Dinosaurs A to Z
Brachiosauridae

Brachiosaurus altithorax BRAK-ee-oh-SOR-us

"Arm lizard"

Brachiosaurus silhouette
You 1.8 m (5.9 ft) tall
Brachiosaurus 22 m (72 ft) long
as long as 13 people standing fingertip to fingertip
silhouette · Will Toosey (CC-BY) via PhyloPic
Length
22 m (72 ft) — About 22 m long, 12 m tall, ~30 tonnes — front legs longer than the back.
Period
Late Jurassic (156–145 Mya)
Place
North America · Western United States
Food
Herbivore — High-growing leaves and branches — neck held up like a giraffe to reach tree tops.
Clade
Brachiosauridae Dinosauria Saurischia Sauropodomorpha Sauropoda Giraffe-postured sauropods with long front legs.

The "arm lizard" whose front legs were longer than its back, tilting its body skyward.

Brachiosaurus was built like a giraffe scaled up to bus-sized. Its front legs were longer than its back legs, so its body sloped upward and its neck rose almost vertically, letting it reach treetops other sauropods couldn’t touch. The huge nostrils on top of the skull confused early scientists, who thought it might have lived in water — modern reconstructions put them firmly on land.

More about Brachiosaurus

Discovery

Named by Elmer S. Riggs · 1903 · The Colorado River valley in western Colorado, United States

American paleontologist Elmer S. Riggs first described the dinosaur in 1903 from fossils found in western Colorado, naming it Brachiosaurus altithorax. He described the holotype material in more detail in 1904, focusing especially on the vertebrae.

The genus name is Greek for "arm lizard", a reference to its proportionately long arms. The species name, altithorax, means "deep chest".

Elmer S. Riggs. An American paleontologist who first described Brachiosaurus and named the family Brachiosauridae, of which it is the namesake genus.

Was Brachiosaurus warm-blooded or cold-blooded?
It has been used as an example of a dinosaur that was most likely ectothermic, because its large size implied a need for sufficient forage. More recent research, however, suggests it was warm-blooded. The question is still being weighed against its enormous body size and dietary needs.
How do we even know how big Brachiosaurus was if it is so rare?
Most size estimates for Brachiosaurus altithorax are based on the better-known relative Giraffatitan, since Brachiosaurus is known from much less complete material. The type specimen also appears to be a subadult, shown by an unfused suture between the coracoid and scapula, so it probably does not reflect the genus's maximum size.

What we know

  • Front legs longer than back legs, giving the body an upward slope.
  • Reached about 12 m tall at the head — taller than a 3-story building.
  • Lived alongside Allosaurus, Stegosaurus, Diplodocus, and Apatosaurus.
  • Lighter, hollow-boned skeleton compared to other giant sauropods of its time.

What we guess

  • Exact neck posture — early models held it nearly vertical; newer work suggests less extreme.
  • How much it ate per day — estimates range from 200 to 400 kg of plants.
  • How it pumped blood up to the head — required either a huge heart or a clever blood-pressure system.