title: Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira.
abstract: The early evolutionary history of Ornithodira (avian-line archosaurs) has hitherto been documented by incomplete (Lagerpeton) or unusually specialized forms (pterosaurs and Silesaurus). Recently, a variety of Silesaurus-like taxa have been reported from the Triassic period of both Gondwana and Laurasia, but their relationships to each other and to dinosaurs remain a subject of debate. Here we report on a new avian-line archosaur from the early Middle Triassic (Anisian) of Tanzania. Phylogenetic analysis places Asilisaurus kongwe gen. et sp. nov. as an avian-line archosaur and a member of the Silesauridae, which is here considered the sister taxon to Dinosauria. Silesaurids were diverse and had a wide distribution by the Late Triassic, with a novel ornithodiran bauplan including leaf-shaped teeth, a beak-like lower jaw, long, gracile limbs, and a quadrupedal stance. Our analysis suggests that the dentition and diet of silesaurids, ornithischians and sauropodomorphs evolved independently from a plesiomorphic carnivorous form. As the oldest avian-line archosaur, Asilisaurus demonstrates the antiquity of both Ornithodira and the dinosaurian lineage. The initial diversification of Archosauria, previously documented by crocodilian-line archosaurs in the Anisian, can now be shown to include a contemporaneous avian-line radiation. The unparalleled taxonomic diversity of the Manda archosaur assemblage indicates that archosaur diversification was well underway by the Middle Triassic or earlier.
authors.affiliation: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA.
authors.forename: Sterling J
authors.initials: SJ
authors.lastname: Nesbitt
authors.forename: Christian A
authors.initials: CA
authors.lastname: Sidor
authors.forename: Randall B
authors.initials: RB
authors.lastname: Irmis
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Angielczyk
authors.forename: Roger M H
authors.initials: RM
authors.lastname: Smith
authors.forename: Linda A
authors.initials: LA
authors.lastname: Tsuji
mesh_terms: D000818:Animals; D001502:Beak / Q000033:anatomy & histology; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology* / Q000145:classification*; D004032:Diet / Q000662:veterinary; D025061:Dinosaurs / Q000033:anatomy & histology / Q000145:classification; D005121:Extremities / Q000033:anatomy & histology; D005580:Fossils; D005843:Geography; D007568:Jaw / Q000033:anatomy & histology; D010802:Phylogeny; D012104:Reptiles / Q000033:anatomy & histology* / Q000145:classification*; D012863:Skeleton; D013636:Tanzania; D014070:Tooth / Q000033:anatomy & histology