Brachiosaurus was a giant sauropod of the Late Jurassic Period in western North America. Its forelimbs were longer than its hind limbs, lifting the shoulders and the base of the neck. This unusual build made it a high browser rather than simply a heavier version of Diplodocus.
Only one species, Brachiosaurus altithorax, is generally recognised. Its type skeleton lacks the skull, neck, most of the tail and ends of the limbs. The famous, more complete Tanzanian animal formerly called Brachiosaurus brancai is now placed in the separate genus Giraffatitan.
Quick facts
| Scientific name | Brachiosaurus altithorax Riggs, 1903 |
|---|---|
| Group | Dinosauria, Saurischia, Sauropodomorpha, Sauropoda, Brachiosauridae |
| Age | Late Jurassic, approximately 154–150 million years ago |
| Range | Western United States, Morrison Formation |
| Length | Approximately 18–22 m |
| Mass | About 25–35 tonnes, depending on reconstruction |
| Diet | Herbivorous, adapted for high browsing |
| Locomotion | Terrestrial quadruped |
| Fossil record | An incomplete postcranial type skeleton and disputed referred bones; skull and neck absent from the holotype |
What belongs to Brachiosaurus?
Vertebrae, ribs, shoulder, upper arm, pelvis and thigh establish the deep torso and long forelimb. They do not preserve the complete body.
A North American brachiosaurid skull is informative, but limited overlap prevents a completely secure assignment to B. altithorax.
The Tanzanian skeleton differs in more than two dozen overlapping anatomical features. It should guide reconstruction without being copied directly.
Length and mass depend on restoring the absent neck and tail and choosing a body outline around the preserved bones.
Name and discovery
Brachiosaurus means “arm lizard”, a reference to the strikingly long upper arm. The species name altithorax means “deep chest” and reflects enormous ribs, one of which measures about 2.75 metres.
An expedition from Chicago’s Field Columbian Museum found the type skeleton near Grand Junction, Colorado, in 1900. Elmer Riggs announced the find in 1901, named B. altithorax in 1903 and published a fuller description in 1904.
The specimen, FMNH PR 25107, includes seven rear dorsal vertebrae, the sacrum, the first two tail vertebrae, part of the shoulder girdle, a right upper arm, pelvic elements, a thigh bone and several huge ribs. It does not include a skull, neck vertebrae, most of the tail or complete hands and feet.
Classification
Brachiosaurus belongs to Brachiosauridae within Macronaria and Titanosauriformes. Brachiosaurids were relatives of the lineage that later produced titanosaurs, but they were not themselves members of Titanosauria. Their exact family tree varies between analyses.
The group is recognised by a combination of vertebral, limb and torso features rather than tall shoulders alone. Its place among sauropods is shown in the guide to dinosaur classification.
Brachiosaurus is not Giraffatitan
Werner Janensch named Tanzanian material Brachiosaurus brancai in 1914. Because it was much more complete than the American skeleton, it supplied the skull, neck and many proportions in museum mounts and books for most of the twentieth century.
Gregory Paul proposed the name Giraffatitan for the African form in 1988. A detailed 2009 comparison by Michael Taylor identified at least 26 differences in overlapping bones, supporting Giraffatitan brancai as a separate genus. It was generally more slender, with different vertebrae, shoulder and pelvic anatomy; Brachiosaurus had a longer, deeper trunk and probably a more substantial tail.
This separation does not make old restorations useless. Giraffatitan remains a close relative that helps fill gaps, but its bones cannot be presented as direct fossils of the North American genus.
Other referred fossils and the missing skull
Isolated upper arms, shoulder bones, vertebrae, pelvic elements and a partial skull from Felch Quarry have been assigned to Brachiosaurus or compared with it. Attribution is difficult when the referred specimen and holotype preserve few of the same bones.
The skull USNM 5730 is clearly brachiosaurid and has been prepared and scanned in detail. Yet the holotype has no skull for a direct comparison, so assigning it specifically to B. altithorax remains uncertain. The exact head shape of the named species is therefore less secure than popular images suggest.
Length, mass and proportions
Most reconstructions place Brachiosaurus around 18–22 metres long. A useful mass range is about 25–35 tonnes. A three-dimensional volumetric model produced about 28.7 tonnes for the holotype and suggested that it might not have been fully mature because parts of the shoulder girdle were not completely fused.
Results change with the restored length of the absent neck and tail, the width of soft tissues and assumed body density. Brachiosaurus could have outweighed the more slender Giraffatitan at a similar height. The size comparison keeps those assumptions visible.
The forelimbs were conspicuously longer than the hind limbs, so the shoulders stood above the hips. The torso was deep, broad and relatively long. Air-filled spaces invaded vertebrae and some ribs, reducing skeletal mass without making the whole animal lightweight.
Neck, head and high browsing
No neck vertebra belongs to the holotype. Close relatives indicate a long neck carried with a substantial upward component in its usual range of motion. Long forelimbs already raised the neck base, allowing access to foliage beyond the reach of many neighbouring herbivores.
A permanently vertical swan-like pose is not demonstrated. Joints, muscles and other soft tissues limited movement, and the head still had to reach water or lower plants. High browsing describes an advantage and a feeding envelope, not one frozen posture.
The probable brachiosaurid skull had a high arch around the bony nasal openings, a broad muzzle and strong spoon-shaped teeth extending along much of the front jaw. High bony openings do not mean the fleshy nostrils formed a snorkel on top of the head.
Diet, movement and breathing
Brachiosaurus was herbivorous. Its teeth could crop conifer shoots, ginkgo foliage, cycads, ferns and other plants, but did not grind food like mammalian molars. Microbes in a large digestive system carried out most breakdown. There is no evidence that it ate only treetops or one plant species.
The animal walked on four column-like limbs. Its hands were near-vertical weight-bearing structures with reduced fingers and a large first-digit claw. No trackway can be assigned securely to the genus, so exact speed and gait are modelled from anatomy rather than measured.
Air spaces created by extensions of a bird-like respiratory system entered the skeleton. They reduced the cost of a huge neck and trunk and supported efficient breathing. They did not make an animal weighing tens of tonnes buoyant, nor support the old idea of a deep-water lifestyle.
Morrison environment and unknown behaviour
The Morrison Formation records river plains, seasonal channels, lakes, woodland patches and open areas. Brachiosaurus lived in the broad regional fauna with Diplodocus, Camarasaurus, Stegosaurus and predators such as Allosaurus. Differences in tooth form and feeding height helped several huge herbivores share ecosystems.
Direct evidence for social life is sparse. Sauropod trackways can record group movement, but usually cannot identify the maker to genus. No egg, nest or hatchling is securely tied to B. altithorax. Herd structure, long-distance migration, courtship, parental care, colours and calls remain unknown.
Evidence, inference and reconstruction
| Evidence level | Examples |
|---|---|
| Directly preserved | Deep ribs, part of the trunk, shoulder and pelvic elements, a long upper arm, thigh bone and pneumatic skeletal structures |
| Strong inference | High shoulders, quadrupedal movement, herbivory, high browsing and an air-sac respiratory system |
| Plausible but unresolved | Exact neck posture, mature body mass, details of the North American skull and social grouping |
| Unknown | Colour, calls, courtship, migration, parental care and the complete proportions of the named species |
Frequently asked questions
When did Brachiosaurus live?
Brachiosaurus lived during the Late Jurassic, approximately 154–150 million years ago, in western North America.
How large was Brachiosaurus?
The American species is commonly reconstructed at about 18–22 metres long and roughly 25–35 tonnes, with uncertainty because much of the skeleton is missing.
Is the famous African skeleton Brachiosaurus?
No. The well-known Tanzanian material once called Brachiosaurus brancai is now generally recognised as the separate genus Giraffatitan.
Why were its shoulders so high?
Its upper arm and forelimbs were proportionally long. They raised the shoulder girdle above the hips and increased the height from which the neck could reach vegetation.