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Apatosaurus excelsius (Jurassic period, about 140 million years ago) was a Sauropod dinosaur, one of the largest land animals that ever existed, about 15 feet (4.6 m) tall at the hips, with a length of up to 35 m. Until recently it was also called Brontosaurus, but the name "Apatosaurus" is technically correct.

In 1877, Othniel Charles Marsh published notes on his discovery of the "Apatosaurus", and then in 1879 described another, larger dinosaur - the "Brontosaurus". In 1903 it was discovered that the Apatosaur was in fact a juvenile Brontosaurus, and the name "Apatosaurus", having been published first, was deemed to have priority as the official name; "Brontosaurus" was relegated to being a synonym.

Like its relative Diplodocus, Apatosaurus was a grazing animal with a very long neck, and a long tail as a counterweight. To aid in processing tough plant fibers, Apatosaurus swallowed gizzard stones (gastroliths).

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

Other Apatosaurus species: