Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira

• Published in: Nature (London) 2010-03, Vol.464 (7285), p.95-98
• Main Authors: Angielczyk, Kenneth D, Tsuji, Linda A, Nesbitt, Sterling J, Sidor, Christian A, Smith, Roger M. H, Irmis, Randall B
• Format: Article
• Language: English
• Subjects: 631/181/759/2467; Animals; Beak - anatomy & histology; Biological Evolution; Birds - anatomy & histology; Birds - classification; Diet - veterinary; Dinosaurs; Dinosaurs - anatomy & histology; Dinosaurs - classification; Distribution; Earth sciences; Earth, ocean, space; Environmental aspects; Exact sciences and technology; Extremities - anatomy & histology; Fossils; Geography; Humanities and Social Sciences; Jaw - anatomy & histology; letter; multidisciplinary; Natural history; Paleontology; Phylogeny; Reptiles; Reptiles - anatomy & histology; Reptiles - classification; Science; Science (multidisciplinary); Skeleton; Tanzania; Taxa; Teeth; Tooth - anatomy & histology; Triassic; Vertebrate paleontology
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature08718

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.