Suuwassea emilieae (Sauropoda: Flagellicaudata): Description and phylogenetic, geologic, and paleobiogeographic implications

A detailed osteology of cranial, axial, and appendicular elements of the flagellicaudatan sauropod Suuwassea emilieae Harris and Dodson, 2004, from the Upper Jurassic Morrison Formation is presented. In addition to numerous autapomorphies, it retains sauropod symplesiomorphies that are unexpected in a Late Jurassic taxon and thus sheds light on the evolutionary origins of the Flagellicaudata. A new phylogenetic analysis of the position of S. emilieae to other sauropods, particularly diplodocoids, recovered the taxon in a trichotomy with other flagellicaudatan terminal taxa (Diplodocidae and Dicraeosauridae), despite an expanded data matrix. This lack of resolution is probably due to a combination of missing data, character conflict, and poor incorporation of specimens referred to diplodocid taxa that differ from their genoholotypes and species holotypes. Paleobiogeographic implications for a basal flagellicaudatan in the Morrison Formation are discussed. S. emilieae is part of a larger pattern of "small" sauropods and other unusual taxa that have begun to be recovered from the northern region of the Morrison Formation. This small fauna may be evidence of a previously unrecognized faunal province within the otherwise largely homogenous Morrison Formation that preferred a wetter paleoenvironment close to the receding Sundance Sea. Lithostratigraphic, biostratigraphic, and radioisotopic intraformational correlations of the Morrison Formation are reviewed and examined for evidence that Morrison Formation deposition was time-transgressive; tests on sediment samples from the Upper Jurassic section that produced the holotype of S. emilieae are proposed to further examine this hypothesis.