We describe five partial middle and posterior dorsal vertebrae from an upper Kimmeridgian exposure of the Upper Jurassic Morrison Formation at the Carnegie Quarry of Dinosaur National Monument (Utah, USA), and refer these vertebrae to the poorly known diplodocid sauropod dinosaur genus Barosaurus Marsh, 1890. The small size and unfused neurocentral junctions of these vertebrae indicate that they belonged to a subadult individual and the osteologically youngest specimen yet described for the genus. We also provide corroborating evidence for the pneumatic hiatus previously hypothesized for Barosaurus and identify a previously unrecognized autapomorphy of the genus, the presence of an anterodorsallyoriented accessory lamina arising from the spinodiapophyseal lamina on dorsal vertebrae. Furthermore, we document a temporal uncoupling of neural spine lamination and pneumatization of the centrum, such that full development of the laminae preceded development of sharp-lipped pleurocoels in dorsal vertebrae.