Aves

From EvoWiki

Contents 1 Definition 2 The modern avian radiation 2.1 Origin 3 Birds of a feather 4 References

Definition

Aves (Linne sensu Chiappe 1992 =Avialae sensu Gauthier, 1986), is the clade which includes the common ancestor of Archaeopteryx and Passer and all its descendants. Members of the group are commonly known as birds.

Birds are the most speciose of all terrestrial vertebrates, and with their cousins the Crocodylia, represent the other great branch of the Mesozoic archosaurian radiation. There are 9,702 species of extant birds in 2,063 genera (Sibley & Monroe 1990) spanning approximately 36 orders (this count varies amongst classifications, this figure comes primarily from the classification presented in Gill, 1995). Roger Tory Peterson referred to birds as "the most splendid expression of creation" and this statement encapsulates the awe humanity has historically had for our feathered companions on this planet. Marvels of evolution and a showcase of diversity, the class Aves presents myriad lessons and intractable riddles to the prospective student of biology.

Avian monophyly is almost universally accepted, and the following characters can be considered synapomorphic of class Aves (after Witmer 2002):

1. Prenarial aspect of premaxilla elongate
2. Caudal tympanic recess opens with the columellar recess (this may or may not in fact be a synapomorphy of Aves, as it is possibly present in Sinovenator changii)
3. Caudal maxillary sinus present
4. Prezygapophyses reduced distally
5. Pes anisodactyl
6. Teeth with characteristic root constriction (after Martin et al. 1980, Gauthier 1986, Chiappe 1995 and Chiappe et al. 1996)

Maryanska et al. (2002) presented in their analysis of oviraptorosaur phylogeny the following synapomorphies of Aves (these authors prefer usage of Gauthier's clade Avialae):

1. Nasal as long as or shorter than frontal
2. Orbit length to antorbital fossa's length ratio at least 1.2
3. Suborbital part of jugal shallow dorsoventrally or rod-shaped
4. Tail including not more than 30 caudals

The author can find no compelling argument to dismiss the validity of these characters, most especially the last two, and thus regards them as equally diagnostic of Aves as those enumerated in Witmer (2002). Chatterjee (1997) presented an extensive list of characters diagnosing Aves, and of these the author regards the following as synapomorphic of birds:

1. Ascending process of jugal lost
2. Brain enlarged such that cerebellum extends to supraoccipital
3. Pectineal process present

Remaining characters in Chatterjee's list are either of questionable significance, distribution or validity, or are plesiomorphic in birds. Chiappe (2002), concluded there were only three synapomorphies underwriting monophyly of Aves, but this conclusion seems difficult to take seriously in light of the difficulties in his analysis (e.g., uncinate processes are coded as present in Confuciusornis sanctus this in spite of no trace of such structures in any of the hundreds of known specimens). The following characters from Chiappe's analysis are credibly presented as synapomorphies of Aves:

1. Prominent acromion process
2. Postacetabular process pointed and shallow, less then 50% the depth of the preacetabular wing at the acetabulum

The modern avian radiation

The classification of Aves presented to the right applies to the crown only, and is thus a classification of Aves sensu Gauthier (1986), i.e., of Neornithes only. The ordinal ranking within this classification is after Wetmore (1960) with the following exceptions:

1. The order Turniciformes sensu Sibley & Ahlquist (1990) is herein recognized due to the conflicting data on the actual phylogenetic affinities of these birds.
2. Phoenicopteridae are placed within their own order, the Phoenicopteriformes in recognition of continued dispute regarding the appropriate phylogenetic relationships of these most peculiar birds. Though this is admittedly not an ideal method, it recognizes the difficulties in resolving flamingo relationships.
3. In general, a conservative listing of ordinal taxa for Aves has been presented here to minimize assumptions about phylogeny. Various disputes about the monophyly of the various orders listed can be found on the respective pages for those groups.

Origin

There is markedly little consensus in neornithine phylogeny, though the basal divergences have widely been agreed upon. Multiple morphological (e.g., Livezey & Zusi 2001) and molecular (e.g., Chubb 2004) analyses have retrieved a fundamental dichotomy at the base of the neornithine tree between a holophyletic Paleognathae (=Eoaves sensu Sibley & Ahlquist 1990), and Neognathae (=Neoaves sensu Sibley & Ahlquist 1990). This divergence is presumed to have occurred near the origin of the neornithine assembalge. Within Neognathae, some data suggest that the most basal divergence therein is between a clade composed of Anseriformes and Galliformes called "Galloanserae", and all other birds (Neoaves of most analyses, e.g., Chubb 2004). The "basal Galloanserae" hypothesis is primarily one based upon molecular data and it is not beyond dispute (e.g., Ericson 1997, see also Galloanserae: A Critical Examination).

Neognaths to the exclusion of the "galloanserines" largely fall out in an unresolved polytomy (e.g., Johansson et al. 2001, Chubb 2004). There is little to no consensus which can withstand critical examination for the higher-level systematics of neognathous birds, and the lament of Streseman (1959), remains largely unchallenged, despite the proliferation of systematic work on birds in the past three decades.

Birds of a feather

A classification of Aves as a whole, is far more contentious. Multiple analyses have been presented in the past 30 years have done little clarify the matter, despite a great deal of discussion about the emergence of "consensus" viewpoints. Three representative classifications are presented below to demonstrate some of the more prominent hypotheses amongst ornithologists and paleontologists, and the conflicts between these viewpoints.

Chiappe (2002) suggested the following topology:

Aves

Un-named node, Confuciusornis + Pygostylia

Pygostylia

Ornithothoraces

Ornithuromorpha

Ornithurae

Carinatae

Neornithes

Given the difficulties with Chiappe's 2002 analysis, including miscoding of characters, it seems best to regard this phylogeny with some skepticism. Not the least of the difficulties is the wholly unwarranted redefinition of Ornithurae from a stem-based taxon conforming with historical usage, to a far more restrictive node-based taxon.

Sereno (1998, 1999a) has presented a far more coherent phylogeny of the class Aves which preserves the historical usage of the term Ornithurae:

Aves

Ornithurae

Ornithothoraces

Euornithes

Feduccia (1996, 1999) and Hou et al. (1996) noting a fundamental dichotomy at the base of the avian radiation presented the following phylogeny for the class Aves:

Aves

Sauriurae

Ornithurae

Neornithes

This hypothesis has met considerable dispute (e.g., Chiappe 2002).

References

1. Chatterjee, S. 1997. The Rise of Birds: 225 Million Years of Evolution. Johns Hopkins University Press, Baltimore.
2. Chiappe, L. 1992. Enantiornithine (Aves) tarsometatarsi and the avian affinities of the Late Cretaceous Avisauridae. Journal of Vertebrate Paleontology 12: 344-350.
3. Chiappe, L. 1995. The first 85 million years of avian evolution. Nature 378: 349-355.
4. Chiappe, L. 2002. Basal bird phylogeny. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 448-472.
5. Chiappe, L. Norell, M. A., & Clarke, J. A. 1996. Phylogenetic position of Mononykus (Aves: Alvarezsauridae) from the Cretaceous of the Gobi Desert. Memoirs of the Queensland Museum 39(3): 557-582.
6. Chubb, A. 2004. New nuclear evidence for the oldest divergence among neognath birds: the phylogenetic utility of ZENK (i). Molecular Phylogenetics and Evolution 30: 140-151.
7. Ericson, P. G. 1997. Systematic position of the Paleogene Family Presbyornithidae (Aves: Anseriformes). Zoological Journal of the Linnean Society 121: 429-483.
8. Feduccia, A. 1996. The Origin and Evolution of Birds, First Edition. Yale University Press, New Haven.
9. Feduccia, A. 1999. The Origin and Evolution of Birds, Second Edition. Yale University Press, New Haven.
10. Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds. In: Padian, K. (ed.), The Origin of Birds and the Evolution of Flight, California Academy of Sciences, San Francisco California, 1-55.
11. Gill, F. B. 1995. Ornithology, Second Edition. W. H. Freeman, New York.
12. Hou, L., Martin, L. D., Zhou, Z., & Feduccia, A. 1996. Early adaptive radiation of birds: evidence from fossil from northeastern China. Science 274: 1164-1167.
13. Johansson, U. S., Parsons, T. j., Irestedt, M. & Ericson, P. G. 2001. Clades within the 'higher land birds', evaluated by nuclear DNA sequences. Zool. Syst. Evol. Research 39: 37-51.
14. Livezey, B. C., & Zusi, R. 2001. Higher order phylogenetics of modern Aves based on comparative anatomy. Netherlands Journal of Zoology 51(2): 179-205.
15. Martin, L. D., Stewart, J. D. & Whetstone, K. N. 1980. The origin of birds: structure of the tarsus and teeth. Auk 97: 86-93.
16. Martin, L. D. & Stewart, J. D. 1999. Implantation and replacement of bird teeth. In: Olson, S. L., (ed.), Avian Paleontology at the Close of the 20th Century: Proceedings of the 4th International Meeting of the Society of Avian Paleontology and Evolution, 295-300.
17. Maryanska, R., Osmolska, H., & Wolsan, M. 2002. Avialian status for Oviraptorosauria. Acta Paleontologica Polonica 47: 97-116.
18. Sereno, P. 1998. A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria. Neues Jahrbuch fur Geologie und Palaontologie 210: 41-83.
19. Sereno, P. 1999a. The evolution of dinosaurs. Science 284: 2137-2147.
20. Sibley, C. G. & Ahlquist, J. E. 1990. Phylogeny and Classification of Birds: A Study in Molecular Evolution. Yale University Press, New Haven.
21. Sibley, C. G. & Monroe, Jr., B. L. 1990. Distribution and Taxonomy of birds of the world. Yale University Press, New Haven.
22. Streseman, E. 1959. The status of avian systematics and its unsolved problems. Auk 76: 269-280.
23. Wetmore, A. 1960. A classification for the birds of the world. Smithsonian Miscellaneous Collections 139(11): 1-37.
24. Witmer, L. 2002. The debate on avian ancestry: phylogeny, function, and fossils. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 3-30.

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JGK