Apatosaurus ajax ah-PAT-oh-SOR-us

"Deceptive lizard"

Apatosaurus silhouette — silhouette · Elihu Guzmán Arroyo (CC-BY) via PhyloPic

Length: 22 m (72 ft) — About 22 m long, ~25 tonnes — heavier and stockier than Diplodocus.
Period: Late Jurassic (155–150 Mya)
Place: North America · Western United States
Food: Herbivore — Low and mid-height plants — peg teeth for stripping leaves and ferns.
Clade: Diplodocidae Dinosauria Saurischia Sauropodomorpha Sauropoda Whip-tailed sauropods: Diplodocus, Apatosaurus, Brontosaurus.

A robust, whip-tailed Jurassic giant whose name and missing skull both fooled science.

Apatosaurus was a thick-bodied cousin of Diplodocus, heavier and shorter overall but built on the same long-necked plan. For most of the 20th century it had the wrong head — early scientists mounted a Camarasaurus skull on Apatosaurus bones. The mistake wasn’t fully corrected in museums until the 1970s, after the right skull was identified.

More about Apatosaurus

Discovery

Named by Othniel Charles Marsh · 1877 · Morrison, in Jefferson County, Colorado, in the eastern foothills of the Rocky Mountains

Local miner Arthur Lakes and his friend Henry C. Beckwith found the first fossils in spring 1877 and wrote to both Marsh and Edward Drinker Cope. Marsh hired Lakes to collect the material, and a well-preserved partial postcranial skeleton (YPM VP 1860) was named Apatosaurus ajax in November 1877.

The name means "deceptive lizard." It comes from the Greek words for "deception" or "deceptive" and the word for "lizard."

Portrait of Othniel Charles Marsh — Othniel Charles Marsh Professor of Paleontology at Yale University who described and named the first known species, and who chose the name based on the animal's unusual chevron bones.

What was that long whip-like tail actually for?

The text notes the tail may have been used as a whip to create loud noises. A more recent suggestion is that it served instead as a sensory organ. Both ideas are offered as possibilities rather than settled fact.

For the curious → Is Apatosaurus really separate from Brontosaurus?

Under the rules of the ICZN, the older name Apatosaurus has priority, so Brontosaurus was long treated as a junior synonym. However, Robert T. Bakker argued in the 1990s that A. ajax and A. excelsus were distinct enough for the latter to merit a separate genus, so the split is debated.

What we know

Same family as Diplodocus, but heavier and more thickly built.
Famously had the wrong skull on display for almost 90 years.
Massive vertebrae with deep hollows to save weight on the long neck.
Found in the Morrison Formation across the American west.

What we guess

Whether Apatosaurus and Brontosaurus are separate genera (the 2015 study says yes).
Whether the thick neck was used in male-on-male combat, slamming necks together.
Lifespan estimates range from 70 to 100 years based on bone histology.