Fossil range: Late Jurassic–Recent, 156-0 Ma
Archaeopteryx BW
Archaeopteryx, the earliest known bird.
Scientific classification



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(Linnaeus, 1758)

Birds (class Aves) are winged, bipedal, endothermic (warm-blooded), vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates.

The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma. Most paleontologists regard birds as the only clade of dinosaurs that survived the Cretaceous–Tertiary extinction event approximately 65.5 Ma.

Evolution and taxonomyEdit

Main article: Bird evolution
Naturkundemuseum Berlin - Archaeopteryx - Eichstätt

Archaeopteryx, the earliest known bird

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[1] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[2] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[3] Aves and a sister group, the clade Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[4]

Archaeopteryx, from the Tithonian stage of the Late Jurassic (some 150–145 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups. This has been done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae[5] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

All modern birds lie within the subclass Neornithes, which has two subdivisions: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds.[3] These two subdivisions are often given the rank of superorder,[6] although Livezey & Zusi assigned them "cohort" rank.[3] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[7] to 10,050.[8]

Dinosaurs and the origin of birdsEdit

Main article: Origin of birds
Confuchisornis sanctus

Confuciusornis, a Cretaceous bird from China

Fossil evidence and intensive biological analyses have demonstrated beyond any reasonable doubt that birds are a specialised sub-group of theropod dinosaurs.[9] More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.[10] As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity.[11]

The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these three form a group called Paraves. The basal dromaeosaur Microraptor has features which may have enabled it to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, and/or may have been able to glide.[12][13]

The Late Jurassic Archaeopteryx is well-known as one of the first transitional fossils to be found and it provided support for the theory of evolution in the late 19th century. Archaeopteryx has clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, but it has finely preserved wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, but is the oldest and most primitive member of Aves or Avialae, and it is probably closely related to the real ancestor. It has even been suggested that Archaeopteryx was a dinosaur that was no more closely related to birds than were other dinosaur groups,[14] and that Avimimus was more likely to be the ancestor of all birds than Archaeopteryx.[15]

Alternative theories and controversiesEdit

There have been many controversies in the study of the origin of birds. Early disagreements included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors.[16] Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently.[17] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae. A few scientists suggest that birds are not dinosaurs, but evolved from early archosaurs like Longisquama.[18][19]

Early evolution of birdsEdit










Basal bird phylogeny simplified after Chiappe, 2007[20]

Birds diversified into a wide variety of forms during the Cretaceous Period.[20] Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,[20] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia.

The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.[20] More advanced lineages also specialised in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").[21] One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specialisations, the Hesperornithiformes represent some of the closest relatives of modern birds.[20]

Radiation of modern birdsEdit

Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis, now known to have evolved into some basic lineages by the end of the Cretaceous[22] and is split into two superorders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the K–T extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.[23] This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.[23][24]

The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,[25] although it is frequently debated and constantly revised. Most evidence seems to suggest that the assignment of orders is accurate,[26] but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.

Prehistoric BirdsEdit

Prehistoric birds are here considered any species of the Aves and some possible close relatives of more uncertain status that have become extinct before recorded by modern sources.

They are known from fossil and subfossil remains only, and in a few cases may be remembered in folk memory. Such birds are here presented in 2 articles, separated according to the rough date of extinction and the possibility of human influence in their extinction:


  1. ^ del Hoyo, Josep; Andy Elliott & Jordi Sargatal (1992). Handbook of Birds of the World, Volume 1: Ostrich to Ducks. Barcelona: Lynx Edicions. ISBN 84-87334-10-5. 
  2. ^ Linnaeus, Carolus (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Holmiae. (Laurentii Salvii). p. 824. 
  3. ^ a b c Livezey, Bradley C.; Richard L. Zusi (January 2007). "Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion". Zoological Journal of the Linnean Society 149 (1): 1–95. doi:10.1111/j.1096-3642.2006.00293.x. 
  4. ^ Padian, Kevin; L.M. Chiappe Chiappe LM (1997). "Bird Origins". in Philip J. Currie & Kevin Padian (eds.). Encyclopedia of Dinosaurs. San Diego: Academic Press. pp. 41–96. ISBN 0-12-226810-5. 
  5. ^ Gauthier, Jacques (1986). "Saurischian Monophyly and the origin of birds". in Kevin Padian. The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Science 8. San Francisco, CA: Published by California Academy of Sciences. pp. 1–55. ISBN 0-940228-14-9. 
  6. ^ "Bird biogeography". Retrieved on 2008-04-10. 
  7. ^ Clements, James F. (2007). The Clements Checklist of Birds of the World (6th ed.). Ithaca: Cornell University Press. ISBN 978-0-8014-4501-9. 
  8. ^ Gill, Frank (2006). Birds of the World: Recommended English Names. Princeton: Princeton University Press. ISBN 978-0-691-12827-6. 
  9. ^ see eg Richard O. Prum "Who's Your Daddy" Science 322 1799-1800, citing also R. O. Prum, Auk 119, 1 (2002)
  10. ^ Paul, Gregory S. (2002). "Looking for the True Bird Ancestor". Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: John Hopkins University Press. pp. 171–224. ISBN 0-8018-6763-0. 
  11. ^ Norell, Mark; Mick Ellison (2005). Unearthing the Dragon: The Great Feathered Dinosaur Discovery. New York: Pi Press. ISBN 0-13-186266-9. 
  12. ^ Turner, Alan H.; Pol, Diego; Clarke, Julia A.; Erickson, Gregory M.; and Norell, Mark (2007). "A basal dromaeosaurid and size evolution preceding avian flight" (PDF). Science 317: 1378–1381. doi:10.1126/science.1144066. PMID 17823350. 
  13. ^ Xing, X., Zhou, Z., Wang, X., Kuang, X., Zhang, F., and Du, X. (2003). "Four-winged dinosaurs from China". Nature 421 (6921): 335–340. doi:10.1038/nature01342. 
  14. ^ Thulborn, R.A. (1984). "The avian relationships of Archaeopteryx, and the origin of birds". Zoological Journal of the Linnean Society 82: 119–158. doi:10.1111/j.1096-3642.1984.tb00539.x. 
  15. ^ Kurzanov, S.M. (1987). "Avimimidae and the problem of the origin of birds". Transactions of the joint Soviet - Mongolian Paleontological Expedition 31: 31–94. 
  16. ^ Heilmann, Gerhard (1927). The Origin of Birds. New York: Dover Publications.
  17. ^ Rasskin-Gutman, Diego; Angela D. Buscalioni (March 2001). "Theoretical morphology of the Archosaur (Reptilia: Diapsida) pelvic girdle". Paleobiology 27 (1): 59–78. doi:10.1666/0094-8373(2001)027<0059:TMOTAR>2.0.CO;2. 
  18. ^ Feduccia, Alan; Theagarten Lingham-Soliar, J. Richard Hinchliffe (November 2005). "Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence". Journal of Morphology 266 (2): 125–66. doi:10.1002/jmor.10382. PMID 16217748. 
  19. ^ This theory is contested by most paleontologists Prum, Richard O. (April 2003). "Are Current Critiques Of The Theropod Origin Of Birds Science? Rebuttal To Feduccia 2002". The Auk 120 (2): 550–61. doi:10.1642/0004-8038(2003)120[0550:ACCOTT]2.0.CO;2.;2-0. 
  20. ^ a b c d e Chiappe, Luis M. (2007). Glorified Dinosaurs: The Origin and Early Evolution of Birds. Sydney: University of New South Wales Press. ISBN 978-0-86840-413-4. 
  21. ^ Clarke, Julia A. (September 2004). "Morphology, Phylogenetic Taxonomy, and Systematics of Ichthyornis and Apatornis (Avialae: Ornithurae)" (PDF). Bulletin of the American Museum of Natural History 286: 1–179. doi:10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2. 
  22. ^ Clarke, Julia A.; Claudia P. Tambussi, Jorge I. Noriega, Gregory M. Erickson and Richard A. Ketcham (January 2005). "Definitive fossil evidence for the extant avian radiation in the Cretaceous" (PDF). Nature 433: 305–308. doi:10.1038/nature03150. PMID 15662422.  Supporting information
  23. ^ a b Ericson, Per G.P.; Cajsa L. Anderson, Tom Britton et al. (December 2006). "Diversification of Neoaves: Integration of molecular sequence data and fossils" (PDF). Biology Letters 2 (4): 543–547. doi:10.1098/rsbl.2006.0523. PMID 17148284. 
  24. ^ Brown, Joseph W.; Robert B. Payne, David P. Mindell (June 2007). "Nuclear DNA does not reconcile 'rocks' and 'clocks' in Neoaves: a comment on Ericson et al.". Biology Letters 3 (3): 257–259. doi:10.1098/rsbl.2006.0611. PMID 17389215. 
  25. ^ Sibley, Charles; Jon Edward Ahlquist (1990). Phylogeny and classification of birds. New Haven: Yale University Press. ISBN 0-300-04085-7. 
  26. ^ Mayr, Ernst; Short, Lester L. (1970). Species Taxa of North American Birds: A Contribution to Comparative Systematics. Publications of the Nuttall Ornithological Club, no. 9. Cambridge, Mass.: Nuttall Ornithological Club. 
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