The first "Barosaurus" remains were discovered in the Morrison Formation of South Dakota by Ms. E. R. Ellerman, postmistress of Pottsville, and excavated by Othniel Charles Marsh and John Bell Hatcher of Yale University in 1889. Only six tail vertebrae were recovered at that time, forming the type specimen (YPM 429) of a new species, which Marsh named "Barosaurus lentus", from the Classical Greek words "βαρυς" (') ("heavy") and "σαυρος" (') ("lizard"), and the Latin word "" ("slow"). The rest of the type specimen was left in the ground under the protection of the landowner, Ms Rachel Hatch, until it was collected nine years later, in 1898, by Marsh's assistant, George Rieber Wieland. These new remains consisted of vertebrae, ribs and limb bones. In 1896 Marsh had placed "Barosaurus" in the Atlantosauridae; in 1898 it was by him classified as a diplodocid for the first time. In his last published paper before his death, Marsh named two smaller metatarsals found by Wieland as a second species, "Barosaurus affinis", but this has long been considered a junior synonym of "B. lentus".