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.

[2] 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.

[8] During a prospecting trip on horseback, Riggs's field assistant Harold William Menke found the humerus of FMNH P 25107,[4] on July 4,[9] exclaiming it was "the biggest thing yet!".

[13] 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 5,700 kilograms (12,500 lb).

[5] 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.

[12] 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,[18] today catalogued as FMNH P25112 and identified as a Brontosaurus exemplar.

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".

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).

[32] 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 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.

[5][24] 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.

[54] B. nougaredi was in 2004 considered to represent a distinct, unnamed brachiosaurid genus,[51] 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.

[51] 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.

[67][68] 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.

[30][34] The dorsal and lateral temporal fenestrae (openings at the upper rear and sides of the skull) were large, perhaps due to the force imparted there by the massive jaw adductor musculature.

[76] Since the 1990s, computer-based cladistic analyses allow for postulating detailed hypotheses on the relationships between species, by calculating those trees that require the fewest evolutionary changes and thus are the most likely to be correct.

[49] This cladogram follows that published by Michael D. D'Emic in 2012:[76] Europasaurus Giraffatitan Brachiosaurus Abydosaurus Cedarosaurus Venenosaurus Cladistic analyses also allow scientists to determine which new traits the members of a group have in common, their synapomorphies.

[5] 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.

[12] Though Riggs's 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.

Christian and Dzemski (2007) estimated that the middle part of the neck in Giraffatitan was inclined by sixty to seventy degrees; a horizontal posture could be maintained only for short periods of time.

[60] 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.

[91] 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.

[99] Sander (2010) found that these calculations were based on incorrect body mass estimates and faulty assumptions on the available cooling surfaces, as the presence of large air sacs was unknown at the time of the study.

[100] The ontogeny of Brachiosaurus has been reconstructed by Carballido and colleagues in 2012 based on Toni (SMA 0009), a postcranial skeleton of a young juvenile with an estimated total body length of just 2 meters (6.6 ft).

The spinodiapophyseal lamina or "SPOL", the ridge normally running from each side of the neural spine toward each diapophysis, the transverse process bearing the contact facet for the upper rib head, is totally lacking.

[27] If the large foot reported from Wyoming (the northernmost occurrence of a brachiosaurid in North America) did belong to Brachiosaurus, the genus would have covered a wide range of latitudes.

[39] Other dinosaurs known from the Morrison Formation include the predatory theropods Koparion, Stokesosaurus, Ornitholestes, Ceratosaurus, Allosaurus, Torvosaurus and Saurophaganax, as well as the herbivorous ornithischians Camptosaurus, Dryosaurus, Othnielia, Gargoyleosaurus and Stegosaurus.

[92] Other vertebrates that shared this paleoenvironment included ray-finned fish, frogs, salamanders, turtles like Dorsetochelys, sphenodonts, lizards, terrestrial and aquatic crocodylomorphs such as Hoplosuchus, and several species of pterosaur like Harpactognathus and Mesadactylus.

When reading a lecture to the inhabitants of Grand Junction, illustrated by lantern slides, on July 27, 1900, 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.