Texas is the second largest state in the United States by both area and population. Geographically located in the South Central region of the country, Texas shares borders with the U. S. states of Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the west, the Mexican states of Chihuahua, Nuevo León, Tamaulipas to the southwest, while the Gulf of Mexico is to the southeast. Houston is the most populous city in Texas and the fourth largest in the U. S. while San Antonio is the second-most populous in the state and seventh largest in the U. S. Dallas–Fort Worth and Greater Houston are the fourth and fifth largest metropolitan statistical areas in the country, respectively. Other major cities include Austin, the second-most populous state capital in the U. S. and El Paso. Texas is nicknamed "The Lone Star State" to signify its former status as an independent republic, as a reminder of the state's struggle for independence from Mexico; the "Lone Star" can be found on the Texan state seal.
The origin of Texas's name is from the word taysha. Due to its size and geologic features such as the Balcones Fault, Texas contains diverse landscapes common to both the U. S. Southern and Southwestern regions. Although Texas is popularly associated with the U. S. southwestern deserts, less than 10% of Texas's land area is desert. Most of the population centers are in areas of former prairies, grasslands and the coastline. Traveling from east to west, one can observe terrain that ranges from coastal swamps and piney woods, to rolling plains and rugged hills, the desert and mountains of the Big Bend; the term "six flags over Texas" refers to several nations. Spain was the first European country to claim the area of Texas. France held a short-lived colony. Mexico controlled the territory until 1836 when Texas won its independence, becoming an independent Republic. In 1845, Texas joined the union as the 28th state; the state's annexation set off a chain of events that led to the Mexican–American War in 1846.
A slave state before the American Civil War, Texas declared its secession from the U. S. in early 1861, joined the Confederate States of America on March 2nd of the same year. After the Civil War and the restoration of its representation in the federal government, Texas entered a long period of economic stagnation. Four major industries shaped the Texas economy prior to World War II: cattle and bison, cotton and oil. Before and after the U. S. Civil War the cattle industry, which Texas came to dominate, was a major economic driver for the state, thus creating the traditional image of the Texas cowboy. In the 19th century cotton and lumber grew to be major industries as the cattle industry became less lucrative, it was though, the discovery of major petroleum deposits that initiated an economic boom which became the driving force behind the economy for much of the 20th century. With strong investments in universities, Texas developed a diversified economy and high tech industry in the mid-20th century.
As of 2015, it is second on the list of the most Fortune 500 companies with 54. With a growing base of industry, the state leads in many industries, including agriculture, energy and electronics, biomedical sciences. Texas has led the U. S. in state export revenue since 2002, has the second-highest gross state product. If Texas were a sovereign state, it would be the 10th largest economy in the world; the name Texas, based on the Caddo word táyshaʼ "friend", was applied, in the spelling Tejas or Texas, by the Spanish to the Caddo themselves the Hasinai Confederacy, the final -s representing the Spanish plural. The Mission San Francisco de los Tejas was completed near the Hasinai village of Nabedaches in May 1690, in what is now Houston County, East Texas. During Spanish colonial rule, in the 18th century, the area was known as Nuevo Reino de Filipinas "New Kingdom of the Philippines", or as provincia de los Tejas "province of the Tejas" also provincia de Texas, "province of Texas", it was incorporated as provincia de Texas into the Mexican Empire in 1821, declared a republic in 1836.
The Royal Spanish Academy recognizes both spellings and Texas, as Spanish-language forms of the name of the U. S. State of Texas; the English pronunciation with /ks/ is unetymological, based in the value of the letter x in historical Spanish orthography. Alternative etymologies of the name advanced in the late 19th century connected the Spanish teja "rooftile", the plural tejas being used to designate indigenous Pueblo settlements. A 1760s map by Jacques-Nicolas Bellin shows a village named Teijas on Trinity River, close to the site of modern Crockett. Texas is the second-largest U. S. state, with an area of 268,820 square miles. Though 10% larger than France and twice as large as Germany or Japan, it ranks only 27th worldwide amongst country subdivisions by size. If it were an independent country, Texas would be the 40th largest behind Zambia. Texas is in the south central part of the United States of America. Three of its borders are defined by rivers; the Rio Grande forms a natural border with the Mexican states of Chihuahua, Nuevo León, Tamaulipas to the south.
The Red River forms a natural border with Arkansas to the north. The Sabine River forms a natural border with Louisiana to the east; the Texas Panhandle has an eastern border with Oklahoma at 100° W, a northern border with Oklahoma at 36°30' N and a western
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
The Pelecaniformes are an order of medium-sized and large waterbirds found worldwide. As traditionally—but erroneously—defined, they encompass all birds that have feet with all four toes webbed. Hence, they were also known by such names as totipalmates or steganopodes. Most have a bare throat patch, the nostrils have evolved into dysfunctional slits, forcing them to breathe through their mouths, they feed on squid, or similar marine life. Nesting is colonial; the young are altricial, hatching from the egg naked in most. They lack a brood patch; the Fregatidae, Phalacrocoracidae and Phaethontidae were traditionally placed in the Pelecaniformes, but molecular and morphological studies indicate they are not such close relatives. They have been placed in their own orders and Phaethontiformes, respectively. Classically, bird relationships were based on morphological characteristics; the Pelecaniformes were traditionally—but erroneously—defined as birds that have feet with all four toes webbed, as opposed to all other birds with webbed feet where only three of four were webbed.
Hence, they were also known by such names as totipalmates or steganopodes. The group included frigatebirds, cormorants and tropicbirds. Sibley and Ahlquist's landmark DNA-DNA hybridisation studies led to them placing the families traditionally contained within the Pelecaniformes together with the grebes, cormorants and spoonbills, New World vultures, penguins, albatrosses and loons together as a subgroup within a expanded order Ciconiiformes, a radical move which by now has been all but rejected: their "Ciconiiformes" assembled all early advanced land- and seabirds for which their research technique delivered insufficient phylogenetic resolution. Morphological study has suggested pelicans are sister to a gannet-cormorant clade, yet genetic analysis groups them with the hamerkop and shoebill, though the exact relationship between the three is unclear. Mounting evidence pointed to the shoebill as a close relative of pelicans; this included microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995, who found that the shells of pelecaniform eggs were covered in a thick microglobular material.
Reviewing genetic evidence to date and colleagues surmised that pelicans were sister to the shoebill with the hamerkop as the next earlier offshoot. Ericson and colleagues sampled five nuclear genes in a 2006 study spanning the breadth of bird lineages, came up with pelicans and hamerkop in a clade. Hackett and colleagues sampled 32 kilobases of nuclear DNA and recovered shoebill and hamerkop as sister taxa, pelicans sister to them, herons and ibises as sister groups to each other with this heron and ibis group a sister to the pelican/shoebill/hamerkop clade; the current International Ornithological Committee classification has pelicans grouped with the shoebill, hamerkop and spoonbills, herons and bitterns. Recent research suggests that the similarities between the Pelecaniformes as traditionally defined are the result of convergent evolution rather than common descent, that the group is paraphyletic. All families in the traditional or revised Pelecaniformes except the Phalacrocoracidae have only a few handfuls of species at most, but many were more numerous in the Early Neogene.
Fossil genera and species are discussed in genus accounts. This is "Sula" ronzoni from Early Oligocene rocks at Ronzon, believed to be a sea-duck and is an ancestral Pelecaniform; the pelecaniform lineages appear to have originated around the end of the Cretaceous. Monophyletic or not, they appear to belong to a close-knit group of "higher waterbirds" which includes groups such as penguins and Procellariiformes. Quite a lot of fossil bones from around the Cretaceous–Paleogene boundary cannot be placed with any of these orders and rather combine traits of several of them; this is, of course, only to be expected, if the theory that most if not all of these "higher waterbird" lineages originated around that time is correct. Of those basal taxa, the following show some similarities to the traditional Pelecaniformes: Lonchodytes Torotix Tytthostonyx Cladornis "Liptornis"—a nomen dubiumThe proposed Elopterygidae—supposedly a family of Cretaceous Pelecaniformes—are neither monophyletic nor does Elopteryx appear to be a modern bird.
Pelecaniformes portal Bourdon, Estelle. J. Vertebr. Paleontol. 25: 157–170. DOI: 10.1671/0272-46340252.0. CO. Journal für Ornithologie 144: 157–175. HTML abstract Mortimer, Michael: The Theropod Database: Phylogeny of taxa. Retrieved 2013-MAR-02. Tree of Life: Pelecaniformes
Polymorphism in biology and zoology is the occurrence of two or more different morphs or forms referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population; the term polyphenism can be used to clarify. Genetic polymorphism is a term used somewhat differently by geneticists and molecular biologists to describe certain mutations in the genotype, such as single nucleotide polymorphisms that may not always correspond to a phenotype, but always corresponds to a branch in the genetic tree. See below. Polymorphism is common in nature. Polymorphism functions to retain variety of form in a population living in a varied environment; the most common example is sexual dimorphism. Other examples are mimetic forms of butterflies, human hemoglobin and blood types. According to the theory of evolution, polymorphism results from evolutionary processes, as does any aspect of a species, it is modified by natural selection.
In polyphenism, an individual's genetic makeup allows for different morphs, the switch mechanism that determines which morph is shown is environmental. In genetic polymorphism, the genetic makeup determines the morph; the term polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals, called zooids, within the same organism. It is a characteristic feature of cnidarians. For example, Obelia has the gastrozooids. Although in general use, polymorphism is a broad term. In biology, polymorphism has been given a specific meaning. A more specific term, when only two forms occur, is dimorphism; the term omits characteristics showing continuous variation. Polymorphism deals with forms in which the variation is discrete or bimodal or polymodal. Morphs must occupy the same habitat at the same time; the use of the words "morph" or "polymorphism" for what is a visibly different geographical race or variant is common, but incorrect. The significance of geographical variation is in that it may lead to allopatric speciation, whereas true polymorphism takes place in panmictic populations.
The term was first used to describe visible forms, but nowadays it has been extended to include cryptic morphs, for instance blood types, which can be revealed by a test. Rare variations are not classified as polymorphisms, mutations by themselves do not constitute polymorphisms. To qualify as a polymorphism, some kind of balance must exist between morphs underpinned by inheritance; the criterion is that the frequency of the least common morph is too high to be the result of new mutations or, as a rough guide, that it is greater than 1%. Polymorphism crosses several discipline boundaries, including ecology and genetics, evolution theory, taxonomy and biochemistry. Different disciplines may give the same concept different names, different concepts may be given the same name. For example, there are the terms established in ecological genetics by E. B. Ford, for classical genetics by John Maynard Smith; the shorter term morphism may be more accurate than polymorphism, but is not used. It was the preferred term of the evolutionary biologist Julian Huxley.
Various synonymous terms exist for the various polymorphic forms of an organism. The most common are morpha, while a more formal term is morphotype. Form and phase are sometimes used, but are confused in zoology with "form" in a population of animals, "phase" as a color or other change in an organism due to environmental conditions. Phenotypic traits and characteristics are possible descriptions, though that would imply just a limited aspect of the body. In the taxonomic nomenclature of zoology, the word "morpha" plus a Latin name for the morph can be added to a binomial or trinomial name. However, this invites confusion with geographically variant ring species or subspecies if polytypic. Morphs have no formal standing in the ICZN. In botanical taxonomy, the concept of morphs is represented with the terms "variety", "subvariety" and "form", which are formally regulated by the ICN. Horticulturists sometimes confuse this usage of "variety" both with cultivar and with the legal concept "plant variety".
Three mechanisms may cause polymorphism: Genetic polymorphism – where the phenotype of each individual is genetically determined A conditional development strategy, where the phenotype of each individual is set by environmental cues A mixed development strategy, where the phenotype is randomly assigned during development Selection, whether natural or artificial, changes the frequency of morphs within a population. A genetic polymorphism persists over many generations, maintained by two or more opposed and powerful selection pressures. Diver found banding morphs in Cepaea nemoralis could be seen in prefossil shells going back to t
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
Gulf Coast of the United States
The Gulf Coast of the United States is the coastline along the Southern United States where they meet the Gulf of Mexico. The coastal states that have a shoreline on the Gulf of Mexico are Texas, Mississippi and Florida, these are known as the Gulf States; the economy of the Gulf Coast area is dominated by industries related to energy, fishing, aerospace and tourism. The large cities of the region are McAllen, Corpus Christi, Galveston, Lafayette, Baton Rouge, New Orleans, Mobile, Pensacola, St. Petersburg and Sarasota. All contain large ports; the Gulf Coast is made of many inlets and lagoons. The coast is intersected by numerous rivers, the largest of, the Mississippi River. Much of the land along the Gulf Coast is, or marshland. Ringing the Gulf Coast is the Gulf Coastal Plain which reaches from Southern Texas to the western Florida Panhandle while the western portions of the Gulf Coast are made up of many barrier islands and peninsulas, including the 130 miles Padre Island and Galveston Island located in the U.
S. State of Texas; these landforms protect numerous inlets providing as a barrier to oncoming waves. The central part of the Gulf Coast, from eastern Texas through Louisiana, consists of marshland; the eastern part of the Gulf Coast, predominantly Florida, is dotted with many inlets. The Gulf Coast climate is humid subtropical, although the southwestern tip of Florida, such as Everglades City, features a tropical climate. Much of the year is warm to hot along the Gulf Coast, while the 3 winter months bring periods of cool weather mixed with mild temperatures; the area is vulnerable to severe thunderstorms. Much of the Gulf Coast has a summer precipitation maximum, with July or August the wettest month due to the combination of frequent summer thunderstorms produced by relentless heat and humidity, tropical weather systems, while winter and early spring rainfall can be heavy; this pattern is evident at Houston, New Orleans, Mobile and Pensacola, Florida. However, the central and southern Florida peninsula has a pronounced winter dry season, as at Tampa and Fort Myers, Florida.
On the central and southern Texas coast, early spring and mid-summer are markedly drier, September is the wettest month on average. Tornadoes do occur. Over most of the Gulf Coast from Houston, Texas eastward, extreme rainfall events are a significant threat from tropical weather systems, which can bring 4 to 10 or more inches of rain in a single day. In August 2017, Hurricane Harvey made landfall along the central Texas coast migrated to and stalled over the greater Houston area for several days, producing extreme, unprecedented rainfall totals of over 40 inches in many areas, unleashing widespread flooding. Earthquakes are rare to the area, but a surprising 6.0 earthquake in the Gulf of Mexico on September 10, 2006, could be felt from the cities of New Orleans to Tampa. The Gulf Coast is a major center of economic activity; the marshlands along the Louisiana and Texas coasts provide breeding grounds and nurseries for ocean life that drive the fishing and shrimping industries. The Port of South Louisiana and the Port of Houston are two of the ten busiest ports in the world by cargo volume.
As of 2004, seven of the top ten busiest ports in the U. S. are on the Gulf Coast. The discovery of oil and gas deposits along the coast and offshore, combined with easy access to shipping, have made the Gulf Coast the heart of the U. S. petrochemical industry. The coast contains nearly 4,000 oil platforms. Besides the above, the region features other important industries including aerospace and biomedical research, as well as older industries such as agriculture and — since the development of the Gulf Coast beginning in the 1920s and the increase in wealth throughout the United States — tourism. Before Europeans arrived in the region, the region was home to several pre-Columbian kingdoms that had extensive trade networks with empires such as the Aztecs and the Mississippi Mound Builders. Shark and alligator teeth and shells from the Gulf have been found as far north as Ohio, in the mounds of the Hopewell culture; the first Europeans to settle the Gulf Coast were the French and the Spanish. The Louisiana Purchase, Adams–Onís Treaty and the Texas Revolution made the Gulf Coast a part of the United States during the first half of the 19th century.
As the U. S. population continued to expand its frontiers westward, the Gulf Coast was a natural magnet in the South providing access to shipping lanes and both national and international commerce. The development of sugar and cotton production allowed the South to prosper. By the mid 19th century the city of New Orleans, being situated as a key to commerce on the Mississippi River and in the Gulf, had become the largest U. S. city not on the Atlantic seaboard and the fourth largest in the U. S. overall. Two major events were turning points in the earlier history of the Gulf Coast region; the first was the American Civil War, which caused severe damage to some economic sectors in the South, including the Gulf Coast. The second event was the Galveston Hurricane of 1900. At the end of
Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates. Included in this definition are the living hagfish and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods; because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces is considered a typological, but not a phylogenetic classification; the earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era.
Many fish of the Paleozoic developed external armor. The first fish with jaws appeared in the Silurian period, after which many became formidable marine predators rather than just the prey of arthropods. Most fish are ectothermic, allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish can communicate in their underwater environments through the use of acoustic communication. Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another; the production of sounds as a means of communication among fish is most used in the context of feeding, aggression or courtship behaviour. The sounds emitted by fish can vary depending on the stimulus involved, they can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and hadal depths of the deepest oceans, although no species has yet been documented in the deepest 25% of the ocean. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates. Fish are an important resource for humans worldwide as food. Commercial and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean, they are caught by recreational fishers, kept as pets, raised by fishkeepers, exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, as the subjects of art and movies. Fish do not represent a monophyletic group, therefore the "evolution of fish" is not studied as a single event. Early fish from the fossil record are represented by a group of small, armored fish known as ostracoderms. Jawless fish lineages are extinct.
An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils; the diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways; the first ancestors of fish may have kept the larval form into adulthood, although the reverse is the case. Fish are a paraphyletic group: that is, any clade containing all fish contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications. Traditional classification divides fish into three extant classes, with extinct forms sometimes classified within the tree, sometimes as their own classes: Class Agnatha Subclass Cyclostomata Subclass Ostracodermi † Class Chondrichthyes Subclass Elasmobranchii Subclass Holocephali Class Placodermi † Class Acanthodii † Class Osteichthyes Subclass Actinopterygii Subclass Sarcopterygii The above scheme is the one most encountered in non-specialist and general works.
Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nomenclature, the fishes has been split up into a more detailed scheme, with the following major groups: Class Myxini Class Pteraspidomorphi † Class Thelodonti † Class Anaspida † Class Petromyzontida or Hyperoartia Petromyzontidae Class Conodonta † Class Cephalaspidomorphi † Galeaspida † Pituriaspida † Osteostraci † Infraphylum Gnathostomata Class Placodermi † Class Chondrichthyes Class Acanthodii † Superclass Osteichthy