The phylogenetic relationships of the Nemegtosauridae (Saurischia, Sauropoda)

The phylogenetic relationships of the Late Cretaceous Mongolian sauropods N emegtosaurus and Quaesitosaurus are controversial. Previous studies have in terpreted nemegtosaurids as members of the Diplodocoidea or Titanosauroidea . The conflicting character sets, which support these alternative hypothese s, are examined and critically evaluated. Only two derived states are uniqu e to nemegtosaurids and one or more titanosauroid taxa. At least five synap omorphies (e.g., transverse narrowing of the rostral end of the premaxilla, elongation of the subnarial foramen, and extreme retraction of the externa l nares) support the inclusion of the nemegtosaurids within the Diplodocoid ea, but the presence of the plesiomorphic state cannot always be confirmed in higher titanosauroids. Cladistic analysis supports the hypothesis that n emegtosaurids are the sister-group to a clade containing Rebbachisaurus, di craeosaurids and diplodocids. Antarctosaurus wichmannianus, a problematic t axon from the Late Cretaceous of Argentina, is placed within the Diplodocoi dea las the sister-taxon to the Nemegtosauridae) by the analysis. Several l ines of evidence suggest, however, that: the holotype of A. wichmannianus i s composed of an amalgam of different sauropod taxa and does not represent a reliable operational taxonomic unit. The distribution of derived states i ndicates a considerable amount of convergence between the skulls of diplodo coids and higher titanosauroids. Contrary to previous suggestions, many of the apomorphic features of the diplodocoid skull were acquired gradually an d do not represent a single complex character.