History of Classification
McIntosh 1990 included both these animals in the family Diplodocidae, subfamily Dicraeosaurinae, as they resemble the skull of Dicraeosaurus, although differing in certain details. Although the skull of Nemegtosaurus was found in the same formation as the headless skeleton of Opisthocoelicaudia, McIntosh rejects the possibility that they belong to the same animal, as Nemegtosaurus is diplodocid-like, but Opisthocoelicaudia camarasaur-like.
Upchurch 1995 placed the two genera in their own family, without giving a cladistic definition. However, the name first appeared in print a few weeks earlier in a paper by Barrett and Upchurch.
Wilson in 2002 referred the two genera to the family Nemegtosauridae, and on the basis of cladistic analysis, transferred them from the Diplodocoidea to the Titanosauria
Apesteguia (2004), in a paper describing a new Patagonian sauropod, Bonitasaura salgadoi, may have been the first to properly define the taxon, although without the use of cladistic analysis: the stemclade consisting of all titanosaurs more closely related to Nemegtosaurus than to Saltasaurus. He argued for a close relationship between Nemegtosaurus, Quaesitosaurus, Rapetosaurus, and Bonitasaura and referred to the previous phylogenetic analysis and use of Nemegtosauridae by Wilson (2002).
Upchurch et al. 2004 returned the Nemegtosauridae to the Diplodocoidea, and defined the former as the stem clade consisting of all Diplodocoidea closer to Nemegtosaurus than to Diplodocus. The only known species were Nemegtosaurus itself and Queasitosaurus.
Curry Rogers 2005, in one of the most complete cladistic analysis of the Titanosauria to date, returns both Nemegtosaurus and Quaesitosaurus to the titanosaurs, but rejects the validity of both the family Nemegtosauridae and the clade concepts given under that name. Quaesitosaurus is placed in the Saltasaurinae and Nemegtosaurus in a new, unnamed "Rapetosaurus clade" (which, under ICZN rules, would, if named, be termed subfamily Nemegtosaurinae or tribe Nemegtosaurini, depending on its position). She places Opisthocoelicaudia in a separate clade, the Opisthocoelicaudiinae. All three clades are included in the Saltasauridae (= Titanosauridae).
Unaware of Apesteguia's work, Wilson (2005) proposed a definition identical to his and further clarified the anatomy and relationships of the central genus Nemegtosaurus. He thus also defines the Nemegtosauridae as a stem-based clade that includes all titanosaurs more closely related to Nemegtosaurus than to Saltasaurus. He also suggests that Opisthocoelicaudia may eventually be shown to be a junior synonym of Nemegtosaurus.
Read more about this topic: Nemegtosauridae
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