Stilts
Stilts are poles, posts or pillars used to allow a person or structure to stand at a height above the ground. Stilts for walking are poles equipped with platforms for the feet to stand on and can be used, depending on the design, with straps to attach them to the user's legs or be held in place by the hands of the user. In flood plains, on beaches or unstable ground, buildings are constructed on stilts to protect them from damage by water, waves or shifting soil or sand. Stilts have been used for many hundreds of years. Hand-held stilts are used as childhood toys and in circus skills workshops and are of two main types: string and can/bucket stilts and pole stilts. Unlike other forms of stilts, hand-held stilts are not strapped to the wearer. Hand-held pole stilts consist of each with a foot support; the stilt walker holds onto the upper end of the pole, rests his feet on the foot plates and pulls upward on the pole while taking a step. A second type of hand-held pole stilts are similar to the first type but ends in a handle so the walker has more control and flexibility to move his stilts.
Those type of stilts can be high. Hand-held string stilts are platforms with strings attached to them; the platforms, most made of tin cans or small plastic upturned buckets hold the stilt walker's weight while the strings are used to pull the cans to the feet as they take a step. Peg stilts known as Chinese stilts, are used by professional performers; these stilts strap on at the foot and just below the knee. Peg stilts are made from wood but can be made of aluminium or tubular steel; this type of stilts are the most lightweight ones and allow a user to walk to turn and to jump rope or dance. The stilt walker must keep moving at all times to keep their balance. Drywall or Dura stilts are designed to allow the stilt walker to walk, they were designed for people to work at an elevated height during drywall or plasterboard construction and other such activities. Drywall stilts are heavier than peg stilts and are mostly made of aluminium; the design means they are safer for walking but means they are less versatile than peg stilts in use.
Spring stilts are spring-loaded stilts that allow the user to run and perform various acrobatics. Spring stilts using fiberglass leaf springs were patented in the United States in 2004 under the trademark "PowerSkip", marketed for recreational and extreme sports use. Using these stilts is called Powerbocking, named for the stilts' inventor, Alexander Boeck. Spring stilts are mostly made of aluminium. Spring stilts using steel coil springs, an antecedent of the pogo stick, were attempted in the 19th century; the Digitigrade stilt is a peg stilt. This allows costumers to mimic any digitigrade animal: horses, goats, etc; because of the extreme stresses on this type of design they tend to be more rare. This type of stilt is similar to drywall stilts in that they allow the stilt walker to stand in one place without shifting their weight from foot to foot. Articulated stilts feature a flexing joint under the ball of the performers foot, in one variant under the performer's heel; these stilts are used in theme parks such as Walt Disney World and Universal Studios because they safely allow performers to dance and perform stunts that would damage other types of stilts.
Two brands of articulated stilts include'Bigfoots' manufactured by Gary Ensmenger of Orlando, FL and'Jay Walkers', manufactured by Stilt Werks of Las Vegas, Nevada. Archaeological ruins and texts show that stiltwalking was practised in ancient Greece as far back as the 6th century BC; the ancient Greek word for a stilt walker was κωλοβαθριστής, from κωλόβαθρον, "stilt", a compound of κῶλον, "limb" and βάθρον, "base, pedestal". Some stilt use traditions are old. In Belgium, stilt walkers of Namur have practiced fights on stilts since 1411; the inhabitants of marshy or flooded areas sometimes use stilts for practical purposes, such as working in swamps or fording swollen rivers. The shepherds of the Landes region of southern France used to watch their flocks while standing on stilts to extend their field of vision, while townspeople used them to traverse the soggy ground in their everyday activities. Stilts were used by workers to attach hop gras to wires at 12 feet above the ground; this technique was documented up to the mid 20.
Century before being superseded. Stilts can be used for various purposes: as a prop in entertainment, as a tool to enable other types of work to be achieved and as part of a hobby or recreation. Stilts are used in many countries for the purpose of entertainment. Stilt walkers perform their skills in parades, street events and at corporate functions; the local festivals of Anguiano feature a dance on stilts in which dancers go down a stepped street while turning. Other stilts walking and dancing festivals are held in Deventer, Netherlands in early July each year, in Namur, Belgium. Early stilt walking acts were of the style of a tall person with the costume having long trousers or skirt to cover the stilts. More stilt walkers have created a wide variety of costumes that do not resemble a tall person. Examples are animals; the tall person type has expanded to include a wide variety of themes. Examples include sportsmen, historical acts based on literary or film characters. One of the most recent varieties of stilt walking acts is a stilt walker riding a'stilt bicycle'.
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Animal
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Insect
Insects or Insecta are hexapod invertebrates and the largest group within the arthropod phylum. Definitions and circumscriptions vary; as used here, the term Insecta is synonymous with Ectognatha. Insects have a chitinous exoskeleton, a three-part body, three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; the total number of extant species is estimated at between ten million. Insects may be found in nearly all environments, although only a small number of species reside in the oceans, which are dominated by another arthropod group, crustaceans. Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts; the immature stages differ from the adults in structure and habitat, can include a passive pupal stage in those groups that undergo four-stage metamorphosis. Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages.
The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm; the most diverse insect groups appear to have coevolved with flowering plants. Adult insects move about by walking, flying, or sometimes swimming; as it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles, composed of the front & rear on one side with the middle on the other side. Insects are the only invertebrates to have evolved flight, all flying insects derive from one common ancestor. Many insects spend at least part of their lives under water, with larval adaptations that include gills, some adult insects are aquatic and have adaptations for swimming; some species, such as water striders, are capable of walking on the surface of water. Insects are solitary, but some, such as certain bees and termites, are social and live in large, well-organized colonies.
Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light. Humans regard certain insects as pests, attempt to control them using insecticides, a host of other techniques; some insects damage crops by feeding on sap, fruits, or wood. Some species are parasitic, may vector diseases; some insects perform complex ecological roles. Insect pollinators are essential to the life cycle of many flowering plant species on which most organisms, including humans, are at least dependent. Many insects are considered ecologically beneficial as predators and a few provide direct economic benefit. Silkworms produce silk and honey bees produce honey and both have been domesticated by humans.
Insects are consumed as food in 80% of the world's nations, by people in 3000 ethnic groups. Human activities have effects on insect biodiversity; the word "insect" comes from the Latin word insectum, meaning "with a notched or divided body", or "cut into", from the neuter singular perfect passive participle of insectare, "to cut into, to cut up", from in- "into" and secare "to cut". A calque of Greek ἔντομον, "cut into sections", Pliny the Elder introduced the Latin designation as a loan-translation of the Greek word ἔντομος or "insect", Aristotle's term for this class of life in reference to their "notched" bodies. "Insect" first appears documented in English in 1601 in Holland's translation of Pliny. Translations of Aristotle's term form the usual word for "insect" in Welsh, Serbo-Croatian, etc; the precise definition of the taxon Insecta and the equivalent English name "insect" varies. In the broadest circumscription, Insecta sensu lato consists of all hexapods. Traditionally, insects defined in this way were divided into "Apterygota" —the wingless insects—and Pterygota—the winged insects.
However, modern phylogenetic studies have shown that "Apterygota" is not monophyletic, so does not form a good taxon. A narrower circumscription restricts insects to those hexapods with external mouthparts, comprises only the last three groups in the table. In this sense, Insecta sensu stricto is equivalent to Ectognatha. In the narrowest circumscription, insects are restricted to hexapods that are either winged or descended from winged ancestors. Insecta sensu strictissimo is equivalent to Pterygota. For the purposes of this article, the middle definition is used; the evolutionary relationship of insects to other animal groups remains unclear. Although traditionally grouped with millipedes and centiped
Spiracle
Spiracles are openings on the surface of some animals, which lead to respiratory systems. The spiracle is a small hole behind each eye. In the primitive jawless fish the first gill opening behind the mouth is similar to the other gill opening. With the evolution of the jaw in the early jawed vertebrates, this gill slit was "caught" between the forward gill-rod and the next rod, the hyomandibular bone, supporting the jaw hinge and anchoring the jaw to the skull proper; the gill opening was closed off from below, the remaining opening was small and hole-like, is termed a "spiracle". The spiracle is still found in all cartilaginous fish except requiem sharks, hammerhead sharks, chimaeras, is found in some primitive bony fishes, it is seen as an otic notch in the skull of the extinct labyrinthodonts, is thought to be associated with the ear opening in amniotes and frogs. Insects and some more derived spiders have spiracles on their exoskeletons to allow air to enter the trachea. In the respiratory system of insects, the tracheal tubes deliver oxygen directly into the animals' tissues.
The spiracles can be closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. In order to open, the muscle relaxes; the closer muscle is controlled by the central nervous system but can react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea; the timing and duration of spiracle closures can affect the respiratory rates of the organism. Spiracles may be surrounded by hairs to minimize bulk air movement around the opening, thus minimize water loss. Chapman, R. F; the Insects. 1998. Cambridge University Press
Ocean
An ocean is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean; these are, in descending order by area, the Pacific, Indian and Arctic Oceans. The word "ocean" is used interchangeably with "sea" in American English. Speaking, a sea is a body of water or enclosed by land, though "the sea" refers to the oceans. Saline water covers 361,000,000 km2 and is customarily divided into several principal oceans and smaller seas, with the ocean covering 71% of Earth's surface and 90% of the Earth's biosphere; the ocean contains 97% of Earth's water, oceanographers have stated that less than 5% of the World Ocean has been explored. The total volume is 1.35 billion cubic kilometers with an average depth of nearly 3,700 meters. As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, influences climate and weather patterns; the World Ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger over two million.
The origin of Earth's oceans is unknown. Extraterrestrial oceans may be composed of water or other compounds; the only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories and Venus are theorized to have had large water oceans; the Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of natural satellites; the Solar System's giant planets are thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone.
Ocean planets are a hypothetical type of planet with a surface covered with liquid. The word ocean comes from the figure in classical antiquity, the elder of the Titans in classical Greek mythology, believed by the ancient Greeks and Romans to be the divine personification of the sea, an enormous river encircling the world; the concept of Ōkeanós has an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases. Though described as several separate oceans, the global, interconnected body of salt water is sometimes referred to as the World Ocean or global ocean; the concept of a continuous body of water with free interchange among its parts is of fundamental importance to oceanography. The major oceanic divisions – listed below in descending order of area and volume – are defined in part by the continents, various archipelagos, other criteria.
Oceans are fringed by smaller, adjoining bodies of water such as seas, bays and straits. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system, part of every ocean and the longest mountain range in the world; the continuous mountain range is 65,000 km long. The total mass of the hydrosphere is about 1.4 quintillion metric tons, about 0.023% of Earth's total mass. Less than 3% is freshwater; the area of the World Ocean is about 361.9 million square kilometers, which covers about 70.9% of Earth's surface, its volume is 1.335 billion cubic kilometers. This can be thought of as a cube of water with an edge length of 1,101 kilometers, its average depth is about 3,688 meters, its maximum depth is 10,994 meters at the Mariana Trench. Nearly half of the world's marine waters are over 3,000 meters deep; the vast expanses of deep ocean cover about 66% of Earth's surface. This does not include seas not connected to the World Ocean, such as the Caspian Sea; the bluish ocean color is a composite of several contributing agents.
Prominent contributors include dissolved organic chlorophyll. Mariners and other seafarers have reported that the ocean emits a visible glow which extends for miles at night. In 2005, scientists announced that for the first time, they had obtained photographic evidence of this glow, it is most caused by bioluminescence. Oceanographers divide the ocean into different vertical zones defined by physical and biological conditions; the pelagic zone includes all open ocean regions, can be divided into further regions categorized by depth and light abundance. The photic zone includes the oceans from the surface to a depth of
Mosquito
Mosquitoes are a group of about 3500 species of small insects that are a type of fly. Within that order they constitute the family Culicidae; the word "mosquito" is Spanish for "little fly". Mosquitoes have a slender segmented body, a pair of wings, three pairs of long hair-like legs, feathery antennae, elongated mouthparts. Mosquitoes diverged from other insects about 226 million years ago. Fossils of primitive mosquitoes have been found; the life cycle consists of the egg, larva and adult. Eggs are laid on the water surface. Females of most species have tube-like mouthparts which can pierce the skin of the host in order to extract blood, which contains protein and iron needed to produce eggs. Thousands of mosquito species feed on the blood of various hosts — vertebrates, including mammals, reptiles and some fish; this loss of blood is of any importance to the host. The saliva of the mosquito transmitted to the host with the bite can cause a rash. In addition, many species of mosquitoes inject or ingest disease-causing organisms with the bite and are thus a vector for the transmission of diseases such as malaria, yellow fever, West Nile virus, dengue fever, Zika virus and other arboviruses.
Mosquitoes kill more people than any other animal: over 700,000 each year. The oldest known mosquito with an anatomy similar to modern species was found in 79-million-year-old Canadian amber from the Cretaceous. An older sister species with more primitive features was found in Burmese amber, 90 to 100 million years old. Two mosquito fossils have been found that show little morphological change in modern mosquitoes against their counterpart from 46 million years ago; these fossils are the oldest found to have blood preserved within their abdomens. Despite no fossils being found earlier than the Cretaceous, recent studies suggest that the earliest divergence of mosquitoes between the lineages leading to Anophelinae and Culicinae occurred 226 million years ago; the mosquito Anopheles gambiae is undergoing speciation into the M and S molecular forms. Some pesticides that work on the M form no longer work on the S form. Over 3,500 species of the Culicidae have been described, they are divided into two subfamilies which in turn comprise some 43 genera.
These figures are subject to continual change, as more species are discovered, as DNA studies compel rearrangement of the taxonomy of the family. The two main subfamilies are the Anophelinae and Culicinae, with their genera as shown in the subsection below; the distinction is of great practical importance because the two subfamilies tend to differ in their significance as vectors of different classes of diseases. Speaking, arboviral diseases such as yellow fever and dengue fever tend to be transmitted by Culicine species, not in the genus Culex; some transmit various species of avian malaria, but it is not clear that they transmit any form of human malaria. Some species do however transmit various forms of filariasis, much as many Simuliidae do. Mosquitoes are members of a family of nematocerid flies: the Culicidae. Superficially, mosquitoes resemble. Anophelinae Culicinae Over 3,500 species of mosquitoes have thus far been described in the scientific literature. Like all flies, mosquitoes go through four stages in their lifecycles: egg, larva and adult or imago.
The first three stages—egg and pupa—are aquatic. These stages last 5 to 14 days, depending on the species and the ambient temperature, but there are important exceptions. Mosquitoes living in regions where some seasons are freezing or waterless spend part of the year in diapause. For instance, Wyeomyia larvae get frozen into solid lumps of ice during winter and only complete their development in spring; the eggs of some species of Aedes remain unharmed in diapause if they dry out, hatch when they are covered by water. Eggs hatch to become larvae; the adult mosquito emerges from the mature pupa. Bloodsucking mosquitoes, depending on species and weather conditions, have potential adult lifespans ranging from as short as a week to as long as several months; some species can overwinter as adults in diapause. In most species, adult females lay their eggs in stagnant water: some lay near the water's edge while others attach their eggs to aquatic plants; each species selects the situation of the water into which it lays its eggs and does so according to its own ecological adaptations.
Some are generalists and are not fussy. Some breed in some in temporary puddles; some breed in some in salt-marshes. Among those that breed in salt water, some are at home in fresh and salt water up to about one-third the concentration of seawater, whereas others must acclimatize themselves to the salinity; such differences are important because certain ecological pre
Bird
Birds known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, a strong yet lightweight skeleton. Birds range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the world's most numerically-successful class of tetrapods, with ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are less developed depending on the species. 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 and diverse endemic island species of birds; the digestive and respiratory systems of birds are uniquely adapted for flight. Some bird species of aquatic environments seabirds and some waterbirds, have further evolved for swimming; the fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs.
The closest living relatives of birds 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. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of powered flight, many retained primitive characteristics like toothy jaws in place of beaks, long bony tails. DNA-based evidence finds that birds diversified 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 those in the southern continents, survived this event and migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics; some birds corvids and parrots, are among the most intelligent animals. Many species annually migrate great distances. Birds are social, communicating with visual signals and bird songs, participating in such social behaviours as cooperative breeding and hunting and mobbing of predators.
The vast majority of bird species are monogamous for one breeding season at a time, sometimes for years, but for life. Other species have breeding systems that are polygynous or polyandrous. Birds produce offspring by laying eggs, they are 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 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 being important sources of eggs and feathers. Songbirds and other species are popular as pets. Guano 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, 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 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 in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. 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 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, 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, assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier identified four different definitions for the same biological name "Aves", 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. Aves can mean all archosaurs closer to birds than to crocodiles Aves can mean those advanced archosaurs with feathers Aves can mean those feathered dinosaurs that fly Aves can mean the last common ancestor of all the living birds and all of its descendants (a "c
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