 There were two strongly supported placements found for "Amphicoelias", as the basalmost genus within Apatosaurinae, and as an apatosaurine nested between "Brontosaurus excelsus" and "Brontosaurus yahnahpin".There were no unambiguous characters found to support an assignment within Apatosaurinae, so the authors instead opted to classify "Amphicoelias" as the basalmost diplodocid outside Apatosaurinae and Diplodocinae, having primitive features, minimal significant differences from either subfamily, a slightly more parsimonious apatosaurine phylogeny, but a slightly more diplodocine morphology.The following cladogram of the Diplodocidae follows the species-level agreement tree of Tschopp "et al."