Brachiosaurus

It is not to be confused with Branchiosaurus. Brachiosaurus is a genus of sauropod dinosaur that lived in North America during the Late Jurassic, about 155.6 to 145.5 million years ago. American paleontologist Elmer S. Riggs first described it in 1903, using fossils found in the Colorado River valley in western Colorado, United States. Riggs named the dinosaur Brachiosaurus altithorax. The generic name comes from Greek and means "arm lizard," because its arms are long compared to the rest of its body. The specific name means "deep chest."

Brachiosaurus is estimated to have been between 18 and 22 meters (59 and 72 ft) long. It had a very long neck, a small skull, and a large body, like other sauropods. Unusually, its front legs were longer than its back legs. This gave it a steep trunk and a shorter tail than many of its relatives. It gives its name to the family Brachiosauridae, which includes a few other similar sauropods. Most popular pictures of Brachiosaurus are actually based on Giraffatitan, a related brachiosaurid from the Tendaguru Formation of Tanzania.

Three other species of Brachiosaurus have been named from fossils found in Africa and Europe. Two are no longer considered valid, and a third has become its own genus, Lusotitan. The type specimen of B. altithorax is still the most complete one known, and only a few other fossils are thought to belong to this genus. This makes it one of the rarer sauropods of the Morrison Formation. It is thought to have been a high browser, possibly eating plants as high as 9 meters (30 ft) off the ground. Unlike other sauropods, it could not rear up on its back legs.

For a long time, Brachiosaurus was used as an example of a dinosaur that was probably ectothermic, because its large size would have needed a lot of forage. However, more recent research suggests it was warm-blooded. Among the most famous dinosaurs, and once thought to be one of the largest dinosaurs, Brachiosaurus has appeared in popular culture, especially in the 1993 film Jurassic Park.

History of discovery

In 1900, a team led by American paleontologist Elmer S. Riggs found part of a skeleton in the valley of the Colorado River near Fruita, Colorado. The team worked for the Field Columbian Museum (now the Field Museum of Natural History) in Chicago. At first, Riggs thought the large upper arm bone came from a different dinosaur. Later, he realized it came from a huge animal. He believed it was the largest land animal ever found. The fossils were packed into many crates and sent by train to Chicago. Only about one-fifth of the skeleton had been found, so it could not be displayed fully at the time. In 1994, a plastic copy of the skeleton went on display at the Field Museum. Riggs named the species in 1903. The name Brachiosaurus means "arm lizard," because its arms were very long. Altithorax means "deep chest." Later, few more of this dinosaur were found in North America. Some species once called Brachiosaurus were later moved into other groups, such as Giraffatitan.

Description

Most estimates of the size of Brachiosaurus altithorax are based on the related brachiosaurid Giraffatitan (formerly known as B. brancai), which is known from much more complete material. The main specimen seems to be a subadult, meaning it was not yet fully grown. This is shown by an unfused suture between the coracoid, a bone of the shoulder girdle that forms part of the shoulder joint, and the scapula (shoulder blade). Its mass has been estimated between 28.3 and 46.9 metric tons, and its length between 18 and 22 meters. Another specimen, HM XV2, has a fibula that is thirteen percent longer than that of a Giraffatitan specimen. This means the largest size of the group is probably not yet known.

Like all sauropod dinosaurs, Brachiosaurus walked on four legs. It had a small head, a long neck, a large body, a long, muscular tail, and slender legs. Air sacs connected to the lungs filled parts of the neck and body. They reached into the vertebrae and ribs through bone resorption, which made the body lighter. The neck is not preserved in the holotype specimen, but in Giraffatitan it was very long. It had thirteen cervical (neck) vertebrae and long neck ribs that helped move the distal parts of the neck. The dorsal (back) of Brachiosaurus had a small shoulder hump. Its forelimb was longer than its hindlimb. Its humerus (upper arm bone) and femur (thigh bone) were about the same length. Its build looks more like a giraffe than any other living animal. Most other sauropods had shorter forelimbs, especially the diplodocoids of the same time. Only one complete caudal (tail) vertebra has been found. This suggests the tail was larger and longer than in Giraffatitan, with a much bigger ligament area. Its metacarpus and body proportions also differed from Giraffatitan. In 2009, paleontologist Mike P. Taylor pointed out that the necks of the two genera may have had identical proportions.

Classification

In 1903, Elmer Riggs first described Brachiosaurus as a clear member of the sauropods. He compared it with Atlantosaurus and Amphicoelias. In 1904, he decided it was more closely related to Haplocanthosaurus than he had first thought. He named a new family, Brachiosauridae, and gave Brachiosaurus as its namesake genus. In 1914, Janensch noticed that a long upper arm bone was shared by Brachiosaurus and Pelorosaurus. He also saw it in Cetiosaurus. In 1929, he grouped these animals into a subfamily called Brachiosaurinae.

During the 1900s, many other sauropods were placed in Brachiosauridae, including Bothriospondylus and Pleurocoelus. These groupings were often based on general similarities rather than clear synapomorphies. Most of those animals are now considered dubious. Since the 1990s, scientists have used computers and cladistic studies to find the most likely family trees for these animals. In 1997, Leonardo Salgado argued that brachiosaurids were not a separate branch. He said they were part of a larger group, the Titanosauriformes. Later studies have found that at least some brachiosaurids belong to a basal branch within that group. In 2009, Taylor moved B. brancai into its own genus, Giraffatitan. He found that the two genera were sister groups. Other studies have included Abydosaurus, Cedarosaurus, and Qiaowanlong in their analyses. This cladogram follows the one published by Michael D. D'Emic in 2012.

Paleobiology

For a long time, people thought huge sauropods like Brachiosaurus were too heavy to hold themselves up on land, so they must have lived partly in water. John Bell Hatcher shared observations that helped Elmer Riggs argue in 1904 that most sauropods lived fully on land. Their hollow bones and long legs with small feet were built for walking. Brachiosaurus had slim legs, a high chest, and a short tail, which fit a life on land. Its zygapophyses were reduced, and its hyposphene-hypantrum complex was well developed. This made its body stiff, so it could not bend easily from side to side. Riggs's idea was forgotten for a while, but since the 1950s it has gained support, and paleontologists now widely accept it. In 1990, Stephen Czerkas said Brachiosaurus might sometimes have entered water to thermoregulate. It is estimated to have walked at about 1.49 meters per second (5.4 km/h; 3.3 mph).

Further information: Sauropod neck posture

Scientists still debate how the necks of brachiosaurids were held, with ideas ranging from nearly upright to horizontal. Brachiosaurus is thought to have been a high browser, feeding on leaves above 5 meters (16 ft) and eating plants like ginkgos, conifers, and large cycads. Like other sauropods, it likely used hindgut fermentation to digest its food. It shared its habitat with many other sauropods. Feeding high up helped it avoid direct competition through niche partitioning. Tall conifers like Sequoiadendron were rare, which may explain why it was less common in its ecosystem than the related Giraffatitan. Its nostrils were likely on top of its head, as the bony neosauropods nasal openings suggest. However, Lawrence M. Witmer argued that living vertebrate land animals have nostrils at the front. Like other sauropods, Brachiosaurus was probably homeothermic and endothermic, which means it controlled its body temperature ("warm-blooded") through a high basic metabolic rate. Its air sacs worked like those of birds, moving air in one direction through the lungs. This helped it take in more oxygen and remove carbon dioxide, making gas exchange more effective. Air sacs also reached into the bones, creating pleurocoels. A young skeleton studied by Carballido and colleagues in 2012 showed how the body changed as it grew, through ontogeny. In 2005, Jack Horner estimated that Brachiosaurus reached adulthood in less than 20 years.

Paleoecology

Brachiosaurus is known only from the Morrison Formation of western North America. This rock layer formed during the Kimmeridgian and Tithonian ages, about 155.6 to 145.5 million years ago. The land was probably semiarid, with clear wet and dry seasons and flat floodplains. Several other sauropods lived there, including Apatosaurus, Barosaurus, Camarasaurus, Diplodocus, Haplocanthosaurus, and Supersaurus. Brachiosaurus was one of the less common ones. In a 2003 survey of more than two hundred fossil sites, John Foster found 12 specimens. This was close to Barosaurus (13) and Haplocanthosaurus (12), but far fewer than Apatosaurus (112), Camarasaurus (179), and Diplodocus (98). Brachiosaurids could chew tough plants with their broad-crowned teeth. This may have let them live in more kinds of plant areas than diplodocids could. Camarasaurids, which had similar teeth, were also widespread and moved with the seasons. Brachiosaurids may have done the same.

Other dinosaurs from the Morrison Formation include the meat-eating theropods Koparion, Stokesosaurus, Ornitholestes, Ceratosaurus, Allosaurus, and Torvosaurus. There were also plant-eating ornithischians like Camptosaurus, Dryosaurus, Othnielia, Gargoyleosaurus, and Stegosaurus. Allosaurus made up about 70 to 75 percent of the theropod fossils found. It was at the top trophic level of the Morrison food web. Ceratosaurus may have hunted large sauropods, including smaller Brachiosaurus individuals. Other animals of this time included ray-finned fish, frogs, salamanders, turtles like Dorsetochelys, sphenodonts, lizards, crocodylomorphs such as Hoplosuchus, and several pterosaur species like Harpactognathus and Mesadactylus. There were also common bivalves and water snails. Plants included green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Along rivers, there were gallery forests with tree ferns and ferns. Further from the water, there were fern savannas with a few trees, such as the Araucaria-like conifer Brachyphyllum.

Cultural significance

At first, Riggs tried to keep the find quiet. In a July 27, 1900 talk in Grand Junction, he explained how dinosaurs had changed over time. He did not mention that he had just found a special fossil. He worried that other museums might learn of the discovery and take the best fossils left behind. A week later, his host Bradbury wrote about the find in the local Grand Junction News. He called it one of the largest dinosaurs ever found. On August 14, The New York Times also reported the story. At that time, people were fascinated by sauropod dinosaurs because of their great size. Newspapers often made them seem even bigger. Riggs also stressed how enormous Brachiosaurus was. Replica skeletons of Brachiosaurus can be seen in Chicago, Illinois. One is outside the Field Museum, and another is inside O'Hare International Airport.

Brachiosaurus is often called one of the most famous dinosaurs. Most popular pictures are based on the African species B. brancai. That species has since been moved to its own genus, Giraffatitan. A main belt asteroid, 1991 GX₇, was named 9954 Brachiosaurus in 1991 to honor the genus. The dinosaur appeared in the 1993 movie Jurassic Park. It was the first computer generated dinosaur shown in the movie, and the effects were considered groundbreaking at the time. The movie's Brachiosaurus moved in a way based on how a giraffe walks, combined with the weight of an elephant. A later scene used an animatronic head and neck when the dinosaur interacted with human characters. The digital model was later the starting point for the ronto models in the 1997 special edition of Star Wars Episode IV: A New Hope.