title: The earliest bird-line archosaurs and the assembly of the dinosaur body plan.
abstract: The relationship between dinosaurs and other reptiles is well established, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies or are known from poorly preserved or incomplete material. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird-crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously recognized. Moreover, several early dinosauromorphs that were previously used as models to understand dinosaur origins may represent specialized forms rather than the ancestral avemetatarsalian morphology.
authors.affiliation: Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA.
authors.forename: Sterling J
authors.initials: SJ
authors.lastname: Nesbitt
authors.affiliation: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Butler
authors.affiliation: School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.|CONICET-Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina.
authors.forename: Martín D
authors.initials: MD
authors.lastname: Ezcurra
authors.affiliation: Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Barrett
authors.affiliation: Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA.
authors.forename: Michelle R
authors.initials: MR
authors.lastname: Stocker
authors.affiliation: Integrative Research Center, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA.
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Angielczyk
authors.affiliation: Evolutionary Studies Institute, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa.|Iziko South African Museum, PO Box 61, Cape Town, South Africa.
authors.forename: Roger M H
authors.initials: RMH
authors.lastname: Smith
authors.affiliation: Burke Museum and Department of Biology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Christian A
authors.initials: CA
authors.lastname: Sidor
authors.affiliation: Department of Organismal Biology, Uppsala University, Norbyvägen 18A, 752 36 Uppsala, Sweden.
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Niedźwiedzki
authors.affiliation: Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya 123, Moscow 117997, Russia.|Kazan Federal University, Kremlyovskaya ul. 18, Kazan 420008, Russia.
authors.forename: Andrey G
authors.initials: AG
authors.lastname: Sennikov
authors.affiliation: Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Charig
mesh_terms: D000489:Alligators and Crocodiles / Q000033:anatomy & histology / Q000145:classification; D000818:Animals; D001717:Birds / Q000033:anatomy & histology / Q000145:classification*; D025061:Dinosaurs / Q000033:anatomy & histology* / Q000145:classification*; D005580:Fossils; D006614:Hindlimb / Q000033:anatomy & histology; D010802:Phylogeny; D012863:Skeleton / Q000033:anatomy & histology; D013636:Tanzania