarasaurus in 2006, finding that the opening for the facial nerve in the braincase was small. The facial nerve was thus not enlarged as in elephants, where it is involved in operating the sophisticated musculature of the proboscis. However, Knoll and colleagues also noted that the facial nerve for Giraffatitan was larger, and could therefore not discard the possibility of a proboscis in this genus.

Metabolism

Like other sauropods, Brachiosaurus was probably homeothermic (maintaining a stable internal temperature) and endothermic (controlling body temperature through internal means) at least while still growing, meaning that it was able to actively control its body temperature ("warm-blooded"), producing the necessary heat through a high basic metabolic rate of its cells. Russel (1989) used Brachiosaurus as an example of a dinosaur for which endothermy is unlikely, because of the combination of great size (leading to overheating) and great caloric needs to fuel endothermy. However, Sander (2010) found that these calculations were based on incorrectly high body mass estimates and incorrect assumptions about the available cooling surfaces, as the presence of large air sacs was unknown at the time of Russell's study. These inaccuracies resulted in the overestimation of heat production and the underestimation of heat loss. The large nasal arch has been postulated as an adaptation for cooling the brain, as a surface for evaporative cooling of the blood.

Growth

Juvenile B. sp. specimen SMA 0009 (the skull is reconstructed), Sauriermuseum Aathal

The ontogeny of Brachiosaurus has been reconstructed by Carballido and colleagues in 2012 based on SMA 0009, a postcranial skeleton of a young juvenile with an estimated total body length of just 2 meters (6.6 ft). This skeleton shares some unique traits with the B. altithorax holotype, indicating it is referable to this species. These commonalities include an elevation on the rear blade of the ilium; the lack of a postspinal lamina; vertical neural spines on the back; an ilium with a subtle notch between the appendage for the ischium and the rear blade; and the lack of a side bulge on the upper thighbone. However, there are also differences. These might indicate that the juvenile is not a B. altithorax individual after all, but belongs to a new species. Alternatively, they might be explained as juvenile traits that would have changed when the animal matured.

Reconstruction of SMA 0009, possibly a juvenile Brachiosaurus, based on the initial diplodocoid identification

Such ontogenetic changes are especially to be expected in the proportions of an organism. The middle neck vertebrae of SMA 0009 are remarkably short for a sauropod, being just 1.8 times longer than high, compared with a ratio of 4.5 in Giraffatitan. This suggests that the necks of brachiosaurids became proportionally much longer while their backs, to the contrary, experienced relative negative growth. The humerus of SMA 0009 is relatively robust: it is more slender than that of most basal titanosauriforms but thicker than the upper arm bone of B. altithorax. This suggests that it was already lengthening in an early juvenile stage and became even more slender during growth. This is in contrast to diplodocoids and basal macronarians, whose slender humeri are not due to such allometric growth. Brachiosaurus also appears to have experienced an elongation of the metacarpals, which in juveniles were shorter compared to the length of the radius; SMA 0009 shows a ratio of just 0.33, the lowest known in the entire Neosauropoda.

Another plausible ontogenetic change is the increased pneumatization of the vertebrae. During growth, the diverticula of the air sacs invaded the bones and hollowed them out. SMA 0009 already has pleurocoels, pneumatic excavations, at the sides of its neck vertebrae. These are divided by a ridge but are otherwise still very simple in structure, compared with the extremely complex ridge systems typically shown by adult derived sauropods. Its back vertebrae still completely lack such pleurocoels.

Two traits are not so obviously linked to ontogeny. The neural spines of the rear back vertebrae and the front sacral vertebrae are extremely compressed transversely, being eight times longer from front to rear than wide from side to side. The spinodiapophyseal lamina or "SPOL", the ridge normally running from each side of the neural spine towards each diapophysis, the transverse process bearing the contact facet for the upper rib head, is totally lacking. Both traits could be autapomorphies, unique derived characters proving that SMA 0009 represents a distinct species. However, there are indications that these traits are growth-related as well. Of the basal sauropod Tazoudasaurus a young juvenile is known that also lacks the spinodiapophyseal lamina, whereas the adult form has an incipient ridge. Furthermore, a very young juvenile of Europasaurus shows a weak SPOL but it is well developed in mature individuals. These two cases represent the only finds in which the condition can be checked; they suggest that the SPOL developed during growth. As this very ridge widens the neural spine, its transverse compression is not an independent trait and the development of the SPOL plausibly precedes the thickening of the neural spine with more mature animals.

Sauropods were likely able to sexually reproduce before they attained their maximum individual size. The maturation rate differed between species. Its bone structure indicates that Brachiosaurus was able to reproduce when it reached 40% of its maximal size.

Paleoecology

Map showing locations of brachiosaurid remains from the Morrison Formation (gray); 5 (middle left) is the B. altithorax type locality

Brachiosaurus is known only from the Morrison Formation of western North America (following the reassignment of the African species). The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains. Several other sauropod genera were present in the Morrison Formation, with differing body proportions and feeding adaptations. Among these were Apatosaurus, Barosaurus, Camarasaurus, Diplodocus, Haplocanthosaurus, and Supersaurus. Brachiosaurus was one of the less abundant Morrison Formation sauropods. In a 2003 survey of over 200 fossil localities, John Foster reported 12 specimens of the genus, comparable to Barosaurus (13) and Haplocanthosaurus (12), but far fewer than Apatosaurus (112), Camarasaurus (179), and Diplodocus (98). Brachiosaurus fossils are found only in the lower-middle part of the expansive Morrison Formation (stratigraphic zones 2–4), dated to about 154–153 million years ago, unlike many other types of sauropod which have been found throughout the formation. If the large foot reported from Wyoming (the nothernmost occurrence of a brachiosaurid in North America) did belong to Brachiosaurus, the genus would have covered a wide range of latitudes. Brachiosaurids could process tough vegetation with their broad-crowned teeth, and might therefore have covered a wider range of vegetational zones than for example diplodocids. Camarasaurids, which have a similar tooth morphology to brachiosaurids, were also widespread and are known to have migrated seasonally, so this might have also been true for brachiosaurids as well.

Other dinosaurs known from the Morrison Formation include the predatory theropods Koparion, Stokesosaurus, Ornitholestes, Ceratosaurus, Allosaurus and Torvosaurus, as well as the herbivorous ornithischians Camptosaurus, Dryosaurus, Othnielia, Gargoyleosaurus and Stegosaurus. Allosaurus accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web. Ceratosaurus might have specialized in attacking large sauropods, including smaller individuals of Brachiosaurus. Other vertebrates that shared this paleoenvironment included ray-finned fishes, frogs, salamanders, turtles like Dorsetochelys, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans such as Hoplosuchus, and several species of pterosaur like Harpactognathus and Mesadactylus. Shells of bivalves and aquatic snails are also common. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests in otherwise treeless settings (gallery forests) with tree ferns, and ferns, to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.

Cultural significance

Child with the Potter Creek humerus

Riggs in the first instance tried to limit public awareness of the find. When reading a lecture to the inhabitants of Grand Junction, illustrated by lantern slides, on 27 July 1901, he explained the general evolution of dinosaurs and the exploration methods of museum field crews but did not mention that he had just found a spectacular specimen. He feared that teams of other institutions might soon learn of the discovery and take away the best of the remaining fossils. His host Bradbury however, a week later published an article in the local Grand Junction News announcing the find of one of the largest dinosaurs ever. On 14 August, the New York Times brought the story. At the time sauropod dinosaurs appealed to the public because of their great size, often exaggerated by sensationalist newspapers. Riggs in his publications played into this by emphasizing the enormous magnitude of Brachiosaurus.

Brachiosaurus has been called one of the most iconic dinosaurs, but most popular depictions are based on the African species B. brancai which has since been moved to its own genus, Giraffatitan. A main belt asteroid, 1991 GX₇, was named 9954 Brachiosaurus in honor of the genus in 1991. Brachiosaurus was featured in the 1993 movie Jurassic Park, as the first computer generated dinosaur shown. These effects were considered ground-breaking at the time, and the awe of the movie's characters upon seeing the dinosaur for the first time was mirrored by audiences. The movements of the movie's Brachiosaurus were based on the gait of a giraffe combined with the mass of an elephant. A scene later in the movie used an animatronic head and neck, for when a Brachiosaurus interacts with human characters. The digital model of Brachiosaurus used in Jurassic Park later became the starting point for the ronto models in the 1997 special edition of the film Star Wars Episode IV: A New Hope.

Bibliography

• Brinkman, P. D. (2010), The Second Jurassic Dinosaur Rush: Museums and Paleontology in America at the Turn of the Twentieth Century, Chicago and London: The University of Chicago Press, ISBN 978-0226074726
• Hallett, M.; Wedel, M. (2016), The Sauropod Dinosaurs: Life in the Age of Giants, Baltimore: Johns Hopkins University Press, ISBN 978-1421420288

External links

• The dictionary definition of Brachiosaurus at Wiktionary
• Media related to Brachiosaurus at Wikimedia Commons
• The First Brachiosaurus – Interview with Joyce Havstad of the Field Museum about Braciosaurus and the concept of holotypes