Apatosaurus ajax
Apatosaurus excelsius
Apatosaurus louisae

Apatosaurus, formerly known as Brontosaurus, is a genus of sauropod dinosaurs that lived about about 140 million years ago, during the Jurassic period. They were some of the largest land animals that ever existed, about 4.5 metres (15 feet) tall at the hips, with a length of up to 25m (80 feet) and a mass up to 35 metric tonnes (40 tons).

Environment

It was believed that Apatosaurus was too massive to support its own weight on dry land, so it was theorized that the sauropod must have lived partly submerged in water, perhaps in a swamp. Recent findings do not support this. In fact, like its relative Diplodocus, Apatosaurus was a grazing animal with a very long neck, and a long tail that served as a counterweight. Fossilized footprints indicate that it probably lived in herds. To aid in processing food, Apatosaurus may have swallowed gizzard stones (gastroliths) the same way many birds do today — its jaws alone were not sufficient to chew tough plant fibers.

Classification and history

In 1877, Othniel Charles Marsh published notes on his discovery of the Apatosaurus, and then in 1879 described another, more complete dinosaur — the Brontosaurus. In 1903, it was discovered that the apatosaur was in fact a juvenile brontosaur, and the name Apatosaurus, having been published first, was deemed to have priority as the official name; Brontosaurus was relegated to being a synonym. The name was not formally removed from the records of paleontology until 1974.

Fossils of this animal have been found in Nine Mile Quarry and Bone Cabin Quarry in Wyoming, and at sites in Colorado, Oklahoma, Utah, USA.

Species

• A. ajax is the type species of the genera, and was named by the paleontologist Othniel Charles Marsh in 1877 after Ajax, the hero from Greek mythology. It is the holotype for the genera, and two partial skeletons have been found including part of a skull.
• A. excelsus (originally Brontosaurus) was named by Marsh in 1879. It is known from six partial skeletons, including part of a skull, which have been found in the United States, in Oklahoma, Utah, and Wyoming.
• A. louisae was named by William Holland, in 1915. It is known from one partial skeleton, which was found in Colorado, in the United States.

Robert T. Bakker made A. yahnahpin the type species of a new genus, Eobrontosaurus in 1998, so it is now properly Eobrontosaurus yahnahpin. It was named by Filla, James and Redman in 1994. One partial skeleton has been found in Wyoming.