Protoavis and the early evolution of birds

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Abstract

Protoavis texensis from the Late Triassic Dockum Group of Texas is identified as the "Urvogel", or the world's oldest known bird. It predates Archaeopteryx by 75 million years, thus pushing the origin of birds back to the Triassic, to the very dawn of the age of the dinosaurs. Yet Protoavis displays a number of avian features throughout the skeleton that place it closer to the ancestry of modern birds than Archaeopteryx. Protoavis was a pheasant-sized volant bird with a long, bony tail that lived in the tropical forests of Texas. The skull is lightly built, pneumatized, with an enormous orbit and inflated braincase. The dentition is reduced where the teeth are retained at the tip of the jaws, but the posterior teeth are lost. The quadrate is streptostylic with an orbital process, and the temporal region is modified in avian fashion to achieve prokinetic movement of the upper jaw for manipulation of food. A relatively large brain with audiovisual acuity indicates that Protoavis had begun to develop the highly specialized central nervous system associated with balance, coordination, muscular control, and proprioception. The well-developed stereoscopic vision and carnivorous teeth at the tip of the jaws evince its predatory nature. In contrast to Archaeopteryx, the postcranial skeleton of Protoavis exhibits several derived features such as heterocoelous cervical vertebrae with hypapophysis, highly enlarged neural canal, pneumatic scapula with a tapering posterior end, strut-like coracoid, triosseal canal for a supracoracoideus pulley, spring-like furcula with a large hypocleidium, keeled sternum, humerus with a well-developed head, bicipital crest and brachial depression, wing folding mechanism, fusion of ilium and ischium enclosing an ilioischiadic fenestra and renal fossa, presence of an antitrochanter around acetabular ring, lack of a distal symphysis on ischium and pubis, femur with lateral condyle having a trochlea for fibula, tibia with lateral cnemial crest, and absence of metatarsal V. The presence of feathers is inferred from the quill knobs on the metacarpals. The flight apparatus of Protoavis is highly derived with the development of strut-like coracoid, triosseal canal, keeled sternum, and spring-like furcula, indicating that the animal was capable of powered horizontal flight and could take off from the ground. A modified version of the arboreal theory is proposed where the crucial stage in the evolution of avian flight is believed to have occurred at the air/ water interface during landing. A sequence of four phylogenetic stages leading to avian flight is recognized: gliding flight, undulating flight, horizontal flight, and maneuvering flight. Numerical cladistic analysis of 84 characters has generated a highly corroborated hypothesis of the major clades of Mesozoic birds. Aves is used in the traditional sense here to include Archaeopteryx as the basal taxon. By using dormaeosaurid theropods such as Velociraptor as the outgroup, the analysis supports the monophyly of Aves. Phylogenetic analysis indicates that Archaeopteryx is the sister-group of all remaining avian taxa, or Metornithes. Mononykus and Avimimus are basal taxa of Metornithes and form the sister-group of all higher birds, Ornithothoraces. Ornithothoraces is supported by 26 synapomorphies. Within Ornithothoraces, Sinornis, Iberomesornis, Protoavis, Cathayornis, and Enatiornithes are successively closer to Ornithurae. The latter taxon is resolved into two lineages, Hesperornithiformes and Patagopteryx in one line, and Carinatae in the other. Carinatae, in turn, are subdivided into two sister-groups: one is formed by Ichthyornithiformes and Ambiortus, and the other by Neornithes or modern birds. The early evolution of birds shows a complex bush-like pattern of radiation where heterochrony seems to have played a major role in clade diversification and morphological discontinuities. Four ecological types can be recognized among Mesozoic taxa: basal land birds, shore birds, foot-propelled divers, and flightless terrestrial birds. Flightlessness occurred several times among avian lineages to save energy and generate morphological discontinuities. Birds are considered glorified theropods - the sole surviving lineage of dinosaurs.

Original languageEnglish
Pages (from-to)1-100
Number of pages100
JournalPalaeontographica, Abteilung A: Palaozoologie - Stratigraphie
Volume254
Issue number1-3
StatePublished - 1999

Keywords

  • Anatomy
  • Birds
  • Evolution
  • Protoavis
  • Triassic

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