Rediscovery of the type localities of the Late Cretaceous Mongolian sauropods Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii: Stratigraphic and taxonomic implications

In 1965, the Polish-Mongolian Palaeontological Expeditions recovered two sauropods from the Nemegt Formation of the Nemegt Basin, Mongolia (Kielan-Jaworowska and Dovchin 1968). One specimen, a nicely preserved, complete skull that in 1971 became the holotype of Nemegtosaurus mongoliensis, was found in Central Sayr at the Nemegt Locality. The other was found at Altan Uul IV and is a nearly complete postcranial skeleton lacking only the skull and neck. In 1977, this skeleton became the holotype of Opisthocoelicaudia skarzynskii. Nemegtosaurus and Opisthocoelicaudia were initially assigned to different sauropod higher taxa, Dicraeosaurinae and Camarasauridae respectively. However, since the late 1990s, both genera have been recognized as members of Titanosauria. Their coincident spatiotemporal distribution and non-overlapping skeletal parts have led to the persistent suspicion that they belong to the same species. Rediscovery of the original quarries and discovery of the postcranial remains attributable to the Nemegtosaurus holotype provides the first opportunity to directly compare these two taxa. Seven additional sites at the Nemegt locality preserve sauropod remains (including vertebrae, humeri, femora, pelvic elements, pedal phalanges, and unguals), and more than 20 sauropod footprint sites have been mapped. None of this material suggests that there is more than one sauropod taxon present in the Nemegt Formation. All localities occur within a discrete stratigraphic interval encompassing the uppermost Baruungoyot (footprints), Baruungoyot-Nemegt interfingering interval (Nemegtosaurus type), and lowermost Nemegt formations. Stratigraphic comparisons indicate the Opisthocoelicaudia locality at Altan Uul IV is within the lower beds of the Nemegt Formation. As sauropod remains are now documented for a total of 34 sites in the Nemegt Formation, a more refined stratigraphic framework may shed new light on the taxonomic inclusiveness of the sample.