The holotypic materials of the sauropod ‘Morosaurus' agilis—a partial skull, proatlases, and first three cervical vertebrae—have been a taxonomic and phylogenetic mystery since their initial description by O. C. Marsh in 1889 and redescription by C. W. Gilmore in 1907. Although most species of Morosaurus were subsumed into Camarasaurus in 1919, ‘M.' agilis was left in the defunct genus without a proper taxonomic assignment. Similarities have been noted between ‘M.' agilis and other Morrison Formation sauropod taxa, including Camarasaurus, Haplocanthosaurus, Diplodocus, and Brachiosauridae, but it had yet to be included in a phylogenetic analysis. Here we present new data following additional preparation and study that suggest ‘M.' agilis is a basally diverging member of Dicraeosauridae, along with the recently described sauropod Kaatedocus. Based upon its recovery as a distinct taxon, we propose that ‘M.’ agilis receive the new generic name Smitanosaurus, yielding the new combination Smitanosaurus agilis.