Brachiosaurus altithorax

Brachiosaurus altithorax was a gigantic brachiosaurid sauropod from Late Jurassic Colorado. Its defining skeleton, FMNH P 25107, preserves the rear seven trunk vertebrae, sacrum, two front tail vertebrae, ribs and selected shoulder, hip and limb bones—but no skull or neck. The long humerus and proportions of the preserved torso show that the shoulders stood high and that a large share of the animal's mass rested over the forelimbs. Much of the familiar popular image of Brachiosaurus actually comes from the more complete Tanzanian animal formerly called Brachiosaurus brancai and now named Giraffatitan. Separating those genera makes B. altithorax both more anatomically distinctive and much less completely known than its fame suggests.

Its fossils occur between approximately 154 and 150 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Holotype FMNH P 25107 includes dorsal vertebrae six through twelve, the sacrum, the first two caudal vertebrae, ribs, a left coracoid, right humerus, right ilium and right femur. The humerus is slightly longer than the femur, creating the elevated shoulder line that inspired the name 'arm lizard'. Long, strongly opisthocoelous trunk vertebrae surrounded a deep chest, while air-filled internal spaces reduced skeletal weight. Taylor's 2009 reconstruction inferred a longer, deeper torso and probably a taller, more substantial tail than Giraffatitan. Because the type contains no cervical vertebrae or skull, the exact neck length, head shape and nostril position of B. altithorax are not directly known from its defining specimen.

• Humerus slightly longer than the femur, producing high shoulders
• Very deep trunk reflected in the name altithorax
• Long and strongly opisthocoelous rear dorsal vertebrae
• Air-filled chambers within the trunk vertebrae
• Large share of body mass carried over the forelimbs
• Longer, deeper torso than the closely related Giraffatitan
• Skull and complete neck absent from the holotype

In 1900, Elmer Riggs and H. William Menke of Chicago's Field Columbian Museum excavated a huge partial sauropod skeleton at Quarry 13 near Grand Junction, Colorado. The vertebral column lay inverted and partly articulated before a sandstone lens cut off the front of the carcass. Riggs recognized that the unusually long upper arm exceeded the femur and named Brachiosaurus altithorax in 1903; a detailed monograph followed in 1904. The quarry is now known as Riggs Hill. The original bones remain at the Field Museum, while public full-body mounts combine casts of the holotype with sculpted parts based on Giraffatitan.

Discovery credit: Elmer S. Riggs, H. William Menke.

Naming authors: Elmer S. Riggs.

Riggs Hill preserves the Brushy Basin Member of the Morrison Formation, deposited across seasonal river floodplains in western Colorado. Sand-rich channels alternated with mud-rich overbank areas, ponds and drier open ground. Conifers, cycads, ferns and horsetails supplied plant food. Brachiosaurus was rare within a community dominated by other sauropods such as Camarasaurus, Apatosaurus and Diplodocus and shared the region with Stegosaurus, small ornithopods and predators including Allosaurus.

Brachiosaurus was a terrestrial quadrupedal herbivore. Its elevated shoulders and inferred long neck made feeding above the normal reach of many Morrison herbivores plausible, but the absent neck and skull prevent a precise feeding-height reconstruction for B. altithorax itself. A tentatively referred skull has broad, spoon-shaped teeth and CT evidence suggests slower tooth replacement than in Camarasaurus or diplodocoids, potentially reflecting a different feeding strategy. No trackway, nest, stomach content or mass assemblage is securely attributable to the species, so gait speed, social structure and preferred plants remain unknown.

• Brachiosaurus means 'arm lizard' because its upper arm bone is longer than its thigh bone—the reverse of the usual sauropod proportion.
• The holotype was discovered in 1900 but received its scientific name in 1903 and a full monographic description in 1904.
• The defining skeleton has no skull or neck, despite the animal's instantly recognisable popular silhouette.
• The famous Field Museum and O'Hare mounts combine casts of American Brachiosaurus bones with sculpted anatomy based on African Giraffatitan.
• The unfused shoulder bones suggest the enormous holotype had not yet reached full skeletal maturity.

A research repository is not necessarily a public exhibit. Loan and display status can change, so check with the institution before visiting.

1. 01
   The Paleobiology Database

   Open source
2. 02
   Brachiosaurus altithorax, the largest known dinosaur

   Elmer S. Riggs · American Journal of Science · 1903

   Open source
3. 03
   Structure and relationships of opisthocoelian dinosaurs, Part II: The Brachiosauridae

   Elmer S. Riggs · Field Columbian Museum Geological Series · 1904

   Open source
4. 04
   A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914)

   Michael P. Taylor · Journal of Vertebrate Paleontology · 2009

   Open source
5. 05
   Redescription of brachiosaurid sauropod dinosaur material from the Upper Jurassic Morrison Formation, Colorado, USA

   Michael D. D'Emic, Matthew T. Carrano · The Anatomical Record · 2020

   Open source
6. 06
   Museum History: Elmer Riggs and the Brachiosaurus holotype

   Field Museum

   Open source
7. 07
   Why We Don't Dress Up SUE (Or Any Other Real Skeletons)

   Field Museum · 2016

   Open source
8. 08
   Dinosaur Trail Hikes in Western Colorado: Riggs Hill

   Julia McHugh · Museums of Western Colorado

   Open source