The systematics (evolutionary relationships) of Diplodocidae are becoming better established. "Diplodocus" has long been regarded as the closest relative of "Barosaurus". "Barosaurus" is monospecific, containing only the type species, "B. lentus", while at least three species belong to the genus "Diplodocus". Another diplodocid genus, "Seismosaurus", is considered by many paleontologists to be a junior synonym of "Diplodocus" as a possible fourth species. "Tornieria" (formerly "Barosaurus" "africanus") and "Australodocus" from the famous Tendaguru Beds of Tanzania in eastern Africa have also been classified as diplodocines. With its elongated neck vertebrae, "Tornieria" may have been particularly closely related to "Barosaurus". The other subfamily of diplodocids is Apatosaurinae, which includes "Apatosaurus" and "Supersaurus". The early genus "Suuwassea" is considered by some to be an apatosaurine, while others regard it as a basal member of the superfamily Diplodocoidea. Diplodocid fossils are found in North America, Europe, and Africa. More distantly related within Diplodocoidea are the families Dicraeosauridae and Rebbachisauridae, found only on the southern continents.