Dinosaur polyphyly and the classification of Archosaurs and birds

Abstract

Evidence relating to dinosaur origins, dinosaur physiology and bird ancestry is reviewed. The thesis that birds evolved directly from theropod dinosaurs is perfectly acceptable. There is no convincing evidence to support the view that dinosaurs were endotherms, with an avian system of thermo-regulation, and the biological success of dinosaurs is explicable in terms of straightforward anatomical adaptations (particularly the development of an enarthrodial hip joint). There is considerable evidence in favour of dinosaurian polyphyly. The dinosaur order Saurischia is apparently diphyletic - sauropods and prosauropods having descended from proterosuchian thecodonts, and theropods probably having evolved from pseudosuchian thecodonts. The ancestry of the dinosaur order Ornithischia remains a matter for speculation. The tarsal and pelvic structures of pseudosuchian thecodonts are not irreconcilable with those of dinosaurs, and the differences that do exist are not sufficient to debar pseudosuchians from consideration as dinosaur ancestors. The common pattern of limb joint structure in Triassic dinosaurs is not conclusive evidence of dinosaur monophyly, and is probably nothing more than a consequence of parallel evolution across the thecodontian- dinosaurian boundary. A new classification of archosaurs and birds is presented, wherein the theropod ancestors of birds are transferred to the class Aves while all other dinosaurs (sauropods, prosauropods and ornithischians) are retained in the reptilian subclass Archosauria. This scheme places full emphasis on the dinosaurian ancestry of birds but still manages to retain the stability of conventional classifications.