Warmbloods are a group of middle-weight horse types and breeds originating in Europe and registered with organizations that are characterized by open studbook policy, studbook selection, the aim of breeding for equestrian sport. The term distinguishes these horses from both heavy draft horses and refined light saddle horses such as the Thoroughbred and Akhal-Teke. Although modern warmbloods are descended from heavier agricultural types systematically upgraded by hotblood influence, the term does not imply that warmbloods are direct crosses of "cold" and "hot". Open studbook policies separate most warmbloods from true "breeds" such as Thoroughbreds, Arabians and Morgans which have a closed stud book and require two purebred parents. Instead, most warmblood registries accept breeding stock from other similar populations to continuously improve their own, do not consider their own horses to be a discrete "breed"; the Trakehner is an exception, as although some other breeds are used within the breeding population, this horse is considered a true breed.
The Hanoverian and Selle Français studbooks are considered less open than others. Most warmblood registries recognize breeding stock from any other registry, a member of the World Breeding Federation for Sport Horses, affiliated with the IOC-recognized International Federation for Equestrian Sports. A defining characteristic of a warmblood registry is studbook selection, though some purebred breeds in Europe use this practice. Studbook selection is the use of external evaluation - critiquing conformation and movement - of potential breeding stock to cull unsuitable breeding horses and direct the evolution towards a particular goal. Today, studbook selection entails a performance proof in addition to external evaluation for stallions. Standards of conformation and movement are not designed to perpetuate a particular ancestral type, but rather to meet a particular need; this concept is illustrated by the history of the Oldenburg horse through the past 150 years: in the late 19th century, the standard called for a heavy but elegant, high-stepping carriage horse, in the early 20th century for a heavier, economical farm and artillery horse, since 1950 for a modern sport horse.
The most critical characteristic of a warmblood registry is that its breeding goal is to breed sport horses. Each registry has a different focus, but most breed for show jumping and dressage. Many include combined eventing as well; the breeding aim is reflective of the needs of the market. In eras and regions which called for cavalry mounts, warmbloods were bred to fit that need; the purposeful evolution of the standard breeding aim is another characteristic of the warmbloods. Warmbloods have become popular since the end of World War II when mechanization made agricultural horses obsolete, recreational riding became more widespread in the western world; the ancestral warmblood types are referred to as the heavy warmbloods and are preserved through special organizations. The heavy warmbloods have found their niche in combined driving. Most warmbloods were developed in continental Europe Germany, it was once thought that the warmblood type, which originated in continental Europe, descended from wild, native proto-warmblood ancestors, called the Forest Horse, though modern DNA studies of early horses have disproven this hypothesis.
The best-known German warmbloods are the Hanoverian, Holsteiner and the purebred Trakehner. Others include the Württemberger, Westphalian, Zweibrücker, Brandenburger and Bavarian Warmblood. Several of these breeds are represented by ancestral types such as the Ostfriesen and Alt-Oldenburger, Alt-Württemberger, Rottaler. Western European warmbloods include the French Selle Français, Belgian Warmblood, Dutch Warmblood, Swiss Warmblood, Austrian Warmblood and Danish Warmblood. Scandinavian countries produce high-quality warmbloods such as the Finnish Warmblood and Swedish Warmblood. Warmblood registries which are not based in continental Europe include those that regulate the breeding of American Warmbloods and Irish Sport Horses. Sport horse List of horse breeds Edwards, E. H; the Encyclopedia of the Horse, London: Dorling Kindersley, ISBN 1-56458-614-6. International Museum of the Horse World Breeding Federation for Sports Horses
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, pectoral fins that are not fused to the head. Modern sharks are the sister group to the rays. However, the term "shark" has been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago. Acanthodians are referred to as "spiny sharks". Since sharks have diversified into over 500 species, they range in size from the small dwarf lanternshark, a deep sea species of only 17 centimetres in length, to the whale shark, the largest fish in the world, which reaches 12 metres in length. Sharks are common to depths of 2,000 metres, they do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can be found in both seawater and freshwater.
Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth. Well-known species such as the great white shark, tiger shark, blue shark, mako shark, thresher shark, hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities; until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in the porbeagle; the etymology of the word "shark" is uncertain, the most etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning "villain, scoundrel", applied to the fish due to its predatory behaviour. A now disproven theory is that it derives from the Yucatec Maya word xok, meaning "fish". Evidence for this etymology came from the Oxford English Dictionary, which notes shark first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted "sharke" to refer to the large sharks of the Caribbean Sea.
However, the Middle English Dictionary records an isolated occurrence of the word shark in a letter written by Thomas Beckington in 1442, which rules out a New World etymology. Evidence for the existence of sharks dates from the Ordovician period, 450–420 million years ago, before land vertebrates existed and before a variety of plants had colonized the continents. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks, suspecting that these scales are those of thelodont agnathans; the oldest accepted shark scales are from about 420 million years ago, in the Silurian period. The first sharks looked different from modern sharks. At this time the most common shark tooth is the cladodont, a style of thin tooth with three tines like a trident to help catch fish; the majority of modern sharks can be traced back to around 100 million years ago. Most fossils are of teeth in large numbers. Partial skeletons and complete fossilized remains have been discovered.
Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils. The teeth consist of fossilized calcium phosphate, an apatite; when a shark dies, the decomposing skeleton breaks up. Preservation requires rapid burial in bottom sediments. Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, found within Paleozoic strata in Ohio and Tennessee. At that point in Earth's history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about 1 metre long with stiff triangular fins and slender jaws, its teeth had several pointed cusps. From the small number of teeth found together, it is most that Cladoselache did not replace its teeth as as modern sharks, its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. The presence of whole fish arranged tail-first in their stomachs suggest that they were fast swimmers with great agility.
Most fossil sharks from about 300 to 150 million years ago can be assigned to one of two groups. The Xenacanthida was exclusive to freshwater environments. By the time this group became extinct about 220 million years ago, they had spread worldwide; the other group, the hybodonts, appeared about 320 million years ago and lived in the oceans, but in freshwater. The results of a 2014 study of the gill structure of an unusually well preserved 325-million-year-old fossil suggested that sharks are not "living fossils", but rather have evolved more extensively than thought over the hundreds of millions of years they have been around. Modern sharks began to appear about 100 million years ago. Fossil mackerel shark teeth date to the Early Cretaceous. One of the most evolved families is the hammerhead shark, which emerged in the Eocene; the oldest white shark teeth date from 60 to 66 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution th
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
Feathers are epidermal growths that form the distinctive outer covering, or plumage, on birds, other extinct species of dinosaurs, pterosaurs. They are considered the most complex integumentary structures found in vertebrates and a premier example of a complex evolutionary novelty, they are among the characteristics. Although feathers cover most of the bird's bodies, they arise only from certain well-defined tracts on the skin, they aid in flight, thermal insulation, waterproofing. In addition, coloration helps in protection. Plumology is the name for the science, associated with the study of feathers. Feathers are among the most complex integumentary appendages found in vertebrates and are formed in tiny follicles in the epidermis, or outer skin layer, that produce keratin proteins; the β-keratins in feathers and claws — and the claws and shells of reptiles — are composed of protein strands hydrogen-bonded into β-pleated sheets, which are further twisted and crosslinked by disulfide bridges into structures tougher than the α-keratins of mammalian hair and hoof.
The exact signals that induce the growth of feathers on the skin are not known, but it has been found that the transcription factor cDermo-1 induces the growth of feathers on skin and scales on the leg. There are two basic types of feather: vaned feathers which cover the exterior of the body, down feathers which are underneath the vaned feathers; the pennaceous feathers are vaned feathers. Called contour feathers, pennaceous feathers arise from tracts and cover the entire body. A third rarer type of feather, the filoplume, is hairlike and are associated with contour feathers and are entirely hidden by them, with one or two filoplumes attached and sprouting from near the same point of the skin as each contour feather, at least on a bird's head and trunk. In some passerines, filoplumes arise exposed beyond the contour feathers on the neck; the remiges, or flight feathers of the wing, rectrices, the flight feathers of the tail are the most important feathers for flight. A typical vaned feather features a main shaft, called the rachis.
Fused to the rachis are a series of branches, or barbs. These barbules have minute hooks called barbicels for cross-attachment. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap air and provide excellent thermal insulation. At the base of the feather, the rachis expands to form the hollow tubular calamus which inserts into a follicle in the skin; the basal part of the calamus is without vanes. This part is embedded within the skin follicle and has an opening at the base and a small opening on the side. Hatchling birds of some species have a special kind of natal down feathers which are pushed out when the normal feathers emerge. Flight feathers are stiffened so as to work against the air in the downstroke but yield in other directions, it has been observed that the orientation pattern of β-keratin fibers in the feathers of flying birds differs from that in flightless birds: the fibers are better aligned along the shaft axis direction towards the tip, the lateral walls of rachis region show structure of crossed fibers.
Feathers insulate birds from water and cold temperatures. They may be plucked to line the nest and provide insulation to the eggs and young; the individual feathers in the wings and tail play important roles in controlling flight. Some species have a crest of feathers on their heads. Although feathers are light, a bird's plumage weighs two or three times more than its skeleton, since many bones are hollow and contain air sacs. Color patterns serve as camouflage against predators for birds in their habitats, serve as camouflage for predators looking for a meal; as with fish, the top and bottom colors may be different, in order to provide camouflage during flight. Striking differences in feather patterns and colors are part of the sexual dimorphism of many bird species and are important in selection of mating pairs. In some cases there are differences in the UV reflectivity of feathers across sexes though no differences in color are noted in the visible range; the wing feathers of male club-winged manakins Machaeropterus deliciosus have special structures that are used to produce sounds by stridulation.
Some birds have a supply of powder down feathers which grow continuously, with small particles breaking off from the ends of the barbules. These particles produce a powder that sifts through the feathers on the bird's body and acts as a waterproofing agent and a feather conditioner. Powder down has evolved independently in several taxa and can be found in down as well as in pennaceous feathers, they may be scattered in plumage as in the pigeons and parrots or in localized patches on the breast, belly, or flanks, as in herons and frogmouths. Herons use their bill to break the powder down feathers and to spread them, while cockatoos may use their head as a powder puff to apply the powder. Waterproofing can be lost by exposure to emulsifying agents due to human pollution. Feathers can become waterlogged, causing the bird to sink, it is very difficult to clean and rescue birds whose feathers have been fouled by oil spills. The feathers of cormorants soak up water and help to reduce buoyancy, thereby allowing the birds to swim submerged.
Bristles are stiff. Rictal bristles are found around bill, they may serve a similar purpose to e
An endotherm is an organism that maintains its body at a metabolically favorable temperature by the use of heat set free by its internal bodily functions instead of relying purely on ambient heat. Such internally generated heat is an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted to heat production. Examples include special-function muscular exertion such as shivering, uncoupled oxidative metabolism such as within brown adipose tissue. Only birds and mammals are extant universally endothermic groups of animals. Certain lamnid sharks and billfishes are endothermic. In common parlance, endotherms are characterized as "warm-blooded"; the opposite of endothermy is ectothermy, although in general, there is no absolute or clear separation between the nature of endotherms and ectotherms. Many endotherms have a larger number of mitochondria per cell than ectotherms; this enables them to generate heat by increasing the rate at which they metabolize sugars.
Accordingly, to sustain their higher metabolism, endothermic animals require several times as much food as ectothermic animals do, require a more sustained supply of metabolic fuel. In many endothermic animals, a controlled temporary state of hypothermia conserves energy by permitting the body temperature to drop nearly to ambient levels; such states may be brief, regular circadian cycles called torpor, or they might occur in much longer seasonal, cycles called hibernation. The body temperatures of many small birds and small mammals fall during daily inactivity, such as nightly in diurnal animals or during the day in nocturnal animals, thus reducing the energy cost of maintaining body temperature. Less drastic intermittent reduction in body temperature occurs in other, larger endotherms. There may be other variations in temperature smaller, either endogenous or in response to external circumstances or vigorous exertion, either an increase or a drop; the resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain.
The brain generates about 16% of the total heat produced by the body. Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume. Small warm-blooded animals have insulation in the form of fur or feathers. Aquatic warm-blooded animals, such as seals have deep layers of blubber under the skin and any pelage that they might have. Penguins have blubber. Penguin feathers serve both for insulation and for streamlining. Endotherms that live in cold circumstances or conditions predisposing to heat loss, such as polar waters, tend to have specialised structures of blood vessels in their extremities that act as heat exchangers; the veins are adjacent to the arteries full of warm blood. Some of the arterial heat is recycled back into the trunk. Birds waders have well-developed heat exchange mechanisms in their legs—those in the legs of emperor penguins are part of the adaptations that enable them to spend months on Antarctic winter ice. In response to cold many warm-blooded animals reduce blood flow to the skin by vasoconstriction to reduce heat loss.
As a result, they blanch. In equatorial climates and during temperate summers, overheating is as great a threat as cold. In hot conditions, many warm-blooded animals increase heat loss by panting, which cools the animal by increasing water evaporation in the breath, and/or flushing, increasing the blood flow to the skin so the heat will radiate into the environment. Hairless and short-haired mammals, including humans sweat, since the evaporation of the water in sweat removes heat. Elephants keep cool by using their huge ears like radiators in automobiles, their ears are thin and the blood vessels are close to the skin, flapping their ears to increase the airflow over them causes the blood to cool, which reduces their core body temperature when the blood moves through the rest of the circulatory system. The major advantage of endothermy over ectothermy is decreased vulnerability to fluctuations in external temperature. Regardless of location, endothermy maintains a constant core temperature for optimum enzyme activity.
Endotherms control body temperature by internal homeostatic mechanisms. In mammals, two separate homeostatic mechanisms are involved in thermoregulation—one mechanism increases body temperature, while the other decreases it; the presence of two separate mechanisms provides a high degree of control. This is important because the core temperature of mammals can be controlled to be as close as possible to the optimum temperature for enzyme activity; the overall rate of an animal's metabolism increases by a factor of about two for every 10 °C rise in temperature, limited by the need to avoid hyperthermia. Endothermy does not provide greater speed in movement than ectothermy —ectothermic animals can move as fast as warm-blooded animals of the same size and build when the ectotherm is near or at its optimum temperature, but cannot maintain high metabolic activity for as long as endotherms. Endothermic/homeothermic animals can be optimally active at more times during the diurnal cycle in places of sharp temperature variations between day and night and
The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period in the Rhaetian stage, about 203 million years ago, they became common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution. Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835; the first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821. Since more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy and way of life. Plesiosaurs had a short tail, their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis.
The flippers made a flying movement through the water. Plesiosaurs breathed air, bore live young. Plesiosaurs showed two main morphological types; some species, with the "plesiosauromorph" build, had small heads. Other species, some of them reaching a length of up to seventeen metres, had the "pliosauromorph" build with a short neck and a large head; the two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea. Modern research, indicates that several "long-necked" groups might have had some short-necked members or vice versa. Therefore, the purely descriptive terms "plesiosauromorph" and "pliosauromorph" have been introduced, which do not imply a direct relationship. "Plesiosauroidea" and "Pliosauroidea" today have a more limited meaning. The term "plesiosaur" is properly used to refer to the Plesiosauria as a whole, but informally it is sometimes meant to indicate only the long-necked forms, the old Plesiosauroidea.
Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such. In 1605, Richard Verstegen of Antwerp illustrated in his A Restitution of Decayed Intelligence plesiosaur vertebrae that he referred to fishes and saw as proof that Great Britain was once connected to the European continent; the Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia from 1699 included depictions of plesiosaur vertebrae that again were considered fish vertebrae or Ichthyospondyli. Other naturalists during the seventeenth century added plesiosaur remains to their collections, such as John Woodward. In 1719, William Stukeley described a partial skeleton of a plesiosaur, brought to his attention by the great-grandfather of Charles Darwin, Robert Darwin of Elston; the stone plate came from a quarry at Fulbeck in Lincolnshire and had been used, with the fossil at its underside, to reinforce the slope of a watering-hole in Elston in Nottinghamshire. After the strange bones it contained had been discovered, it was displayed in the local vicarage as the remains of a sinner drowned in the Great Flood.
Stukely affirmed its "diluvial" nature but understood it represented some sea creature a crocodile or dolphin. The specimen is today preserved in the Natural History Museum, its inventory number being BMNH R.1330. It is the earliest discovered less complete fossil reptile skeleton in a museum collection, it can be referred to Plesiosaurus dolichodeirus. During the eighteenth century, the number of English plesiosaur discoveries increased, although these were all of a more or less fragmentary nature. Important collectors were the reverends William Mounsey and Baptist Noel Turner, active in the Vale of Belvoir, whose collections were in 1795 described by John Nicholls in the first part of his The History and Antiquities of the County of Leicestershire. One of Turner's partial plesiosaur skeletons is still preserved as specimen BMNH R.45 in the British Museum of Natural History. In the early nineteenth century, plesiosaurs were still poorly known and their special build was not understood. No systematic distinction was made with ichthyosaurs, so the fossils of one group were sometimes combined with those of the other to obtain a more complete specimen.
In 1821, a partial skeleton discovered in the collection of Colonel Thomas James Birch, was described by William Conybeare and Henry Thomas De la Beche, recognised as representing a distinctive group. A new genus was named, Plesiosaurus; the generic name was derived from the Greek πλήσιος, plèsios, "closer to" and the Latinised saurus, in the meaning of "saurian", to express that Plesiosaurus was in the Chain of Being more positioned to the Sauria the crocodile, than Ichthyosaurus, which had the form of a more lowly fish. The name should thus be rather read as "approaching the Sauria" or "near reptile" than as "near lizard". Parts of the specimen are still present in the Oxford University Museum of Natural History. Soon afterwards, the morphology became much better known. In 1823, Thomas Clark reported an complete skull belonging to Thalassiodracon, which i