38-473: Avicephala (" bird heads") is a potentially polyphyletic grouping of extinct diapsid reptiles that lived during the Late Permian and Triassic periods characterised by superficially bird-like skulls and arboreal lifestyles. As a clade , Avicephala is defined as including the gliding weigeltisaurids and the arboreal drepanosaurs to the exclusion of other major diapsid groups. This relationship
76-729: A better sense of smell. A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula , and the loss of grasping hands. † Anchiornis † Archaeopteryx † Xiaotingia † Rahonavis † Jeholornis † Jixiangornis † Balaur † Zhongjianornis † Sapeornis † Confuciusornithiformes † Protopteryx † Pengornis Ornithothoraces † Enantiornithes Pterosaurs Too Many Requests If you report this error to
114-684: A clade together by John Merck in an abstract presented to the 2003 annual meeting of the Society of Vertebrate Paleontology . The same conclusion would be reached by Phil Senter in a paper in 2004, who named this clade Avicephala and found it to be the sister taxon to Neodiapsida (defined wherein as the clade containing "Younginiforms" and all living diapsids). Within Avicephala, Senter found two clades, one that he named Simiosauria (equivalent to Drepanosauromorpha), and another containing Coelurosauravus and Longisquama . The cladogram below depicts
152-473: A definition similar to "all theropods closer to birds than to Deinonychus ", with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus . 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
190-442: A group called Paraves . Some basal members of Deinonychosauria, 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. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that
228-634: A group of warm-blooded vertebrates constituting the class Aves ( Latin: [ˈaveːs] ), characterised by feathers , toothless beaked 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 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich . There are over 11,000 living species and they are split into 44 orders . More than half are passerine or "perching" birds. Birds have wings whose development varies according to species;
266-638: A new clade, Drepanosauromorpha, to replace Simiosauria following the guidelines of the PhyloCode ). They instead found drepanosauromorphs to be archosauromorphs , possibly related to Prolacertiformes , and unrelated to weigeltisaurids or other early diapsids. Drepanosauromorphs would later be argued to be early-diverging diapsids once again following the description of the three-dimensionally preserved skull of Avicranium in 2017, interpreted as possessing archaic features of their skull anatomy. Still, drepanosaurs and weigeltisaurids remained as separate lineages in
304-400: A novel position for them. The simplified results of this analysis are shown in the right cladogram below, highlighting 'avicephalans'. Buffa et al. argued that most of the shared traits both Senter and Pritchard proposed to unite Avicephala were commonly distributed enough throughout diapsids to be congruent with multiple possible evolutionary scenarios, while others were less similar between
342-406: A sister group, the order 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 . However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and
380-717: A time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (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. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds , parrots, and other species are popular as pets. Guano (bird excrement)
418-540: Is called ornithology . Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs . Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians . Birds are descendants of the primitive avialans (whose members include Archaeopteryx ) which first appeared during the Late Jurassic . According to recent estimates, modern birds ( Neornithes ) evolved in
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#1732773318413456-448: Is especially striking in some drepanosaurs such as Megalancosaurus which possess long, pointed snouts and expanded, rounded craniums. The three-dimensionally preserved Avicranium has even been proposed to have been toothless and had forward-facing eyes, although the construction of its skull is debated. These superficial similarities to birds led some scientists to hypothesise that birds were derived from various avicephalans, although
494-429: Is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 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. The first classification of birds
532-503: Is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs , i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex . The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After
570-501: Is not recovered in the majority of phylogenetic analyses of early diapsids and so Avicephala is typically regarded as an artificial or unnatural grouping. However, the clade was recovered again in 2021 following a redescription of Weigeltisaurus , raising the possibility that the clade may be valid after all, although subsequent analyses have not supported this result. Avicephalans were named in reference to their pointed, lightly constructed, superficially bird-like skulls. The resemblance
608-516: Is synonymous to Avifilopluma. † Scansoriopterygidae † Eosinopteryx † Jinfengopteryx † Aurornis † Dromaeosauridae † Troodontidae Avialae Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs and, more specifically, members of Maniraptora , a group of theropods which includes dromaeosaurids and oviraptorosaurs , among others. As scientists have discovered more theropods closely related to birds,
646-535: Is used by many scientists including adherents to the PhyloCode . 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 broader group Avialae, on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives. Gauthier and de Queiroz identified four different definitions for
684-722: The Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non- ornithuran dinosaurs. Many social species preserve knowledge across generations ( culture ). Birds are social, communicating with visual signals, calls, and songs , and participating in such behaviour as cooperative breeding and hunting, flocking , and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous , usually for one breeding season at
722-669: 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 . The well-known probable 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
760-833: The absence of both cervical and dorsal intercentra, a length/height ratio for the scapula between 0.4 and 0.25, and no outer process on the fifth metatarsal . Consequently, they provided an updated definition for Avicephala as the modern definition of Neodiapsida now includes both drepanosaurs and weigeltisaurids. Avicephala is now cladistically defined as "including all taxa more closely related to Weigeltisaurus jaekeli Weigelt 1930 and Drepanosaurus unguicaudatus Pinna 1979 than to Petrolacosaurus kansensis Lane 1945, Orovenator mayorum Reisz, Modesto & Scott, 2011, Claudiosaurus germaini Carroll, 1978, Youngina capensis Broom 1914, or Sauria Macartney 1802". Despite this result, they noted that should Avicephala not be recovered as monophyletic in future analyses they recommend that
798-428: The appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. The integument evolved into complex, pennaceous feathers . The oldest known paravian (and probably the earliest avialan) fossils come from
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#1732773318413836-509: The birds that descended from them. Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition
874-665: The earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These 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. † Coelurus † Ornitholestes † Ornithomimosauria † Alvarezsauridae † Oviraptorosauria Paraves Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used
912-451: The first avialans were omnivores . 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 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
950-677: The hands and feet and prehensile tails—tipped with a bony hook in some genera like Drepanosaurus . Weigeltisaurids on the other hand possessed wing-like gliding membranes ( patagia ) supported by elongated bony rods along their bodies, novel structures analogous to ribs of the modern gliding Draco lizard. Weigeltisaurids also had toes that were similarly proportioned to living arboreal lizards. Although both groups are highly derived, they do share some similarities, namely they have relatively unossified skeletons lacking intercentra between their vertebrae, as well as proportionately tall, thin shoulder blades . The enigmatic Triassic reptile Longisquama
988-523: The only known groups without wings are the extinct moa and elephant birds . Wings, which are modified forelimbs , gave birds the ability to fly, although further evolution has led to the loss of flight in some birds , including ratites , penguins , and diverse endemic island species. 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 study of birds
1026-402: The outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle , an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved
1064-536: The previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs , contributed to this ambiguity. 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 . Together, these form
1102-561: The result of his analysis: Longisquama Coelurosauravus Vallesaurus Hypuronector Drepanosaurus Dolabrosaurus Megalancosaurus Later research, including Renesto and Binelli (2006) and Renesto et al . (2010), argued that the phylogenetic analysis of Senter (2004) was flawed due to the absence of available data for drepanosaur skulls at the time and the exclusion of particular diapsid groups, such as pterosaurs . Renesto and colleagues (2010) recommended abandoning Avicephala for these reasons (as well as defining
1140-452: The same biological name "Aves", which is a problem. The authors 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. They assigned other names to the other groups. Lizards & snakes Turtles Crocodiles Birds Under the fourth definition Archaeopteryx , traditionally considered one of
1178-551: 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. Avialans diversified into a wide variety of forms during the Cretaceous period. 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). Increasingly stiff tails (especially
Avicephala - Misplaced Pages Continue
1216-422: The study's phylogenetic analysis, with each being successively closer to living diapsids. Avicephala would not be recovered again until 2021 following the redescription of Weigeltisaurus by Adam Pritchard and colleagues, wherein the phylogenetic analysis found a clade comprising drepanosauromorphs and weigeltisaurids. They identified four unambiguous synapomorphies (shared unique traits) supporting Avicephala:
1254-731: The taxon be abandoned once again. Their results are shown simplified in the left cladogram below. A study by Valentin Buffa and colleagues published in 2024 set out to test the revived monophyly of Avicephala following their reinterpretation of the skull of the drepanosaur Avicranium . Like most previous analyses they did not recover a monophyletic Avicephala, with weigeltisaurids once again recovered as stem -saurians (albeit more crownward than typical of prior analyses) and drepanosauromorphs likewise as archosauromorphs, although uniquely close relatives of Trilophosauridae within Allokotosauria ,
1292-484: The two than previously thought (such as the nature of the inclined rear margin of the skull). Only two of Senter's original synapomorphies are common to both groups but rare in other Permo–Triassic: the subequal lengths of the third and fourth metapodials (and consequent symmetrical hands and feet). However, they may be attributable to convergent evolution due to similar arboreal lifestyles, among other shared traits. Constraining Avicephala to be monophyletic in their dataset
1330-419: The well established relationship of birds to theropod dinosaurs indicates that these similarities arose only through convergent evolution . Although bird-like in some respects, avicephalans also possess a variety of archaic and unique characteristics as well, including some associated with an arboreal lifestyle. Drepanosaurs possess a suite of chameleon -like skeletal features, such as opposable digits on
1368-466: Was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae . Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy . Phylogenetic taxonomy places Aves in the clade Theropoda as an infraclass or a subclass, more recently a subclass. Aves and
1406-877: Was four steps less parsimonious, and significantly reduced the overall resolution of diapsid relationships. Both of these results suggest that Avicephala is not likely to be a true clade. Pritchard et al. (2021): Monophyletic Avicephala Petrolacosaurus Orovenator Claudiosaurus Hypuronector Vallesaurus Dolabrosaurus Megalancosaurus Drepanosaurus Avicranium Coelurosauravus Weigeltisaurus Rautiania Sauria Younginidae Tangasauridae Buffa et al. (2024): Non-monophyletic Avicephala Younginiformes Claudiosaurus Rautiania Coelurosauravus Weigeltisaurus Lepidosauromorpha Icarosaurus Protorosaurus Tanystropheidae Azendohsauridae Trilophosauridae Hypuronector Avicranium Bird Birds are
1444-501: Was initially included as an avicephalan when the clade was first defined as a relative of Coelurosauravus . However, due to the limited knowledge of its anatomy, Longisquama has been excluded from many subsequent phylogenetic analyses as its true relationships are difficult to discern. The phylogenetic relationships of both drepanosaurs and weigeltisaurs, as well as Longisquama , have historically been difficult to pin down. Drepanosaurs and weigeltisaurids were first suggested to form
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