Barosaurus is a genus of large sauropod dinosaur that lived in present-day North America during the Late Jurassic period. It was described by American paleontologist Othniel Charles Marsh in 1890. The genus contains a single valid species, B. lentus, though the African species B. africanus has since been assigned to Tornieria. B. lentus' holotype (name-bearing) specimen initially consisted of four caudal (tail) vertebrae that were found in 1889 in South Dakota by Ms. Isabella R. Ellerman. However, later excavations found vertebrae, limb bones, and other postcranial remains from the same individual. Several other specimens have been collected outcrops of the Morrison Formation in Utah and Montana, though fossils potentially from the genus have been reported from Colorado, Wyoming, and Oklahoma as well. Barosaurus is known from the Kimmeridgian stage of the Late Jurassic period, which dates to around 152 to 150 million years ago.