DOI: 10.6084/m9.figshare.30327866
title: Critical reappraisal of a putative dicraeosaurid sauropod dinosaur from the Middle Jurassic of Gondwana and a revised view of diplodocoid evolutionary relationships and biogeography

description: Diplodocoidea is a diverse clade of sauropod dinosaurs that comprises three major lineages: Diplodocidae, Dicraeosauridae and Rebbachisauridae, with the former two united as Flagellicaudata. There has been a recent spate of newly described diplodocoid taxa, as well as additional information on existing species. Of particular significance is <i>Tharosaurus indicus</i>, from the Middle Jurassic of India, which was argued to represent a dicraeosaurid and to potentially evidence a Gondwanan origin for Flagellicaudata. Here, we critically reappraise the anatomy of <i>Tharosaurus</i> and use new morphological data to re-evaluate the phylogenetic relationships and biogeographical history of Diplodocoidea. We incorporated <i>Tharosaurus</i> and 12 diplodocoid operational taxonomic units (OTUs) into the largest existing character matrix for eusauropods, added new characters, and revised the characters and scores for previously included OTUs. The final matrix (563 characters scored for 139 OTUs) includes 38 uncontroversial diplodocoids. Topological results from phylogenetic analyses under maximum parsimony are sensitive to the application of equal versus extended implied weighting, but consistently agree that <i>Tharosaurus</i> is an indeterminate eusauropod that lacks diplodocoid synapomorphies. Favouring results produced under extended implied weighting with a concavity constant of 12, we present a revised view of diplodocoid relationships, provide new diagnoses and illustrate synapomorphies of major clades. The Upper Jurassic Morrison Formation sauropod <i>Haplocanthosaurus</i> is recovered as a non-diplodocimorph diplodocoid. In contrast with previous analyses, the Middle Jurassic Chinese diplodocoid <i>Lingwulong</i> is placed as a stem flagellicaudatan, outside of the diplodocid–dicraeosaurid split. We recover a diverse Dicraeosauridae that includes three Morrison Formation OTUs (<i>Smitanosaurus</i>, <i>Suuwassea</i>, BYU 17096) forming its earliest-branching members. Phylogenetically more-nested dicraeosaurids are all Late Jurassic–Early Cretaceous Gondwanan taxa, for which we formally define the clade Dicraeosaurinae. Diplodocidae includes the Morrison Formation taxon <i>Kaatedocus</i> as an early-diverging member, whilst <i>Tornieria</i> + <i>Leinkupal</i> forms a phylogenetically nested diplodocine clade. The Morrison Formation sauropod <i>Amphicoelias</i> is recovered as an early-branching diplodocoid of uncertain affinities, with equally parsimonious placement as a flagellicaudatan, rebbachisaurid, or non-diplodocimorph diplodocoid. Early-diverging rebbachisaurids include Gondwana taxa, as well as the earliest Cretaceous UK taxon <i>Xenoposeidon</i>. Within Khebbashia, Limaysaurinae is restricted to South America, whereas Rebbachisaurinae is present in Europe, North Africa, and South America. The stratigraphically youngest known flagellicaudatans are from the Barremian, whereas rebbachisaurids survived until the Turonian or Coniacian. The extinction of diplodocoids appears to have been spatiotemporally staggered. Our results reinforce the view that Flagellicaudata (and probably also Rebbachisauridae) likely originated in Laurasia, but the presence of diplodocoids across Eurasia in the Middle Jurassic suggests a potentially widespread distribution early in their evolutionary history that is likely obscured by sampling failure.