The skull and neck of a new flagellicaudatan sauropod from the Morrison Formation and its implication for the evolution and ontogeny of diplodocid dinosaurs.

A new taxon of diplodocid sauropod,Kaatedocus siberigen. et sp. nov., is recognized based on well-preserved cervical vertebrae and skull from the Morrison Formation (Kimmeridgian, Late Jurassic) of northern Wyoming, USA. A phylogenetic analysis places it inside Diplodocinae (Sauropoda: Flagellicaudata: Diplodocidae), as a sister taxon to a clade unitingTornieria africanaand the classical diplodocinesBarosaurus lentusandDiplodocus. The taxon is diagnosed by a unique combination of plesiomorphic and derived traits, as well as the following unambiguous autapomorphies within Diplodocidae: frontals separated anteriorly by a U-shaped notch; squamosals restricted to the post-orbital region; presence of a postparietal foramen; a narrow, sharp and distinct sagittal nuchal crest; the paired basal tuber with a straight anterior edge in ventral view; anterior end of the prezygapophyses of mid- and posterior cervical vertebrae is often an anterior extension of the pre-epipophysis, which projects considerably anterior to the articular facet; anterodorsal corner of the lateral side of the posterior cervical vertebrae marked by a rugose tuberosity; posterior margin of the prezygapophyseal articular facet of posterior cervical vertebrae bordered posteriorly by conspicuous transverse sulcus; posterior cervical neural spines parallel to converging. The inclusion ofK. siberiand several newly described characters into a previously published phylogenetic analysis recovers the new taxon as basal diplodocine, which concurs well with the low stratigraphical position of the holotype specimen.DinheirosaurusandSupersaurusnow represent the sister clade toApatosaurusand Diplodocinae and therefore the most basal diplodocid genera. The geographical location in the less known northern parts of the Morrison Fm., whereK. siberiwas found, corroborates previous hypotheses on faunal provinces within the formation. The probable subadult ontogenetic stage of the holotype specimen allows analysis of ontogenetic changes and their influence on diplodocid phylogeny. [ABSTRACT FROM PUBLISHER]