Gruiformes Classification Essay

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See also: Heron

Cranes are a family, Gruidae, of large, long-legged and long-necked birds in the group Gruiformes. There are fifteen species of crane in four genera. Unlike the similar-looking but unrelated herons, cranes fly with necks outstretched, not pulled back. Cranes live on all continents except Antarctica and South America.

They are opportunistic feeders that change their diet according to the season and their own nutrient requirements. They eat a range of items from suitably sized small rodents, fish, amphibians, and insects to grain, berries, and plants.

Cranes construct platform nests in shallow water, and typically lay two eggs at a time. Both parents help to rear the young, which remain with them until the next breeding season.[1]

Some species and populations of cranes migrate over long distances; others do not migrate at all. Cranes are solitary during the breeding season, occurring in pairs, but during the non-breeding season they are gregarious, forming large flocks where their numbers are sufficient.

Most species of cranes have been affected by human activities and are at the least classified as threatened, if not critically endangered. The plight of the whooping cranes of North America inspired some of the first US legislation to protect endangered species.

Description

The cranes are large to very large birds, including the world's tallest flying bird. They range in size from the demoiselle crane, which measures 90 cm (35 in) in length, to the sarus crane, which can be up to 176 cm (69 in), although the heaviest is the red-crowned crane, which can weigh 12 kg (26 lb) prior to migrating. They are long-legged and long-necked birds with streamlined bodies and large rounded wings. The males and females do not vary in external appearance, but on average males tend to be slightly larger than females.[2]

The plumage of the cranes varies by habitat. Species inhabiting vast open wetlands tend to have more white in the plumage than do species that inhabit smaller wetlands or forested habitats, which tend to be more grey. These white species are also generally larger. The smaller size and colour of the forest species is thought to help them maintain a less conspicuous profile while nesting; two of these species (the common and sandhill cranes) also daub their feathers with mud to further hide while nesting.

Most species of crane have some areas of bare skin on the face; the only two exceptions are the blue and demoiselle cranes. This skin is used in communication with other cranes, and can be expanded by contracting and relaxing muscles, and change the intensity of colour. Feathers on the head can be moved and erected in the blue, wattled and demoiselle cranes for signalling as well.

Also important to communication is the position and length of the trachea. In the two crowned-cranes the trachea is shorter and only slightly impressed upon the bone of the sternum, whereas the trachea of the other species is longer and penetrates the sternum. In some species the entire sternum is fused to the bony plates of the trachea, and this helps amplify the crane's calls, allowing them to carry for several kilometres.[2][3]

Distribution and habitat

See also: List of Gruiformes by population

The cranes have a cosmopolitan distribution, occurring across most of the world continents. They are absent from Antarctica and, mysteriously, South America. East Asia is the centre of crane diversity, with eight species, followed by Africa, which holds five resident species and wintering populations of a sixth. Australia, Europe and North America have two regularly-occurring species each. Of the four crane genera, Balearica (two species) is restricted to Africa, and Leucogeranus (one species) is restricted to Asia; the other two genera, Grus (including Anthropoides and Bugeranus) and Antigone, are both widespread.[2][4]

Most species of crane are dependent on wetlands and require large areas of open space. Most species of crane nest in shallow wetlands. Some species nest in wetlands but move their chicks up onto grasslands to feed (while returning to wetlands at night), whereas others remain in wetlands for the entirety of the breeding season. Even the demoiselle crane and blue crane, which may nest and feed in grasslands (or even arid grasslands or deserts), require wetlands for roosting in during the night. The only two species that do not always roost in wetlands are the two African crowned cranes (Balearica), which are the only cranes to roost in trees.[2]

Some crane species are sedentary, remaining in the same area throughout the year, while others are highly migratory, travelling thousands of kilometres each year from their breeding sites. A few species have both migratory and sedentary populations.

Behaviour and ecology

The cranes are diurnal birds that vary in their sociality by season. During the breeding season they are territorial and usually remain on their territory all the time. In contrast in the non-breeding season they tend to be gregarious, forming large flocks to roost, socialise and in some species feed. Species that feed predominately on vegetable matter in the non-breeding season feed in flocks to do so, whereas those that feed on animals will usually feed in family groups, joining flocks only during resting periods, or in preparation for travel during migration. Large aggregations of cranes are important for safety when resting and also as places for young unmated birds to meet others.[2]

Calls and communication

Cranes are highly vocal and have a large vocabulary of specialized calls. The vocabulary begins soon after hatching with low, purring contact calls for maintaining contact with their parents, as well as food begging calls. Other calls used as chicks include alarm calls and "flight intention" calls, both of which are maintained into adulthood. The cranes' duet calls are most impressive. They can be used for individual recognition (see below external link).[5]

Feeding

The cranes as a family consume a wide range of food, ranging from animal to plant matter. When feeding on land they consume seeds, leaves, nuts and acorns, berries, fruit, insects, worms, snails, small reptiles, mammals and birds. In wetlands roots, rhizomes, tubers and other parts of emergent plants, other molluscs, small fish and amphibians are also consumed as well. The exact composition of the diet varies by location, season and availability. Within the wide range of items consumed there are some patterns; the shorter-billed species usually feed in drier uplands while the longer-billed species feed in wetlands.[2]

Cranes employ different foraging techniques for different food types. Tubers and rhizomes are dug for and a crane digging for them will remain in place for some time digging and then expanding a hole to find them. In contrast both to this and the stationary wait and watch hunting methods employed by many herons, they forage for insects and animal prey by slowly moving forwards with their heads lowered and probing with their bills.[2]

Where more than one species of crane exists in a locality, each species will adopt separate niches in order to minimise competition and niche overlap. At one important lake in Jiangxi Province in China the Siberian cranes feed on the mudflats and in shallow water, the white-naped cranes on the wetland borders, the hooded cranes on sedge meadows and the last two species also feed on the agricultural fields along with the common cranes.[2]

Breeding

Cranes are perennially monogamous breeders, establishing long-term pair bonds that may last the lifetime of the birds. Pair bonds begin to form in the second or third years of life, but it may be several years before the first successful breeding season. Initial breeding attempts often fail, and in many cases newer pair bonds will dissolve (divorce) after unsuccessful breeding attempts. Pairs that are repeatedly successful at breeding will remain together for as long as they continue to do so.[2] In a study of sandhill cranes in Florida, seven out of the 22 pairs studied remained together for an 11-year period. Of the pairs that separated 53% were due to the death of one of the pair, 18% due to divorce and the fate of 29% of pairs were unknown.[6] Similar results had been found by acoustic monitoring (sonography / frequency analysis of duett and guard calls) in 3 breeding areas of common cranes in Germany over 10 years.[7]

Cranes are territorial and generally seasonal breeders. Seasonality varies both between and within species, dependent on local conditions. Migratory species begin breeding upon reaching their summer breeding grounds, between April and June. The breeding season of tropical species, however, is usually timed to coincide with the wet or monsoon seasons. Territory sizes also vary depending on location. Tropical species can maintain very small territories, for example sarus cranes in India can breed on territories as small as one hectare where the area is of sufficient quality and disturbance by humans is minimised. In contrast red-crowned crane territories may require 500 hectares, and pairs may defend even larger territories than that, up to several thousand hectares. Territory defence is usually performed by the male.[2] Because of this females are much less likely to retain the territory than males in the event of the death of a partner.[6]

Taxonomy and systematics

There are 15 living species of cranes in four genera.[4] A molecular phylogenetic study published in 2010 found that the genus Grus, as then defined, was polyphyletic.[8] In the resulting rearrangement to create monophyletic genera, the Siberian crane was moved to the resurrected monotypic genus Leucogeranus while the sandhill crane, the white-naped crane, the sarus crane and the brolga were moved to the resurrected genus Antigone.[4] Some authorities recognize the additional genera Anthropoides (for the demoiselle crane and blue crane) and Bugeranus (for the wattled crane) on morphological grounds.

SUBFAMILY BALEARICINAE – crowned cranes

SUBFAMILY GRUINAE – typical cranes

  • Genus Leucogeranus
  • Genus Antigone
  • Genus Grus
    • Wattled crane, Grus carunculata
    • Blue crane, Grus paradisea
    • Demoiselle crane, Grus virgo
    • Red-crowned crane, Grus japonensis
    • Whooping crane, Grus americana
    • Common crane, Grus grus, also known as the Eurasian crane
    • Hooded crane, Grus monacha
    • Black-necked crane, Grus nigricollis

The fossil record of cranes leaves much to be desired. Apparently, the subfamilies were well distinct by the Late Eocene (around 35 mya). The present genera are apparently some 20 mya old. Biogeography of known fossil and the living taxa of cranes suggests that the group is probably of (Laurasian?) Old World origin. The extant diversity at the genus level is centered on (eastern) Africa, making it all the more regrettable that no decent fossil record exists from there. On the other hand, it is peculiar that numerous fossils of Ciconiiformes are documented from there; these birds presumably shared much of their habitat with cranes back then already. Cranes are sister taxa to Eogruidae, a lineage of flightless birds; as predicted by the fossil record of true cranes, eogruids were native to the Old World.

Fossil genera are tentatively assigned to the present-day subfamilies:

Gruinae

  • Palaeogrus (Middle Eocene of Germany and Italy – Middle Miocene of France)
  • Pliogrus (Early Pliocene of Eppelsheim, Germany)
  • Camusia (Late Miocene of Menorca, Mediterranean)
  • "Grus" conferta (Late Miocene/Early Pliocene of Contra Costa County, US)[9]

Sometimes considered Balearicinae

  • Geranopsis (Hordwell Late Eocene – Early Oligocene of England)
  • Anserpica (Late Oligocene of France)

Sometimes considered Gruidae incertae sedis

  • Eobalearica (Ferghana Late? Eocene of Ferghana, Uzbekistan)
  • Probalearica (Late Oligocene? – Middle Pliocene of Florida, US, France?, Moldavia and Mongolia) – A nomen dubium?
  • Aramornis (Sheep Creek Middle Miocene of Snake Creek Quarries, US)

In mythology and symbolism

Further information: Crane in Chinese mythology

The cranes' beauty and their spectacular mating dances have made them highly symbolic birds in many cultures with records dating back to ancient times. Crane mythology is widely spread and can be found in areas such as the India, Aegean, South Arabia, China, Korea, Japan and in the Native American cultures of North America. In northern Hokkaidō, the women of the Ainu people performed a crane dance that was captured in 1908 in a photograph by Arnold Genthe. In Korea, a crane dance has been performed in the courtyard of the Tongdosa Temple since the Silla Dynasty (646 CE).

The Sanskrit epic poet Valmiki was inspired to write the first śloka couplet by the pathos of seeing a male sarus crane shot while mating.[10][11]

In Mecca, in pre-Islamic South Arabia, Allāt, Uzza, and Manāt were believed to be the three chief goddesses of Mecca, they were called the "three exalted cranes" (gharaniq, an obscure word on which 'crane' is the usual gloss). See The Satanic Verses for the best-known story regarding these three goddesses.

In China, several styles of kung fu take inspiration from the movements of cranes in the wild, the most famous of these styles being Wing Chun, Hung Gar (tiger crane), and the Shaolin Five Animals style of fighting. Crane movements are well known for their fluidity and grace.

The Greek for crane is Γερανος (Geranos), which gives us the cranesbill, or hardy geranium. The crane was a bird of omen. In the tale of Ibycus and the cranes, a thief attacked Ibycus (a poet of the 6th century BCE) and left him for dead. Ibycus called to a flock of passing cranes, who followed the attacker to a theater and hovered over him until, stricken with guilt, he confessed to the crime.

Pliny the Elder wrote that cranes would appoint one of their number to stand guard while they slept. The sentry would hold a stone in its claw, so that if it fell asleep it would drop the stone and waken. A crane holding a stone in its claw is a well-known symbol in heraldry, and is known as a crane in its vigilance.

Aristotle describes the migration of cranes in the History of Animals,[12] adding an account of their fights with Pygmies as they wintered near the source of the Nile. He describes as untruthful an account that the crane carries a touchstone inside it that can be used to test for gold when vomited up. (This second story is not altogether implausible, as cranes might ingest appropriate gizzard stones in one locality and regurgitate them in a region where such stone is otherwise scarce.)

Greek and Roman myths often portrayed the dance of cranes as a love of joy and a celebration of life, and the crane was often associated with both Apollo and Hephaestus.

Throughout Asia, the crane is a symbol of happiness and eternal youth. In Japan, the crane is one of the mystical or holy creatures (others include the dragon and the tortoise) and symbolizes good fortune and longevity because of its fabled life span of a thousand years. The crane is a favourite subject of the tradition of origami or paper folding. An ancient Japanese legend promises that anyone who folds a thousand origami cranes will be granted a wish by a crane. After World War II, the crane came to symbolize peace and the innocent victims of war through the story of schoolgirl Sadako Sasaki and her thousand origami cranes. Suffering from leukemia as a result of the atomic bombing of Hiroshima and knowing she was dying, she undertook to make a thousand origami cranes before her death at the age of 12. After her death, she became internationally recognised as a symbol of the innocent victims of war and remains a heroine to many Japanese girls.

  • Pine, Plum and Cranes, 1759, by Shen Quan (1682—1760). Hanging scroll, ink and colour on silk. The Palace Museum, Beijing.

References

  1. ^Archibald, George W. (1991). Forshaw, Joseph, ed. Encyclopaedia of Animals: Birds. London: Merehurst Press. pp. 95–96. ISBN 1-85391-186-0. 
  2. ^ abcdefghijArchibald, George; Meine, Curt (1996). "Family Gruidae (Cranes)". In del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi. Handbook of the Birds of the World. Volume 3, Hoatzin to Auks. Barcelona: Lynx Edicions. pp. 60–81. ISBN 84-87334-20-2. 
  3. ^Gaunt, Abbot; Sandra L. L. Gaunt; Henry D. Prange; Jeremy S. Wasser (1987). "The effects of tracheal coiling on the vocalizations of cranes (Aves; Gruidae)". Journal of Comparative Physiology A. 161 (1): 43–58. doi:10.1007/BF00609454. 
  4. ^ abcGill, Frank; Donsker, David, eds. (2017). "Rails, gallinules, trumpeters & cranes". World Bird List Version 7.2. International Ornithologists' Union. Retrieved 28 June 2017. 
  5. ^"craneworld.de". craneworld.de. Retrieved 2012-07-29. 
  6. ^ abNesbitt, Stephen A. (1989). "The Significance of Mate Loss in Florida Sandhill Cranes"(PDF). Wilson Bulletin. 101 (4): 648–651. 
  7. ^Wessling, B. (2003). "Acoustic individual monitoring over several years (mainly Common Crane and Whooping Crane)". Craneworld.de. 
  8. ^Krajewski, C.; Sipiorski, J.T.; Anderson, F.E. (2010). "Mitochondrial genome sequences and the phylogeny of cranes (Gruiformes: Gruidae)". Auk. 127 (2): 440–452. doi:10.1525/auk.2009.09045. 
  9. ^Miller, Alden H.; Sibley, Charles G. (1942). "A New Species of Crane from the Pliocene of California"(PDF). Condor. 44 (3): 126–127. doi:10.2307/1364260. 
  10. ^Leslie, J. (1998). "A bird bereaved: The identity and significance of Valmiki's kraunca". Journal of Indian Philosophy. 26 (5): 455–487. doi:10.1023/A:1004335910775. 
  11. ^Hammer, Niels (2009). "Why Sārus Cranes epitomize Karuṇarasa in the Rāmāyaṇa". Journal of the Royal Asiatic Society of Great Britain & Ireland. (Third Series). 19 (2): 187–211. doi:10.1017/S1356186308009334. 
  12. ^"The Internet Classics Archive | The History of Animals by Aristotle". Classics.mit.edu. Retrieved 2012-07-29. 

Bibliography

  • Hayes, M.A. (2005): Divorce and extra-pair paternity as alternative mating strategies in monogamous sandhill cranes. MS thesis, University of South Dakota, Vermilion, S.D.. 86 p. PDF fulltext at the International Crane Foundation's Library

External links

Myths and lores

The bare area of skin on the face of a sandhill crane. This can change colour or even expand in area when the bird is excited
The long coiled trachea that produces the trumpeting calls of cranes (sarus crane, Antigone antigone)
Common cranes in Israel. Many species of crane gather in large groups during migration and on their wintering grounds
Blue crane (Grus paradisea) feeding in Diergaarde Blijdorp (Rotterdam Zoo)
Eogruidae is an extinct lineage of mostly flightless stem-cranes. Pictured is the two-toed Ergilornis.
Illustration in the English manuscript Harley Bestiary (13th century) of the legend of the vigilant cranes. At night cranes take turns keeping watch for enemies.
1909 illustration of the fable of the geese and the cranes, from Aesop's Fables. The geese and the cranes were feeding in the same meadow, when a birdcatcher came to ensnare them in his nets. The cranes, being light of wing, fled away at his approach, while the geese, being slower of flight and heavier in their bodies, were captured.

For other uses, see Bird (disambiguation) and Birds (disambiguation).

"Aves" and "Avifauna" redirect here. For other uses, see Aves (disambiguation) and Avifauna (disambiguation).

Birds (Aves) are a group of endothermicvertebrates, characterised by feathers, toothlessbeaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the world’s most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have which are more or less developed depending on the species; the only known groups without wings are the extinctmoa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.

The fossil record indicates that birds evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischiandinosaurs; their closest living relatives are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago.[1] Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.[1] DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling.[3] This makes them the sole surviving dinosaurs according to cladistics.[4]

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous (referring to social living arrangement, distinct from genetic monogamy), usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous (arrangement of one male with many females) or, rarely, polyandrous (arrangement of one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.

Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds (poultry and game) being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.

Evolution and classification

Main article: Evolution of birds

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[5]Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[6] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur cladeTheropoda.[7]

Definition

Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[8] However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae,[9] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Gauthier[10] identified four different definitions for the same biological name "Aves", which is a problem. Gauthier proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups.

  1. Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
  2. Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
  3. Aves can mean those feathered dinosaurs that fly (alternately Avialae)
  4. Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group", in this sense synonymous with Neornithes)

Under the fourth definition Archaeopteryx is an avialan, and not a member of Aves. Gauthier's proposals have been adopted by many researchers in the field of palaeontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.[11]

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus."[12][13] Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.[10][14]

Dinosaurs and the origin of birds

Main article: Origin of birds

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropoddinosaurs,[17] and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.[18] As scientists have discovered more theropods 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 many small theropod feathered dinosaurs, contribute to this ambiguity.[19][20][21]

The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids.[22] Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both.[23][24] Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores.[25]

The Late JurassicArchaeopteryx 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 was the first fossil to display both clearly traditional reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.[26]

Early evolution

See also: List of fossil bird genera

The earliest known avialan fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui.[11]

The well-known early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.[28]

Avialans diversified into a wide variety of forms during the Cretaceous Period.[29] Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,[29] the tails of more advanced avialans were shortened with the advent of the pygostyle bone in the group Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved a better sense of smell.[30]

Early diversity of bird ancestors

The first large, diverse lineage of short-tailed avialans 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. While they were the dominant group of avialans during the Cretaceous period, enantiornithes became extinct along with many other dinosaur groups at the end of the Mesozoic era.[29]

Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.

The later included the superficially gull-like Ichthyornis,[32] the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.[29] The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).[33] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,[34] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.[28]

Diversification of modern birds

See also: Sibley–Ahlquist taxonomy of birds and dinosaur classification

All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds.[35] These two subdivisions are often given the rank of superorder,[36] although Livezey and Zusi assigned them "cohort" rank.[7] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[37] to 10,050.[38]

The discovery of Vegavis, a late Cretaceous member of the Anatidae, proved that the diversification of modern birds started before the Cenozoic.[39] The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago,[40] are still too controversial to provide a fossil evidence of modern bird diversification.

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Middle Cretaceous[1] to the latest Late Cretaceous.[41] Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the Cretaceous–Palaeogene extinction event.[42] This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous radiation, while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy). Previous attempts to reconcile molecular and fossil evidence have proved controversial,[42][43] but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating molecular clocks, showed that while modern birds originated early in the Late Cretaceous, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event.[44]

Classification of bird orders

See also: List of birds

Cladogram of modern bird relationships based on Prum, R.O. et al. (2015)[41] with some clade names after Yuri, T. et al. (2013).[45]

Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a beak.[27]
Ichthyornis, which lived 93 million years ago, was the first known prehistoric bird relative preserved with teeth.
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