Brachiosaurus

''Brachiosaurus'' () is a genus of sauropod dinosaur that lived in North America during the Late Jurassic, about 154to 150million 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, 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 been between 18 and 22 meters (59 and 72ft) long; body mass estimates of the subadult holotype specimen range from 28.3 to 46.9 metric tons (31.2 and 51.7 short tons). It had a disproportionately long neck, small skull, and large overall size, all of which are typical for sauropods. Atypically, ''Brachiosaurus'' had longer forelimbs than hindlimbs, which resulted in a steeply inclined trunk, and a proportionally shorter tail.

''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. ''Giraffatitan'' 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. Three other species of ''Brachiosaurus'' have been named based on fossils found in Africa and Europe; two are no longer considered valid, and a third has become a separate genus, '' Lusotitan''.

The type specimen of ''B. altithorax'' is still the most complete specimen, and only a few other specimens are thought to belong to the genus, making it one of the rarer sauropods of the Morrison Formation. It is regarded as a high browser, possibly cropping or nipping vegetation as high as 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 suggests it was warm-blooded. Among the most iconic and initially thought to be one of the largest dinosaurs, ''Brachiosaurus'' has appeared in popular culture, notably in the 1993 film ''Jurassic Park''.

History of discovery

Holotype specimen

The genus ''Brachiosaurus'' is based on a partial postcranial skeleton discovered in 1900 in the valley of the Colorado River near Fruita, Colorado. This specimen, which was later declared the holotype, comes from rocks of the Brushy Basin Member of the Morrison Formation, and therefore is late Kimmeridgian in age, about 154to 153million years old. Discovered 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 FMNHP25107.

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 skeptical 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 June 20, 1900 they set camp at the abandoned Goat Ranch. During a prospecting trip on horseback, Riggs's field assistant Harold William Menke found the humerus of FMNHP25107, on July 4, 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 July 26 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.

The site, Riggs Quarry 13, is located on a small hill later known as Riggs Hill; it is today marked by a plaque. More ''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 this seemed to be confirmed when an equally-sized, well-preserved real 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 August 17, 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 . On September 10, 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 several 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 downward. 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 presacral 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 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. In 1903, he named the type species ''Brachiosaurus altithorax''. Riggs derived the genus name from the Greek ''brachion''/βραχίων meaning "arm" and ''sauros''/ 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. The titles of Riggs's 1901 and 1903 articles emphasized that the specimen was the "largest known dinosaur". Riggs followed his 1903 publication 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. No mount of ''Brachiosaurus'' was attempted because only twenty percent of the skeleton had been recovered. In 1993, the holotype bones were molded and cast, and the missing bones were sculpted based on material of the related ''Brachiosaurus brancai'' (now '' Giraffatitan'') in Museum für Naturkunde, 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 BConcourse 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) which is on display outside the museum on the NW terrace.
Another outdoor cast was sent to Disney's Animal Kingdom to serve as a gateway icon for the "DinoLand, U.S.A." area, known as the "Oldengate Bridge" that connects the two halves of the fossil quarry themed Boneyard play area.

Assigned material

Further discoveries of ''Brachiosaurus'' material in North America have been uncommon and consist of a few bones. To date, material can be unambiguously ascribed only to the genus when overlapping with the holotype material, and any referrals of elements form the skull, neck, anterior dorsal region, or distal limbs or feet remain tentative. Nevertheless, material has been described from Colorado, Oklahoma, Utah, and Wyoming, and undescribed material has been mentioned from several other sites.

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 cervical vertebra, which was destroyed during an attempt to collect it. The skull was cataloged 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 dentaries, and a possible partial pterygoid. The bones were roughly prepared for Marsh, which led to some damage. Felch also collected several postcranial fossils, including a partial cervical vertebra and partial forelimb. 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 cataloged 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 ''Brachiosaurus'' sp. (of uncertain species), since it is impossible to determine whether it belonged to the species ''B. altithorax'' itself (as there is no overlapping material between the two specimens). They based the skull's assignment to ''Brachiosaurus'' on its similarity to that of ''B. brancai'', later known as ''Giraffatitan''. In 2019, American paleontologists Michael D. D'Emic and Matthew T. Carrano re-examined the Felch Quarry skull after having it further prepared and CT-scanned (while consulting historical illustrations that showed earlier states of the bones), and concluded that a quadrate bone and dentary tooth considered part of the skull by Carpenter and Tidwell did not belong to it. The quadrate is too large to articulate with the squamosal, is preserved differently from the other bones, and was found several meters away. The tooth does not resemble those within the jaws (as revealed by CT data), is larger, and was therefore assigned to ''Camarasaurus'' sp. (other teeth assignable to that genus are known from the quarry). They also found it most parsimonious to assign the skull to ''B. altithorax'' itself rather than an unspecified species, as there is no evidence of other brachiosaurid taxa in the Morrison Formation (and adding this and other possible elements to a phylogenetic analysis did not change the position of ''B. altithorax'').

A shoulder blade with coracoid 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 had already received the name. Later study showed that the "ultrasaur" material mostly belonged to ''Supersaurus'', though the shoulder blade did not. Because the holotype of ''Ultrasauros'', a dorsal 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), was in 1996 assigned to a ''Brachiosaurus'' sp. (of uncertain species) by Brian Curtice and colleagues; in 2009 Michael P. Taylor concluded that it could not be referred to ''B. altithorax''. 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's original specimen of ''Brachiosaurus''.

Several additional specimens were briefly described by Jensen in 1987. One of these finds, the humerus USNM 21903, was discovered in ca. 1943 by uranium prospectors Vivian and Daniel Jones in the Potter Creek Quarry in western Colorado, and donated to the Smithsonian Institution. Originally, this humerus was part of a poorly preserved partial skeleton that was not collected. According to Taylor in 2009, it is not clearly referable to ''Brachiosaurus'' despite its large size of . Jensen himself worked at the Potter Creek site in 1971 and 1975, excavating the disarticulated specimen BYU 4744, which contains a mid-dorsal vertebra, an incomplete left ilium, a left radius and a right metacarpal. According to Taylor in 2009, this specimen can be confidently referred to ''B. altithorax'', as far as it is overlapping with its type specimen. Jensen further mentioned a specimen discovered near Jensen, Utah, that includes a rib in length, an anterior cervical vertebra, part of a scapula, and a coracoid, although he did not provide a description. In 2001, Curtice and Stadtman ascribed two articulated dorsal vertebrae (specimen BYU 13023) from Dry Mesa Quarry to ''Brachiosaurus''. Taylor, in 2009, noted that these vertebrae are markedly shorter than those of the ''B. altithorax'' holotype, although otherwise being similar.

In 2012, José Carballido and colleagues reported a nearly complete postcranial skeleton of a small juvenile approximately in length. This specimen, nicknamed "Toni" and cataloged as SMA 0009, stems from the Morrison Formation of the Bighorn Basin in north-central Wyoming. Although originally thought to belong to a diplodocid, it was later reinterpreted as a brachiosaurid, probably belonging to ''B. altithorax''. In 2018, the largest sauropod foot ever found was reported from the Black Hills of Weston County, Wyoming. The femur is not preserved but comparisons suggest that it was about two percent longer than that of the ''B. altithorax'' holotype. Though possibly belonging to ''Brachiosaurus'', the authors cautiously classified it as an indeterminate brachiosaurid. However, the assignment of these two specimens to their respective clades was later questioned by D'Emic and Carrano in 2019. They considered the referral of "Toni" to ''B. altithorax'' be based on mistaken interpretations of the species' unique features or of the specimen itself, and deemed it worthy of further study. Analyzing photos of the large foot, D'Emic and Carrano noted that the only feature that allowed referral to Brachiosauridae may have been influenced by damage to the bone it was found on, but did state that "general similarities" with ''Sonorasaurus'' and '' Giraffatitan'' suggested brachiosaurid affinities, but this, the authors stated, would be confirmed only through further study.

Formerly assigned species

''Brachiosaurus brancai'' and ''Brachiosaurus fraasi''

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 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 shared characters between ''Brachiosaurus brancai'' (which he now considered to include ''B. fraasi'') and ''B. altithorax''. Taylor, in 2009, considered only four of these characters as valid; six pertain to groups more inclusive 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 in 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 material was collected by the British Museum of Natural History's Tendaguru expedition, including a nearly complete skeleton (BMNH R5937) collected by F.W.H. Migeod in 1930. This specimen is now believed to represent a new species, awaiting description.

Janensch based his description of ''B. brancai'' on "Skelett S" (skeleton S) from Tendaguru, but later realized that it comprised two partial individuals: SI and SII. He at first did not designate them as a syntype series, but in 1935 made SI (presently MB.R.2180) the lectotype. Taylor in 2009, unaware of this action, proposed the larger and more complete SII (MB.R.2181) as the lectotype. It includes, among other bones, several dorsal vertebrae, the left scapula, both coracoids, the breastbones, both humeri, both ulnae and radii (lower arm bones), a right hand, a partial left hand, both hip bones and the right femur, tibia and fibula (shank bones). Later in 2011, Taylor realized that Janensch had designated the smaller skeleton SI as the lectotype in 1935.

In 1988 Gregory S. Paul published a new reconstruction of the skeleton of ''B. brancai'', highlighting differences in proportion between it and ''B. altithorax''. Chief among them was a distinction in the way the trunk vertebrae vary: they are fairly uniform in length in the African material, but vary widely in ''B. altithorax''. Paul believed that the limb and girdle elements of both species were very similar, and therefore suggested they be separated not at genus, but only at subgenus level, as ''Brachiosaurus (Brachiosaurus) altithorax'' and ''Brachiosaurus (Giraffatitan) brancai''. ''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 divergences 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 between ''Diplodocus'' and ''Barosaurus'', and therefore argued that the African material should indeed be placed in its own genus (''Giraffatitan'') as ''Giraffatitan brancai''. An important contrast between the two genera is their overall body shape, with ''Brachiosaurus'' having a 23 percent longer dorsal vertebral series and a 20 to 25 percent longer and also taller tail. The split was rejected by Daniel Chure in 2010, but from 2012 onward most studies recognized the name ''Giraffatitan''.

''Brachiosaurus 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 the 2004 edition of '' The Dinosauria'' 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 ( MIGM4978, 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 tilting upward indicate that ''Lusotitan'' is a brachiosaurid, which was confirmed by some later studies, such as an analysis in 2013.

''Brachiosaurus nougaredi''

In 1958 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''. 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 , compared to with ''B. altithorax''. This would make it larger than any other sauropod sacrum ever found, except those of '' Argentinosaurus'' and ''Apatosaurus''.

Description

Size

Most estimates of ''Brachiosaurus altithorax'' size are based on the related brachiosaurid ''Giraffatitan'' (formerly known as ''B. brancai''), which is known from much more complete material than ''Brachiosaurus''. The two species are the largest brachiosaurids of which relatively extensive remains have been discovered. There is another 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 the holotype specimen has been estimated within the range of . Benson et al. suggested a maximum body mass of , but these estimates were questioned due to a very large error range and lack of precision. The length of ''Brachiosaurus'' has been estimated at 20–22 meters (66–72ft) per Paul and , and its height at and 12–13 meters (39–43ft).

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 was somewhat lighter than the ''Brachiosaurus'' specimen given its proportional differences. In studies including estimates for both genera, ''Giraffatitan'' was estimated at , , , , and . As with the main ''Brachiosaurus'' specimen, ''Giraffatitan'' specimen MB.R.2181 likely does not reflect the maximum size of the genus, as a fibula (specimen HMXV2) is thirteen percent longer than that of MB.R.2181.

General build

Like all sauropod dinosaurs, ''Brachiosaurus'' was a quadruped 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 by bone resorption, greatly reducing the overall density of the body. The neck is not preserved in the holotype specimen, but was very long even by sauropod standards in the closely related ''Giraffatitan'', consisting of thirteen elongated cervical (neck) vertebrae. The neck was held in a slight S-curve, with the lower and upper sections bent and a straight middle section. ''Brachiosaurus'' likely shared with ''Giraffatitan'' the very elongated neck ribs, which ran 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 trunk, lightening the distal neck portions.

''Brachiosaurus'' and ''Giraffatitan'' probably had a small shoulder hump between the third and fifth dorsal (back) vertebra, where the sideward- and upward-directed vertebral processes were longer, providing additional surface for neck muscle attachment. The ribcage was deep compared to other sauropods. Though 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 of other brachiosaurids. This resulted in an inclined trunk with the shoulder much higher than the hips, and the neck exiting the trunk at a steep angle. The overall build of ''Brachiosaurus'' resembles a giraffe more than any other living animal. In contrast, most other sauropods had a shorter forelimb than hindlimb; the forelimb is especially short in contemporaneous diplodocoids.

''Brachiosaurus'' differed in its body proportions from the closely related ''Giraffatitan''. The trunk was about 25 to 30 percent longer, resulting in a dorsal vertebral column longer than the humerus. Only a single complete caudal (tail) vertebra has been discovered, but its great height suggests that the tail was larger than in ''Giraffatitan''. This vertebra had a much greater area for ligament attachment due to a broadened neural spine, indicating that the tail was also longer than in ''Giraffatitan'', possibly by 20 to 25percent. In 1988, paleontologist Gregory S. Paul suggested that the neck of ''Brachiosaurus'' was shorter than that of ''Giraffatitan'', but in 2009, paleontologist Mike P. Taylor pointed out that two cervical vertebrae likely belonging to ''Brachiosaurus'' had identical proportions. Unlike ''Giraffatitan'' and other sauropods, which had vertically oriented forelimbs, the arms of ''Brachiosaurus'' appear to have been slightly sprawled at the shoulder joints, as indicated by the sideward orientation of the joint surfaces of the coracoids. The humerus was less slender than that of ''Giraffatitan'', while the femur had 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

Though the vertebral column of the trunk or torso is incompletely known, the back of ''Brachiosaurus'' most likely comprised twelve dorsal vertebrae; this can be inferred from the complete dorsal vertebral column preserved in 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), the lower part of the vertebrae, were more elongated and roughly circular in cross-section, while those of ''Giraffatitan'' were broader than tall. 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 upward. At their ends, these projections articulated with the ribs; the articular surface was not distinctly triangular as in ''Giraffatitan''. In side view, the upward-projecting neural spines stood vertically and were twice as wide at the base than at the top; those of ''Giraffatitan'' tilted backward and did not broaden at their base. When seen in front or back view, the neural spines widened toward 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 back sides, the neural spines featured large, triangular and rugose surfaces, which in ''Giraffatitan'' were semicircular and much smaller. 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 processes at the back of the vertebrae, and therefore terminated at mid-height of the neural spines. In ''Giraffatitan'', both laminae were not conflated, and the spinodiapophyseal laminae reached up to the top of the neural spines. ''Brachiosaurus'' is further distinguished from ''Giraffatitan'' in lacking three details in the laminae of the dorsal vertebrae that are unique to the latter genus.

Air sacs not only invaded 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 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'' 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 had a short neural spine that was rectangular and tilted backward. In contrast to the second caudal vertebra of ''Giraffatitan'', that of ''Brachiosaurus'' had a proportionally taller neural arch, making the vertebra about thirty percent taller. The centrum lacked depressions on its sides, in contrast to ''Giraffatitan''. In front or back view, the neural spine broadened toward its tip to approximately three times its minimum width, but no broadening is apparent in ''Giraffatitan''. The neural spines were also inclined backward by about 30°, more than in ''Giraffatitan'' (20°). The caudal ribs projected laterally and were not tilted backward as in ''Giraffatitan''. The articular facets of the articular processes at the back of the vertebra were directed downward, while those of ''Giraffatitan'' faced more toward 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''.

The coracoid was semicircular and taller than broad. Differences from ''Giraffatitan'' are related to its shape in side view, including the straighter suture with the scapula. Moreover, the articular surface that forms part of the shoulder joint was thicker and directed more sideward than in ''Giraffatitan'' and other sauropods, possibly indicating a more sprawled forelimb. The humerus, as preserved, measures in length, though part of its lower end was lost to erosion; its original length is estimated at . This bone was more slender in ''Brachiosaurus'' than in most other sauropods, measuring only in width at its narrowest part. It was, however, more robust than that of ''Giraffatitan'', being about ten percent 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 downward projecting extension connecting to the ischium, reaches farther downward than in ''Giraffatitan''. While the latter genus had 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 ilium, ''Brachiosaurus'' had a pronounced tubercle that is absent in other sauropods. Of the hindlimb, the femur was very similar to that of ''Giraffatitan'' although slightly more robust, and measured long. As in ''Giraffatitan'', it was strongly elliptical in cross-section, being more than twice as wide in front or back view than in side view. The fourth trochanter, a prominent bulge on the back side of the femoral shaft, was more prominent and located further downward. This bulge served as anchor point for the most important locomotory muscle, the caudofemoralis, which was situated in the tail and pulled the upper thigh backward when contracted. At the lower end of the femur, the pair of condyles

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases