Origin of avian flight

"Evolution of flight" redirects here. See also flying and gliding animals and insect flight.

Around 350 BCE, Aristotle and other philosophers of the time attempted to explain the aerodynamics of avian flight. Even after the discovery of the ancestral bird Archaeopteryx over 150 years ago, debates still persist regarding the evolution of flight. There are three leading hypotheses pertaining to avian flight: Pouncing Proavis model, Cursorial model, and Arboreal model. Archaeopteryx, the oldest known ancestor of modern birds, could provide clues to the origin of avian flight.

In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.[1][2]

Archaeopteryx lithographica (Berlin specimen)
The Berlin Archaeopteryx, one of the earliest known birds.

Flight characteristics

For flight to occur in Aves, four physical forces (thrust and drag, lift and weight) must be favorably combined. In order for birds to balance these forces, certain physical characteristics are required. Asymmetrical wings, found on all flying birds with the exception of hummingbirds, help in the production of thrust and lift. Anything that moves through the air produces drag due to friction. The aerodynamic body of a bird can reduce drag, but when stopping or slowing down a bird will use its tail and feet to increase drag. Weight is the largest obstacle birds must overcome in order to fly. An animal can more easily attain flight by reducing its absolute weight. Birds evolved from other theropod dinosaurs that had already gone through a phase of size reduction during the Middle Jurassic, combined with rapid evolutionary changes.[3] Flying birds during their evolution further reduced relative weight through several characteristics such as the loss of teeth, gonadal hypertrophy, and fusion of bones. Teeth were replaced by a lightweight bill made of keratin, and chewing moved to the bird's gizzard. Other advanced physical characteristics evolved for flight are a keel for the attachment of flight muscles and an enlarged cerebellum for fine motor coordination. These were gradual changes, though, and not strict conditions for flight: the first birds had teeth, at best a small keel and relatively unfused bones. Pneumatic bone, that is hollow or filled with air sacs, has often been seen as an adaptation reducing weight, but it was already present in non-flying dinosaurs, and birds on average do not have a lighter skeleton than mammals of the same size. The same is true for the furcula, a bone which enhances skeletal bracing for the stresses of flight.

The mechanics of an avian's wings involve a complex interworking of forces, particularly at the shoulder where most of the wings' motions take place. These functions depend on a precise balance of forces from the muscles, ligaments, and articular cartilages as well as inertial, gravitational, and aerodynamic loads on the wing.[4]

Birds have two main muscles in their wing that are responsible for flight: the pectoralis and the supracoracoideus. The pectoralis is the largest muscle in the wing and is the primary depressor and pronator of the wing. The supracoracoideus is the second largest and is the primary elevator and supinator. In addition, there are distal wing muscles that assist the bird in flight.[5]

Prior to their existence on birds, feathers were present on the bodies of many dinosaur species. Through natural selection, feathers became more common among the animals as their wings developed over the course of tens of millions of years.[6] The smooth surface of feathers on a bird's body helps to reduce friction while in flight. The tail, also consisting of feathers, helps the bird to maneuver and glide.[7]

Hypotheses

Pouncing Proavis model

A theory of a pouncing proavis was first proposed by Garner, Taylor, and Thomas in 1999:[8]

We propose that birds evolved from predators that specialized in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack. Drag–based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack. Selection for enhanced lift-based control led to improved lift coefficients, incidentally turning a pounce into a swoop as lift production increased. Selection for greater swooping range would finally lead to the origin of true flight.

The authors believed that this theory had four main virtues:

  • It predicts the observed sequence of character acquisition in avian evolution.
  • It predicts an Archaeopteryx-like animal, with a skeleton more or less identical to terrestrial theropods, with few adaptations to flapping, but very advanced aerodynamic asymmetrical feathers.
  • It explains that primitive pouncers (perhaps like Microraptor) could coexist with more advanced fliers (like Confuciusornis or Sapeornis) since they did not compete for flying niches.
  • It explains that the evolution of elongated rachis-bearing feathers began with simple forms that produced a benefit by increasing drag. Later, more refined feather shapes could begin to also provide lift.[8]

Cursorial model

A cursorial, or "running" model was originally proposed by Samuel Wendell Williston in 1879. This theory states that "flight evolved in running bipeds through a series of short jumps". As the length of the jumps extended, the wings were used not only for thrust but also for stability, and eventually eliminated the gliding intermediate. This theory was modified in the 1970s by John Ostrom to describe the use of wings as an insect-foraging mechanism which then evolved into a wing stroke. Research was conducted by comparing the amount of energy expended by each hunting method with the amount of food gathered. The potential hunting volume doubles by running and jumping. To gather the same volume of food, Archaeopteryx would expend less energy by running and jumping than by running alone. Therefore, the cost/benefit ratio would be more favorable for this model. Due to Archaeopteryx's long and erect leg, supporters of this model say the species was a terrestrial bird. This characteristic allows for more strength and stability in the hindlimbs. Thrust produced by the wings coupled with propulsion in the legs generates the minimum velocity required to achieve flight. This wing motion is thought to have evolved from asymmetrical propulsion flapping motion.[9] Thus, through these mechanisms, Archaeopteryx was able to achieve flight from the ground up.

Although the evidence in favor of this model is scientifically plausible, the evidence against it is substantial. For instance, a cursorial flight model would be energetically less favorable when compared to the alternative hypotheses. In order to achieve liftoff, Archaeopteryx would have to run faster than modern birds by a factor of three, due to its weight. Furthermore, the mass of Archaeopteryx versus the distance needed for minimum velocity to obtain liftoff speed is proportional, therefore, as mass increases, the energy required for takeoff increases. Other research has shown that the physics involved in cursorial flight would not make this a likely answer to the origin of avian flight. Once flight speed is obtained and Archaeopteryx is in the air, drag would cause the velocity to instantaneously decrease; balance could not be maintained due to this immediate reduction in velocity. Hence, Archaeopteryx would have a very short and ineffective flight. In contrast to Ostrom’s theory regarding flight as a hunting mechanism, physics again does not support this model. In order to effectively trap insects with the wings, Archaeopteryx would require a mechanism such as holes in the wings to reduce air resistance. Without this mechanism, the cost/benefit ratio would not be feasible.

The decrease in efficiency when looking at the cursorial model is caused by the flapping stroke needed to achieve flight. This stroke motion needs both wings to move in a symmetrical motion, or together. This is opposed to an asymmetrical motion like that in humans' arms while running. The symmetrical motion would be costly in the cursorial model because it would be difficult while running on the ground, compared to the arboreal model where it is natural for an animal to move both arms together when falling. There is also a large fitness reduction between the two extremes of asymmetrical and symmetrical flapping motion so the theropods would have evolved to one of the extremes.[10]

Wing-assisted incline running

The WAIR hypothesis, a version of the "cursorial model" of the evolution of avian flight, in which birds' wings originated from forelimb modifications that provided downforce, enabling the proto-birds to run up extremely steep slopes such as the trunks of trees, was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. Note that in this scenario birds need downforce to give their feet increased grip.[11][12] It has been argued that early birds, including Archaeopteryx, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR.[13] However, a study that found lift generated from wings to be the primary factor for successfully accelerating a body toward a substrate during WAIR indicated the onset of flight ability was constrained by neuromuscular control or power output rather than by external wing morphology itself and that partially developed wings not yet capable of flight could indeed provide useful lift during WAIR.[14] Additionally, examination of the work and power requirements for extant bird pectoralis contractile behavior during WAIR at different angles of substrate incline demonstrated incremental increases in these requirements, both as WAIR angles increased and in the transition from WAIR to flapping flight. This provides a model for an evolutionary transition from terrestrial to aerial locomotion as transitional forms incrementally adapted to meet the work and power requirements to scale steeper and steeper inclines using WAIR and the incremental increases from WAIR to flight.[15]

Birds use wing-assisted inclined running from the day they hatch to increase locomotion. This can also be said for birds or feathered theropods whose wing muscles cannot generate enough force to fly, and shows how this behavior could have evolved to help these theropods then eventually led to flight.[16] The progression from wing-assisted incline running to flight can be seen in the growth of birds, from when they are hatchlings to fully grown. They begin with wing-assisted incline running and slowly alter their wing strokes for flight as they grow and are able to make enough force. These transitional stages that lead to flight are both physical and behavioral. The transitions over a hatchling's life can be correlated with the evolution of flight on a macro scale. If protobirds are compared to hatchlings their physical traits such as wing size and behavior may have been similar. Flapping flight is limited by the size and muscle force of a wing. Even while using the correct model of arboreal or cursorial, protobirds' wings were not able to sustain flight, but they did most likely gain the behaviors needed for the arboreal or cursorial model like today's birds do when hatched. There are similar steps between the two.[17] Wing-assisted incline running can also produce a useful lift in babies but is very small compared to that of juveniles and adult birds. This lift was found responsible for body acceleration when going up an incline and leads to flight as the bird grows.[18]

Arboreal model

This model was originally proposed in 1880 by Othniel C. Marsh. The theory states Archaeopteryx was a reptilian bird that soared from tree to tree. After the leap, Archaeopteryx would then use its wings as a balancing mechanism. According to this model, Archaeopteryx developed a gliding method to conserve energy. Even though an arboreal Archaeopteryx exerts energy climbing the tree, it is able to achieve higher velocities and cover greater distances during the gliding phase, which conserves more energy in the long run than a cursorial bipedal runner. Conserving energy during the gliding phase makes this a more energy-efficient model. Therefore, the benefits gained by gliding outweigh the energy used in climbing the tree. A modern behavior model to compare against would be that of the flying squirrel. In addition to energy conservation, arboreality is generally associated positively with survival, at least in mammals.[19]

The evolutionary path between arboreality and flight has been proposed through a number of hypotheses. Dudley and Yanoviak proposed that animals that live in trees generally end up high enough that a fall, purposeful or otherwise, would generate enough speed for aerodynamic forces to have an effect on the body. Many animals, even those which do not fly, demonstrate the ability to right themselves and face the ground ventrally, then exhibiting behaviors that act against aerodynamic forces to slow their rate of descent in a process known as parachuting.[19] Arboreal animals that were forced by predators or simply fell from trees that exhibited these kinds of behaviors would have been in a better position to eventually evolve capabilities that were more akin to flight as we know them today.

Researchers in support of this model have suggested that Archaeopteryx possessed skeletal features similar to those of modern birds. The first such feature to be noted was the supposed similarity between the foot of Archaeopteryx and that of modern perching birds. The hallux, or modified of the first digit of the foot, was long thought to have pointed posterior to the remaining digits, as in perching birds. Therefore, researchers once concluded that Archaeopteryx used the hallux as a balancing mechanism on tree limbs. However, study of the Thermopolis specimen of Archeopteryx, which has the most complete foot of any known, showed that the hallux was not in fact reversed, limiting the creature's ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle.[20] Another skeletal feature that is similar in Archaeopteryx and modern birds is the curvature of the claws. Archaeopteryx possessed the same claw curvature of the foot to that of perching birds. However, the claw curvature of the hand in Archaeopteryx was similar to that in basal birds. Based upon the comparisons of modern birds to Archaeopteryx, perching characteristics were present, signifying an arboreal habitat. The ability for takeoff and flight was originally thought to require a supracoracoideus pulley system (SC). This system consists of a tendon joining the humerus and coracoid bones, allowing rotation of the humerus during the upstroke. However, this system is lacking in Archaeopteryx. Based on experiments performed by M. Sy in 1936,[21] it was proven that the SC pulley system was not required for flight from an elevated position but was necessary for cursorial takeoff.

See also

  • Tetrapteryx, a four-winged stage proposed by William Beebe; hindlimb feathers on Microraptor and Anchiornis have been interpreted as evidence of four-winged gliding.

Footnotes

  1. ^ Voeten, Dennis F.A.E.; et al. (13 March 2018). "Wing bone geometry reveals active flight in Archaeopteryx". Nature Communications. 9 (923): 923. Bibcode:2018NatCo...9..923V. doi:10.1038/s41467-018-03296-8. PMC 5849612. PMID 29535376.
  2. ^ Guarino, Ben (13 March 2018). "This feathery dinosaur probably flew, but not like any bird you know". The Washington Post. Retrieved 13 March 2018.
  3. ^ Lee, Michael S.Y.; Cau, Andrea; Naish, Darren; Dyke, Gareth J. (2014). "Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds". Science. 345 (6196): 562–566. Bibcode:2014Sci...345..562L. doi:10.1126/science.1252243. PMID 25082702.
  4. ^ Baier, David B.; Gatesy, Stephen M.; Jenkins, Farish A. (2006). "A critical ligamentous mechanism in the evolution of avian flight" (PDF). Nature. 445 (7125): 307–310. Bibcode:2007Natur.445..307B. doi:10.1038/nature05435. PMID 17173029.
  5. ^ Tobalske, Bret (2007). "Biomechanics of bird flight". Journal of Experimental Biology. 210 (18): 3135–3146. doi:10.1242/jeb.000273. PMID 17766290.
  6. ^ Heers, Ashley; Dial, Kenneth (2012). "From extant to extinct: locomotor ontogeny and the evolution of avian flight". Trends in Ecology & Evolution. 27 (5): 296–305. doi:10.1016/j.tree.2011.12.003. PMID 22304966.
  7. ^ "Bird Anatomy & Bird Parts". All-Birds. Retrieved 9 April 2016.
  8. ^ a b Garner, J. P.; Taylor, G. K.; Thomas, A. L. R. (1999). "On the origins of birds: the sequence of character acquisition in the evolution of avian flight". Proceedings of the Royal Society B: Biological Sciences. 266 (1425): 1259–1266. doi:10.1098/rspb.1999.0772. PMC 1690052.
  9. ^ Nudds, R.; Dyke, G. (2009). "Forelimb posture in dinosaurs and the evolution of the avian flapping flight-stroke". Evolution. 63 (4): 994–1002. doi:10.1111/j.1558-5646.2009.00613.x. PMID 19154383.
  10. ^ Dyke, G. J.; Nudds, R. L. (2009). "Forelimb Posture In Dinosaurs And The Evolution Of The Avian Flapping Flight-Stroke". Evolution. 63 (4): 994–1002. doi:10.1111/j.1558-5646.2009.00613.x. PMID 19154383.
  11. ^ Dial, K.P. (2003). "Wing-Assisted Incline Running and the Evolution of Flight". Science. 299 (5605): 402–404. Bibcode:2003Sci...299..402D. doi:10.1126/science.1078237. PMID 12532020. Summarized in Morelle, Rebecca (24 January 2008). "Secrets of bird flight revealed" (Web). Scientists believe they could be a step closer to solving the mystery of how the first birds took to the air. BBC News. Retrieved 25 January 2008.
  12. ^ Bundle, M.W & Dial, K.P. (2003). "Mechanics of wing-assisted incline running (WAIR)" (PDF). The Journal of Experimental Biology. 206 (Pt 24): 4553–4564. doi:10.1242/jeb.00673. PMID 14610039.
  13. ^ Senter, P. (2006). "Scapular orientation in theropods and basal birds, and the origin of flapping flight" (Automatic PDF download). Acta Palaeontologica Polonica. 51 (2): 305–313.
  14. ^ Tobalske, B. W. & Dial, K. P. (2007). "Aerodynamics of wing-assisted incline running in birds". The Journal of Experimental Biology. 210 (Pt 10): 1742–1751. doi:10.1242/jeb.001701. PMID 17488937.
  15. ^ Jackson, B. E., Tobalske, B. W. and Dial, K. P. (2011). "The broad range of contractile behaviour of the avian pectoralis: functional and evolutionary implications" (Automatic PDF download). The Journal of Experimental Biology. 214 (Pt 14): 2354–2361. doi:10.1242/jeb.052829. PMID 21697427.CS1 maint: Multiple names: authors list (link)
  16. ^ Dial, K. P. (2003). "Wing-assisted incline running and the evolution of flight". Science. 299 (5605): 402–4. Bibcode:2003Sci...299..402D. doi:10.1126/science.1078237. PMID 12532020.
  17. ^ Dial, K. P.; Jackson, B. E.; Segre, P. (2008). "A fundamental avian wing-stroke provides a new perspective on the evolution of flight". Nature. 451 (7181): 985–9. Bibcode:2008Natur.451..985D. doi:10.1038/nature06517. PMID 18216784.
  18. ^ Tobalske, B. W.; Dial, K. P. (2007). "Aerodynamics of wing-assisted incline running in birds". The Journal of Experimental Biology. 210 (10): 1742–51. doi:10.1242/jeb.001701. PMID 17488937.
  19. ^ a b Dudley, R.; Yanoviak, S. P. (2011). "Animal Aloft: The Origins of Aerial Behavior and Flight" (PDF). Integrative and Comparative Biology. 51 (6): 926–936. doi:10.1093/icb/icr002. PMID 21558180.
  20. ^ MAYR, GERALD; POHL, BURKHARD; HARTMAN, SCOTT; PETERS, D. STEFAN (2007). "The tenth skeletal specimen of Archaeopteryx" (PDF). Zoological Journal of the Linnean Society. 149 (1): 97–116. doi:10.1111/j.1096-3642.2006.00245.x.
  21. ^ Sy, Maxheinz (1936). "Funktionell-anatomische Untersuchungen am Vogelflügel". Journal für Ornithologie. 84 (2): 199–296. doi:10.1007/BF01906709.

References

External links

Alan Feduccia

John Alan Feduccia (born 25 April 1943) is a paleornithologist, specializing in the origins and phylogeny of birds. He is Professor Emeritus at the University of North Carolina. Feduccia's principal authored works include two books, The Age of Birds and The Origin and Evolution of Birds, and numerous papers in various ornithological and biological journals.

Feduccia opposes the scientific consensus that birds originated from and are deeply nested within Theropoda, and are therefore living theropod dinosaurs. He has argued for an alternative theory in which birds share a common stem-ancestor with theropod dinosaurs among more basal archosaurian lineages, with birds originating from small arboreal archosaurs in the Triassic.

Anchiornis

Anchiornis is a genus of small, four-winged paravian dinosaur. The genus Anchiornis contains only the type species Anchiornis huxleyi, named for its similarity to modern birds. Anchiornis fossils have been only found in the Tiaojishan Formation of Liaoning, China, in rocks dated to the Late Jurassic, about 160 million years ago. Anchiornis is known from hundreds of specimens, and given the exquisite preservation of some of these fossils, it became the first Mesozoic dinosaur species for which almost the entire life appearance could be determined, and an important source of information on the early evolution of birds. Anchiornis huxleyi translates to "T.H. Huxley's near-bird" in Latin.

Deinonychus

Deinonychus ( dy-NON-i-kəs; from Greek: δεινός deinós, 'terrible' and ὄνυξ ónux, genitive ὄνυχος ónuchos 'claw') is a genus of dromaeosaurid theropod dinosaur with one described species, Deinonychus antirrhopus. This species, which could grow up to 3.4 metres (11 ft) long, lived during the early Cretaceous Period, about 115–108 million years ago (from the mid-Aptian to early Albian stages). Fossils have been recovered from the U.S. states of Montana, Utah, Wyoming, and Oklahoma, in rocks of the Cloverly Formation, Cedar Mountain Formation and Antlers Formation, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, leading to the "dinosaur renaissance" and igniting the debate on whether dinosaurs were warm-blooded or cold-blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted the small body, sleek, horizontal posture, ratite-like spine, and especially the enlarged raptorial claws on the feet, which suggested an active, agile predator."Terrible claw" refers to the unusually large, sickle-shaped talon on the second toe of each hind foot. The fossil YPM 5205 preserves a large, strongly curved ungual. In life, archosaurs have a horny sheath over this bone, which extends the length. Ostrom looked at crocodile and bird claws and reconstructed the claw for YPM 5205 as over 120 millimetres (4.7 in) long. The species name antirrhopus means "counter balance", which refers to Ostrom's idea about the function of the tail. As in other dromaeosaurids, the tail vertebrae have a series of ossified tendons and super-elongated bone processes. These features seemed to make the tail into a stiff counterbalance, but a fossil of the very closely related Velociraptor mongoliensis (IGM 100/986) has an articulated tail skeleton that is curved laterally in a long S-shape. This suggests that, in life, the tail could bend to the sides with a high degree of flexibility. In both the Cloverly and Antlers formations, Deinonychus remains have been found closely associated with those of the ornithopod Tenontosaurus. Teeth discovered associated with Tenontosaurus specimens imply they were hunted, or at least scavenged upon, by Deinonychus.

Dorsal fin

A dorsal fin is a fin located on the back of most marine and freshwater vertebrates such as fishes, cetaceans (whales, dolphins, and porpoises), and the (extinct) ichthyosaur. Most species have only one dorsal fin, but some have two or three.

Wildlife biologists often use the distinctive nicks and wear patterns which develop on the dorsal fins of large cetaceans to identify individuals in the field.

The bony or cartilaginous bones that support the base of the dorsal fin in fish are called pterygiophores.

Evolution of birds

The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropoda dinosaurs named Paraves. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Paleognathae), ducks and relatives (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves).

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still under debate.

Fin and flipper locomotion

Fin and flipper locomotion occurs mostly in aquatic locomotion, and rarely in terrestrial locomotion. From the three common states of matter — gas, liquid and solid, these appendages are adapted for liquids, mostly fresh or saltwater and used in locomotion, steering and balancing of the body. Locomotion is important in order to escape predators, acquire food, find mates and bury for shelter, nest or food. Aquatic locomotion consists of swimming, whereas terrestrial locomotion encompasses walking, 'crutching', jumping, digging as well as covering. Some animals such as sea turtles and mudskippers use these two environments for different purposes, for example using the land for nesting, and the sea to hunt for food.

Fish fin

Fins are usually the most distinctive anatomical features of a fish. They are composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as seen in sharks. Apart from the tail or caudal fin, fish fins have no direct connection with the spine and are supported only by muscles. Their principal function is to help the fish swim. Fins located in different places on the fish serve different purposes such as moving forward, turning, keeping an upright position or stopping. Most fish use fins when swimming, flying fish use pectoral fins for gliding, and frogfish use them for crawling. Fins can also be used for other purposes; male sharks and mosquitofish use a modified fin to deliver sperm, thresher sharks use their caudal fin to stun prey, reef stonefish have spines in their dorsal fins that inject venom, anglerfish use the first spine of their dorsal fin like a fishing rod to lure prey, and triggerfish avoid predators by squeezing into coral crevices and using spines in their fins to lock themselves in place.

Index of biophysics articles

This is a list of articles on biophysics.

Index of physics articles (O)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Longisquama

Longisquama is a genus of extinct reptile. There is only one species, Longisquama insignis, known from a poorly preserved skeleton and several incomplete fossil impressions from the Middle to Late Triassic Madygen Formation in Kyrgyzstan. It is known from a type fossil specimen, slab and counterslab (PIN 2548/4 and PIN 2584/5) and five referred specimens of possible integumentary appendages (PIN 2584/7 through 9). All specimens are in the collection of the Paleontological Institute of the Russian Academy of Sciences in Moscow.

Longisquama means "long scales"; the specific name insignis refers to its small size. The Longisquama holotype is notable for a number of long structures that appear to grow from its skin. The current opinion is that Longisquama is an ambiguous diapsid and has no bearing on the origin of birds.

Microraptor

Microraptor (Greek, μικρός, mīkros: "small"; Latin, raptor: "one who seizes") is a genus of small, four-winged paravian dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They date from the early Cretaceous Jiufotang Formation (Aptian stage), 120 million years ago. Three species have been named (M. zhaoianus, M. gui, and M. hanqingi), though further study has suggested that all of them represent variation in a single species, which is properly called M. zhaoianus. Cryptovolans, initially described as another four-winged dinosaur, is usually considered to be a synonym of Microraptor.Like Archaeopteryx, well-preserved fossils of Microraptor provide important evidence about the evolutionary relationship between birds and dinosaurs. Microraptor had long pennaceous feathers that formed aerodynamic surfaces on the arms and tail but also on the legs. This led paleontologist Xu Xing in 2003 to describe the first specimen to preserve this feature as a "four-winged dinosaur" and to speculate that it may have glided using all four limbs for lift. Subsequent studies have suggested that Microraptor was capable of powered flight as well.

Microraptor was among the most abundant non-avialan dinosaurs in its ecosystem, and the genus is represented by more fossils than any other dromaeosaurid, with possibly over 300 fossil specimens represented across various museum collections.

Origin of birds

The scientific question of within which larger group of animals birds evolved, has traditionally been called the origin of birds. The present scientific consensus is that birds are a group of theropod dinosaurs that originated during the Mesozoic Era.

A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany. Birds and extinct non-avian dinosaurs share many unique skeletal traits. Moreover, fossils of more than thirty species of non-avian dinosaur have been collected with preserved feathers. There are even very small dinosaurs, such as Microraptor and Anchiornis, which have long, vaned, arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna also shows these long foot feathers. Paleontologist Lawrence Witmer concluded in 2009 that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage. Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastroliths in the digestive system, nest-building and brooding behaviors.

Although the origin of birds has historically been a contentious topic within evolutionary biology, only a few scientists still debate the dinosaurian origin of birds, suggesting descent from other types of archosaurian reptiles. Within the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed. The origin of bird flight is a separate but related question for which there are also several proposed answers.

Outline of birds

The following outline is provided as an overview of and topical guide to birds:

Birds (class Aves) – winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species, making them the most varied of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich.

Paraves

Paraves are a widespread group of theropod dinosaurs that originated in the Late Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and scansoriopterygids, the group also contains the avialans, among which are the over ten thousand species of living birds. Primitive members of Paraves are well known for the possession of an enlarged claw on the second digit of the foot, which was held off the ground when walking in some species.

Pennaraptora

Pennaraptora (Latin penna "bird feather" + raptor "thief", from rapere "snatch"; a feathered bird-like predator) is a clade defined as the most recent common ancestor of Oviraptor philoceratops, Deinonychus antirrhopus, and Passer domesticus (the house sparrow), and all descendants thereof, by Foth et al., 2014. The earliest known definitive member of this clade is Anchiornis, from the late Jurassic period of China, about 160 million years ago.

The clade "Aviremigia" was conditionally proposed along with several other apomorphy-based clades relating to birds by Jacques Gauthier and Kevin de Queiroz in a 2001 paper. Their proposed definition for the group was "the clade stemming from the first panavian with ... remiges and rectrices, that is, enlarged, stiff-shafted, closed-vaned (= barbules bearing hooked distal pennulae), pennaceous feathers arising from the distal forelimbs and tail".

Praeornis

Praeornis is a dubious genus of early avialan or bird-like dinosaur, named on the basis of a single feather discovered in the Balabansai Formation of Kazakhstan by Sharov in 1971. A second specimen was discovered in 2010 by Dzik et al. The feathers of Praeornis likely represent modified tail feathers used for display or balance, similar to those found in some other early avialans. The feathers of Praeornis are unique thanks to their extremely thick central quill (rachis) and stiffened barbs.In 1978, Rautian officially named the feather (cataloged as specimen PIN 2585/32) Praeornis sharovi. He believed it belonged to a bird more primitive than Archaeopteryx, and assigned it to its own sublcass (Praeornithes), order (Praeornithiformes) and family (Praeornithidae). However, in 1986, Bock published a paper arguing that the "feather" was in fact the leaf of a cycad. This opinion was followed by Doludenko and colleagues in 1990, who noted that it was similar to the leaves of the cycad species Paracycas harrisii. L.A. Nessov, in 1992, also suggested that it belonged to a cycad, but synonymized it with the species Cycadites saportae. The opinion that it represents a leaf has since been followed by Alan Feduccia in 1996 and Peter Wellnhofer in 2004.Three studies since the original description has supported the identification of Praeornis as a feather, rather than a leaf. In 1991, Glazunova and colleagues examined the specimen using an electron microscope, and found that the microstructure had features in common with the "primitive" feathers of ratite birds [since ratites are now known to be secondarily flightless paleognathous birds, their feathers are not primitive but degenerate flight and contour feathers]. In a 2001 paper, Kurochkin also accepted its identity as a feather. A more comprehensive study was published in 2010 by Dzik et al., in which the authors conducted a biochemical analysis of a Praeornis feather and other fossils from the same site, including plants and fish. The analysis showed that the chemical markers of the Praeornis fossil was more similar to the fish scales than to the plant leaves, supporting the hypothesis that the feathers were animal in origin. Besides identifying Praeornis as a feather, Dzik et al. also noticed similarities between the purported feathers of Longisquama and those of Praeornis.In 2017, a fossil of an enantiornith found in Brazil was shown to have a pair of rachis-dominated tail feathers very similar to the type specimen of Praeornis, making it likely that Praeornis represents an enantiornith or similar species.

Proavis

Proavis refers to a hypothetical extinct species or hypothetical extinct taxon and was coined in the early 20th century in an attempt to support and explain the hypothetical evolutionary steps and anatomical adaptations leading from non-avian theropod dinosaurs to birds. The term has also been used by defenders of the thecodontian origin of birds. The concept should not be confused with the genus Protoavis.

Sinornithosaurus

Sinornithosaurus (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period (early Aptian) of the Yixian Formation in what is now China. It was the fifth non–avian feathered dinosaur genus discovered by 1999. The original specimen was collected from the Sihetun locality of western Liaoning. It was found in the Jianshangou beds of the Yixian Formation, dated to 124.5 million years ago. Additional specimens have been found in the younger Dawangzhangzi bed, dating to around 122 million years ago.Xu Xing described Sinornithosaurus and performed a phylogenetic analysis which demonstrated that it is basal, or primitive, among the dromaeosaurs. He has also demonstrated that features of the skull and shoulder are very similar to Archaeopteryx and other Avialae. Together these two facts demonstrate that the earliest dromaeosaurs were more like birds than the later dromaeosaurs were.

Sinornithosaurus was among the smallest dromaeosaurids, with a length of about 90 centimetres (3.0 ft). In 2010, Gregory S. Paul gave higher estimations of 1.2 metres and three kilogrammes.

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