The phylogenetic relationships of the Late Cretaceous Mongolian sauropods Nemegtosaurus and Quaesitosaurus are controversial. Previous studies have interpreted nemegtosaurids as members of the Diplodocoidea or Titanosauroidea. The conflicting character sets, which support these alternative hypotheses, are examined and critically evaluated. Only two derived states are unique to nemegtosaurids and one or more titanosauroid taxa. At least five synapomorphies (e.g., transverse narrowing of the rostral end of the premaxilla, elongation of the subnarial foramen, and extreme retraction fo the external nares) support the inclusion of the nemegtosaurids within the Diplodocoidea, but the presence of the plesiomorphic state cannot always be confirmed in higher titanosauroids. Cladistic analysis supports the hypothesis that nemegtosaurids are the sister-goup to a clade containing rebbachisaurus, dicraeosaurids and diplodocids. Antarctosaurus wichmannianus, a problematic taxon from the Late Cretaceous of Argentina, is placed within the Diplodocoidea (as the sister-taxon to the Nemegtosauridae) by the analysis. Several lines of evidence suggest, however, that the holotype of A. wichmannianus is composed of an amalgam of different sauropod taxa and does not represent a reliable operational taxonomic unit. The distribution of derived states indicates a considerable amount of convergence between the skulls of diplodocoids and higher titanosauroids. Contrary to previous suggestions, many of the apomorphic features of the diplodocoid skull were acquired gradually and do not represent a single complex character.
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