「Vertebrate」の版間の差分

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{{For|an explanation of similar terms|Craniate}}{{Use dmy dates|date=August 2013}}
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<article xmlns:fn="http://www.w3.org/2005/xpath-functions" class="content clearfix" role="main">
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{{Automatic taxobox
 
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| name = Vertebrate
 
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| fossil_range = <br>[[Cambrian]]&ndash;[[Holocene|Present]],<ref name = "Shu et al. 1999"/>
 
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{{fossil range|520|0|ref=<ref name="Peterson2008">{{cite journal | last1 = Peterson | first1 = Kevin J. | last2 = Cotton | first2 = James A. | last3 = Gehling | first3 = James G. | last4 = Pisani | first4 = Davide | date = 27 April 2008 | title = The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records | journal = Philosophical Transactions of the Royal Society B: Biological Sciences | volume = 363 | issue = 1496 | pages = 1435–1443 | doi = 10.1098/rstb.2007.2233 | pmid=18192191 | pmc=2614224 }}</ref>}}
 
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| image = Vertebrates.png
 
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| image_caption = Individual organisms from each major vertebrate group. Clockwise, starting from top left:
 
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[[Fire salamander]] (Amphibia), [[saltwater crocodile]] (Reptilia), [[southern cassowary]] (Aves), [[Rhynchocyon petersi|black-and-rufous giant elephant shrew]] (Mammalia), [[ocean sunfish]] (Osteichthyes)
 
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| taxon = Vertebrata
 
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| authority = [[Jean-Baptiste Lamarck|J-B. Lamarck]], 1801<ref name="Nielsen2012">{{cite journal
 
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| author=Nielsen, C.
 
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| date=July 2012
 
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| title=The authorship of higher chordate taxa
 
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| journal=Zoologica Scripta
 
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| volume=41 | issue=4 | pages=435–436
 
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| doi=10.1111/j.1463-6409.2012.00536.x
 
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}}</ref>
 
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| subdivision_ranks = Simplified grouping (see text)
 
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| subdivision =
 
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*[[Fish]]es ([[Cladistics|cladistically]] including the [[Tetrapod]]s)
 
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| synonyms = Ossea <small>Batsch, 1788</small><ref name="Nielsen2012" />
 
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}}
 
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'''Vertebrates''' {{IPAc-en|ˈ|v|ɜːr|t|ᵻ|b|r|ᵻ|t|s}} comprise all species of [[animal]]s within the [[subphylum]] '''Vertebrata''' {{IPAc-en|-|eɪ}} ([[chordates]] with [[vertebral column|backbones]]). Vertebrates represent the overwhelming majority of the [[phylum]] [[Chordata]], with currently about 66,000 [[species]] described.<ref name=IUCN1014>The World Conservation Union. 2014. ''[[IUCN Red List of Threatened Species]]'', 2014.3. Summary Statistics for Globally Threatened Species. [http://cmsdocs.s3.amazonaws.com/summarystats/2014_3_Summary_Stats_Page_Documents/2014_3_RL_Stats_Table_1.pdf Table 1: Numbers of threatened species by major groups of organisms (1996–2014)].</ref> Vertebrates include the [[Agnatha|jawless fish]] and the [[Gnathostomata|jawed vertebrates]], which include the [[Chondrichthyes|cartilaginous fishes]] ([[shark]]s, [[Batoidea|rays]], and ratfish) and the [[Osteichthyes|bony fishes]].
 
  
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A bony [[fish]] clade known as the [[Sarcopterygii|lobe-finned fishes]] is included with [[tetrapods]], which are further divided into [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s. [[Extant taxon|Extant]] vertebrates range in size from the [[frog]] species ''[[Paedophryne amauensis]]'', at as little as {{convert|7.7|mm|in|abbr=on}}, to the [[blue whale]], at up to {{convert|33|m|ft|abbr=on}}. Vertebrates make up less than five percent of all described [[animal species]]; the rest are [[invertebrates]], which lack [[vertebral column]]s.
 
  
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The vertebrates traditionally include the [[hagfish]], which do not have proper [[vertebra]]e due to their loss in evolution,<ref>{{Cite journal|last=Ota|first=Kinya G.|last2=Fujimoto|first2=Satoko|last3=Oisi|first3=Yasuhiro|last4=Kuratani|first4=Shigeru|date=2017-01-25|title=Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish|journal=Nature Communications|volume=2|pages=373|doi=10.1038/ncomms1355|issn=2041-1723|pmc=3157150|pmid=21712821|bibcode=2011NatCo...2E.373O}}</ref> though their closest living relatives, the [[lamprey]]s, do.<ref name=hagfish>{{cite journal|title=Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes | author=Kuraku |date=December 1999|journal=Journal of Molecular Evolution | volume = 49 | pages = 729–35|doi=10.1007/PL00006595|pmid=10594174|last2=Hoshiyama|first2=D|last3=Katoh|first3=K|last4=Suga|first4=H|last5=Miyata|first5=T|issue=6|displayauthors=1 |bibcode=1999JMolE..49..729K}}</ref> Hagfish do, however, possess a [[cranium]]. For this reason, the vertebrate subphylum is sometimes referred to as "[[Craniata]]" when discussing morphology.
 
  
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Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,<ref>{{cite journal|last=Stock|first=D.|author2=Whitt, G.S. |title=Evidence from 18S ribosomal RNA sequences that lampreys and hagfish form a natural group|journal=Science|date=7 August 1992|volume=257|issue=5071|doi=10.1126/science.1496398|url=http://www.sciencemag.org/content/257/5071/787.short|accessdate=22 November 2011|pages=787–9|pmid=1496398|bibcode = 1992Sci...257..787S }}</ref> and so also are vertebrates in a [[monophyletic]] sense. Others consider them a sister group of vertebrates in the common taxon of craniata.<ref>{{cite journal|last=Nicholls|first=H.|title=Mouth to Mouth|date=10 September 2009|journal=Nature|volume=461|issue=7261|pages=164–166|doi=10.1038/461164a|pmid=19741680}}</ref>
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<section class="toc-anchor" id="intro" style="padding-top: 46px; margin-top: -46px;"><header><h1 data-bind="string: 'xsl.article.introduction'">Introduction</h1></header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=121619" data-article-id="110433" data-assembly-id="121619" data-show-google-classroom="false" data-credit="Encyclopædia Britannica, Inc." data-large-path="https://media1.britannica.com/eb-media/46/93546-050-597FCB04.jpg" title="vertebrate: major groups of vertebrates" data-caption="The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals. " data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="110" data-original="https://media1.britannica.com/eb-media/46/93546-004-F9C81CA9.jpg" alt="vertebrate: major groups of vertebrates" data-generated-alt="true" src="https://media1.britannica.com/eb-media/46/93546-004-F9C81CA9.jpg" style="display: block;"><span class="visuallyhidden">Encyclopædia Britannica, Inc.</span>]]<figcaption>Encyclopædia Britannica, Inc.</figcaption></figure><p class="p-first">vertebrate,
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            <span class="alternates">also called <span class="alternate-title">Craniata, </span></span>
  
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== Etymology ==
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any animal of the subphylum Vertebrata, the predominant subphylum of the phylum Chordata. They have backbones, from which they derive their name. The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone.</p></section>
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The word origin of ''vertebrate'' derives from the [[Latin]] word ''vertebratus'' ([[Natural History (Pliny)|Pliny]]), meaning ''joint of the spine.''<ref>{{cite web | title=vertebrate |publisher=Dictionary.com. |work=Online Etymology Dictionary |author= |url=http://dictionary.reference.com/browse/vertebrate}}</ref> The [[Proto-Indo-European]] language origins are still unclear.
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<p>The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals).</p>
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<section class="toc-anchor" id="49518.toc" style="padding-top: 46px; margin-top: -46px;"><header><h1>General features<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"><i class="fontello-icon-list"></i></button></h1></header><p>Although the vertebral column is perhaps the most obvious vertebrate feature, it was not present in the first vertebrates, which probably had only a notochord. The vertebrate has a distinct head, with a differentiated tubular brain and three pairs of sense organs (nasal, optic, and otic). The body is divided into trunk and tail regions. The presence of pharyngeal slits with gills indicates a relatively high metabolic rate. A well-developed notochord enclosed in perichordal connective tissue, with a tubular spinal cord in a connective tissue canal above it, is flanked by a number of segmented muscle masses. A sensory ganglion develops on the dorsal root of the spinal nerve, and segmental autonomic ganglia grow below the notochord. The trunk region is filled with a large, bilateral body cavity (coelom) with contained viscera, and this coelom extends anteriorly into the visceral arches. A digestive system consists of an esophagus extending from the pharynx to the stomach and a gut from the stomach to the anus. A distinct heart, anteroventral to the liver, is enclosed in a pericardial sac. A basic pattern of closed circulatory vessels is largely preserved in most living forms. Unique, bilateral kidneys lie retroperitoneally (dorsal to the main body cavity) and serve blood maintenance and excretory functions. Reproductive organs are formed from tissue adjacent to the kidneys; this original close association is attested by the tubular connections seen in males of living forms. The ducts of the excretory organs open through the body wall into a cloacal chamber, as does the anus of the digestive tract. Reproductive cells are shed through nearby abdominal pores or through special ducts. A muscular tail continues the axial musculature of the trunk.</p><p>Approximately 45,000 living species constitute the vertebrates. Species of several classes are found from the high Arctic or Antarctic to the tropics around the Earth; they are missing only from interior Antarctica and Greenland and from the North Polar ice pack. In size, vertebrates range from minute fishes to elephants and whales (of up to 100 tons), the largest animals ever to have existed. Vertebrates are adapted to life underground, on the surface, and in the air. They feed upon plants, invertebrate animals, and one another. Vertebrate faunas are important to humans for food and recreation.</p></section>
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<section class="toc-anchor" id="49519.toc" style="padding-top: 46px; margin-top: -46px;"><header><h1>Natural history<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"><i class="fontello-icon-list"></i></button></h1></header><p>In order to give a broad and comparative view of their life histories, the vertebrates are subdivided here into major groups based on morphology: the cyclostomes (jawless fishes), the chondrichthyes (cartilaginous fishes), the teleostomes (bony fishes), and the tetrapods.</p><section class="toc-anchor" id="49520.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>The <a class="reflink" href="/levels/collegiate/article/cyclostome/28385">cyclostomes]]</h2></header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=70896" data-article-id="110433" data-assembly-id="70896" data-show-google-classroom="false" data-credit="Oxford Scientific Films/Bruce Coleman Ltd." title="lamprey" data-caption="Lamprey (<i>Lampetra</i>) on rainbow trout." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="132" data-original="https://media1.britannica.com/eb-media/17/37317-004-B45DFB50.jpg" title="lamprey" alt="lamprey" data-generated-alt="true" src="https://media1.britannica.com/eb-media/17/37317-004-B45DFB50.jpg" style="display: block;"><span class="visuallyhidden">Oxford Scientific Films/Bruce Coleman Ltd.</span><div id="media-metadata-70896" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/agnathan/111050" rel="" title="" data-article-id="111050" data-article-url-title="${article.urlTitle}">3</span>agnathan]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/fish/111049" rel="" title="" data-article-id="111049" data-article-url-title="${article.urlTitle}">3</span>fish]]</li></ul>]]<figcaption>Oxford Scientific Films/Bruce Coleman Ltd.</figcaption></figure><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=18450" data-article-id="110433" data-assembly-id="18450" data-show-google-classroom="false" data-credit="Encyclopædia Britannica, Inc." title="hagfish" data-caption="Hagfishes are primitive, jawless fish." data-media-type="VIDEO" data-full-path="G0mRrZXP" data-width="352" data-height="240" data-duration="0:28" data-jwplayer-id="G0mRrZXP" data-closed-caption-id="xskpgOoT" data-closed-caption-language-id="engus" data-transcript="NARRATOR: Hagfishes, together with lampreys, are considered the most primitive living fishes. The hagfish has a scaleless, eel-like body and a cartilaginous skeleton. It lacks paired fins and jaws. Its mouth is a round, suckerlike opening surrounded by short, horny teeth. The hagfish usually feeds on dead fish and marine invertebrates."><div class="video-thumb-container"><img class="lazy" width="220" height="165" data-original="https://media1.britannica.com/eb-media/60/22260-049-5F15EEF4.jpg" alt="hagfish" data-generated-alt="true" src="https://media1.britannica.com/eb-media/60/22260-049-5F15EEF4.jpg" style="display: block;"><span class="fontello-icon-play"></span><span class="video-duration large" id="video-duration-18450">0:28</span><span class="visuallyhidden">Encyclopædia Britannica, Inc.</span><div id="media-metadata-18450" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This video also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/agnathan/111050" rel="" title="" data-article-id="111050" data-article-url-title="${article.urlTitle}">3</span>agnathan]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/fish/274326" rel="" title="" data-article-id="274326" data-article-url-title="${article.urlTitle}">2</span>fish]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/hagfish/38776" rel="" title="" data-article-id="38776" data-article-url-title="${article.urlTitle}">3</span>hagfish]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/Silurian-Period/117444" rel="" title="" data-article-id="117444" data-article-url-title="${article.urlTitle}">3</span>Silurian Period]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/vertebrate/277578" rel="" title="" data-article-id="277578" data-article-url-title="${article.urlTitle}">2</span>vertebrate]]</li></ul>]]<figcaption>Encyclopædia Britannica, Inc.</figcaption></figure><p>The cyclostomes include two classes of living, jawless fishes (agnathous)—Petromyzontiformes (<a class="reflink" href="/levels/collegiate/article/lamprey/46983">lamprey]] eels) and Myxiniformes (<a class="reflink" href="/levels/collegiate/article/hagfish/38776">hagfishes]]). The hagfishes are totally marine, often living in deep waters associated with muddy bottoms. The lampreys may be marine as adults but spawn in fresh waters, where the larvae spend some time before metamorphosing to the adult. Some lampreys live entirely in fresh water and may change only slightly in habit as a result of metamorphosis. Without lateral fins, lampreys swim by undulations of the body and can control direction only for short distances.</p><p>The living agnaths are predatory, the lampreys being well known for attacking salmonoid fishes. The lamprey attaches to its prey using its round, suctorial mouth, and it rasps a hole through the outer tissues using a tongue armed with keratinized teeth. It suctions off bits of tissue, blood, and body fluids. The hagfishes feed somewhat similarly, but on a variety of prey—invertebrates (worms and soft-bodied forms) and dead fishes.</p><p>The lampreys produce small eggs, which develop directly into larvae that burrow into the muddy bottom of the stream. With its mouth at the surface of the mud, the larva filter feeds until large enough to metamorphose and swim off as a small adult. In contrast, the hagfishes produce relatively large encapsulated, yolky eggs up to two centimetres in length. When laid, these eggs attach to any available object by terminal hooks. The encased egg develops more or less directly into a miniature adult.</p></section><section class="toc-anchor" id="49521.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>The <a class="reflink" href="/levels/collegiate/article/chondrichthian/111051">chondrichthyes]]</h2></header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=92147" data-article-id="110433" data-assembly-id="92147" data-show-google-classroom="false" data-credit="Georgette Douwma/Nature Picture Library" data-large-path="https://media1.britannica.com/eb-media/03/93103-050-AE6EAE5D.jpg" title="southern stingray" data-caption="Southern stingrays (<i>Dasyatis americana</i>)." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="148" data-original="https://media1.britannica.com/eb-media/03/93103-004-2C368938.jpg" alt="southern stingray" data-generated-alt="true" src="https://media1.britannica.com/eb-media/03/93103-004-2C368938.jpg" style="display: block;"><span class="visuallyhidden">Georgette Douwma/Nature Picture Library</span><div id="media-metadata-92147" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/chondrichthian/111051" rel="" title="" data-article-id="111051" data-article-url-title="${article.urlTitle}">3</span>chondrichthian]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/fish/111049" rel="" title="" data-article-id="111049" data-article-url-title="${article.urlTitle}">3</span>fish]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/vertebrate/277578" rel="" title="" data-article-id="277578" data-article-url-title="${article.urlTitle}">2</span>vertebrate]]</li></ul>]]<figcaption>Georgette Douwma/Nature Picture Library</figcaption></figure><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=4721" data-article-id="110433" data-assembly-id="4721" data-show-google-classroom="false" data-credit="Bob Abrams—Bruce Coleman Inc." title="bull shark" data-caption="Bull shark (<i>Carcharhinus leucas</i>)." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="165" data-original="https://media1.britannica.com/eb-media/72/13072-004-507560C0.jpg" alt="bull shark" data-generated-alt="true" src="https://media1.britannica.com/eb-media/72/13072-004-507560C0.jpg" style="display: block;"><span class="visuallyhidden">Bob Abrams—Bruce Coleman Inc.</span><div id="media-metadata-4721" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/carcharhinid/20275" rel="" title="" data-article-id="20275" data-article-url-title="${article.urlTitle}">3</span>carcharhinid]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/chondrichthian/111051" rel="" title="" data-article-id="111051" data-article-url-title="${article.urlTitle}">3</span>chondrichthian]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/shark/67153" rel="" title="" data-article-id="67153" data-article-url-title="${article.urlTitle}">3</span>shark]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/shark/277020" rel="" title="" data-article-id="277020" data-article-url-title="${article.urlTitle}">2</span>shark]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/vertebrate/277578" rel="" title="" data-article-id="277578" data-article-url-title="${article.urlTitle}">2</span>vertebrate]]</li></ul>]]<figcaption>Bob Abrams—Bruce Coleman Inc.</figcaption></figure><p>
  
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''Vertebrate'' is derived from the word ''[[vertebra]]'', which refers to any of the bones or segments of the [[spinal column]].<ref>{{cite web | title=vertebra |publisher=Dictionary.com. |work=Online Etymology Dictionary |author=  |url=http://dictionary.reference.com/browse/vertebra}}</ref>
 
  
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== Anatomy and morphology ==
 
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{{see also|Vertebrate anatomy}}
 
  
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All vertebrates are built along the basic chordate [[body plan]]: a stiff rod running through the length of the animal (vertebral column and/or [[notochord]]),<ref>{{cite web|last=Waggoner|first=Ben|title=Vertebrates: More on Morphology|url=http://www.ucmp.berkeley.edu/vertebrates/vertmm.html|publisher=UCMP|accessdate=13 July 2011}}</ref> with a hollow tube of nervous tissue (the [[spinal cord]]) above it and the [[Gut (anatomy)|gastrointestinal tract]] below.
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The <a class="reflink" href="/levels/collegiate/article/shark/67153">sharks]], rays, and chimaerids are usually marine, but some sharks have entered fresh waters (the Amazon) or even live there permanently (Lake Nicaragua). In size, sharks range from the whale shark, nearly 10 metres in length, to rather small species, three centimetres in length. They usually weigh 25 to 200 kilograms (55 to 440 pounds). Sharks are predatory animals. Some large shark species (basking and whale sharks) filter feed on small crustaceans. Herbivorous sharks are unknown. Sharks swim by undulations of the tail, but rays “fly” through the water by undulations of the pectoral fins. Most species occur in near-shore waters, but some range widely throughout the oceans. A few are found in deep water.</p><p>A few sharks produce live young (viviparous) after internal fertilization. The posterior angle of the male’s pelvic fins are modified into a clasper, which acts as an intromittent organ in copulating with the female. Most sharks lay large yolky, encapsulated eggs with hooks for attachment. The young develop directly and begin life as miniature adults. The young that develop in the mother’s uterus obtain nutrients from the large yolk sac until they are born alive. In a few cases, the uterine wall secretes nutrients.</p><p>The teleostome, or osteichthyian, fishes (those having an internal bony skeleton) can be divided into two groups: the subclasses Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). The latter group includes the <a class="reflink" href="/levels/collegiate/article/lungfish/111053">lungfishes]], which live in marshes, ponds, or streams, and are frequent air breathers. They lay fairly large eggs, with a limited amount of yolk, that are enclosed in jelly coats like those of an amphibian. The eggs develop into small fishes that feed on live prey. The larvae of the African lungfish have external gills to supplement oxygen intake.</p></section><section class="toc-anchor" id="49522.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>The <a class="reflink" href="/levels/collegiate/article/bony-fish/80646">teleostomes]]</h2></header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=6809" data-article-id="110433" data-assembly-id="6809" data-show-google-classroom="false" data-credit="Russ Kinne/Photo Researchers" title="American paddlefish" data-caption="American paddlefish (<i>Polyodon spathula</i>)" data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="88" data-original="https://media1.britannica.com/eb-media/20/3320-004-252CEC54.jpg" alt="American paddlefish" data-generated-alt="true" src="https://media1.britannica.com/eb-media/20/3320-004-252CEC54.jpg" style="display: block;"><span class="visuallyhidden">Russ Kinne/Photo Researchers</span><div id="media-metadata-6809" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/paddlefish/57942" rel="" title="" data-article-id="57942" data-article-url-title="${article.urlTitle}">3</span>paddlefish]]</li></ul>]]<figcaption>Russ Kinne/Photo Researchers</figcaption></figure><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=6620" data-article-id="110433" data-assembly-id="6620" data-show-google-classroom="false" data-credit="Bob Evans/Peter Arnold, Inc." data-large-path="https://media1.britannica.com/eb-media/44/2744-050-C113EF5E.jpg" title="common mola" data-caption="Common mola, or ocean sunfish (<i>Mola mola</i>)." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="193" data-original="https://media1.britannica.com/eb-media/44/2744-004-06414188.jpg" alt="common mola" data-generated-alt="true" src="https://media1.britannica.com/eb-media/44/2744-004-06414188.jpg" style="display: block;"><span class="visuallyhidden">Bob Evans/Peter Arnold, Inc.</span><div id="media-metadata-6620" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/bony-fish/80646" rel="" title="" data-article-id="80646" data-article-url-title="${article.urlTitle}">3</span>bony fish]]</li><li><a class="media-metadata-level two" href="/levels/collegiate/article/fish/274326" rel="" title="" data-article-id="274326" data-article-url-title="${article.urlTitle}">2</span>fish]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/mola/53219" rel="" title="" data-article-id="53219" data-article-url-title="${article.urlTitle}">3</span>mola]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/tetraodontiform/111069" rel="" title="" data-article-id="111069" data-article-url-title="${article.urlTitle}">3</span>tetraodontiform]]</li></ul>]]<figcaption>Bob Evans/Peter Arnold, Inc.</figcaption></figure><p>Actinopterygian fishes are the common bony fishes of modern aquatic environments. They range in size from fishes that are only millimetres in size to those two or more metres (6.6 or more feet) in length, weighing 500 kilograms or more. Large species (sturgeons) are found in fresh waters (several other large species are found in the Amazon) as well as in marine environments. The diet may include plants, animals, and carrion. Most species are midwater swimmers, but many spend much time lying on the bottom. Tail, pectoral, and even dorsal fins are used in swimming. Reproduction in this group is by way of large numbers of small eggs, which produce small larvae or develop directly to the adult.</p></section><section class="toc-anchor" id="49523.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>The tetrapods</h2></header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=31150" data-article-id="110433" data-assembly-id="31150" data-show-google-classroom="false" data-credit="Joe B. Blossom/Photo Researchers" title="European pond turtle" data-caption="European pond turtle (<i>Emys orbicularis</i>)." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="192" data-original="https://media1.britannica.com/eb-media/79/38579-004-7F823F22.jpg" alt="European pond turtle" data-generated-alt="true" src="https://media1.britannica.com/eb-media/79/38579-004-7F823F22.jpg" style="display: block;"><span class="visuallyhidden">Joe B. Blossom/Photo Researchers</span><div id="media-metadata-31150" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/pond-turtle/60770" rel="" title="" data-article-id="60770" data-article-url-title="${article.urlTitle}">3</span>pond turtle]]</li><li><a class="media-metadata-level three" href="/levels/collegiate/article/turtle/110252" rel="" title="" data-article-id="110252" data-article-url-title="${article.urlTitle}">3</span>turtle]]</li></ul>]]<figcaption>Joe B. Blossom/Photo Researchers</figcaption></figure><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=106374" data-article-id="110433" data-assembly-id="106374" data-show-google-classroom="false" data-credit="© ElementalImaging/iStock.com" data-large-path="https://media1.britannica.com/eb-media/84/65384-050-A9528785.jpg" title="sandhill crane" data-caption="Sandhill cranes (<i>Grus canadensis</i>)." data-media-type="IMAGE"><div class=""><img class="lazy" width="220" height="138" data-original="https://media1.britannica.com/eb-media/84/65384-004-BA3249AF.jpg" alt="sandhill crane" data-generated-alt="true" src="https://media1.britannica.com/eb-media/84/65384-004-BA3249AF.jpg" style="display: block;"><span class="visuallyhidden">© ElementalImaging/iStock.com</span><div id="media-metadata-106374" class="gallery-media-articles-wrapper hidden"><div class="media-metadata"><p>This image also in:</p><ul><li><a class="media-metadata-level three" href="/levels/collegiate/article/crane/26752" rel="" title="" data-article-id="26752" data-article-url-title="${article.urlTitle}">3</span>crane]]</li></ul>]]<figcaption>© ElementalImaging/iStock.com</figcaption></figure><p>
  
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In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the [[anus]] opens to the exterior before the end of the body. The remaining part of the body continuing after the anus forms a [[tail]] with vertebrae and spinal cord, but no gut.<ref name=Romer>Romer, A.S. (1949): ''The Vertebrate Body.'' W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)</ref>
 
  
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===Vertebral column===
 
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The defining characteristic of a vertebrate is the [[vertebral column]], in which the [[notochord]] (a stiff rod of uniform composition) found in all [[Chordata|chordates]] has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord).
 
  
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However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the [[sturgeon]]<ref>{{cite book|title=Functional anatomy of the vertebrates: an evolutionary perspective|year=2001|publisher=Harcourt College Publishers|isbn=978-0-03-022369-3|author=Liem, K.F.|author2=Walker, W.F. |page=277}}</ref> and [[coelacanth]]. [[Gnathostomata|Jawed vertebrates]] are typified by paired appendages (fins or legs, which may be secondarily lost), but this trait is not required in order for an animal to be a vertebrate.
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The tetrapods live primarily on land and are rather similar in habit. Members include the <a class="reflink" href="/levels/collegiate/article/amphibian/110233">amphibians]], reptiles, birds, and mammals. Amphibians are widespread in the warmer parts of the continents, being absent only in the far north and in the Antarctic. Three orders are recognized: Candata (the salamanders), the frogs and toads (Anura, or Salientia), and the Apoda or Gymnophiona (caecilians). Modification takes many forms, from the moist glandular skin (some scale remnants persist in apodans) to the loss of many of the bones of the skull. Like their ancestors, amphibians are cold-blooded and tend to be aquatic or limited to moist surroundings. <a class="reflink" href="/levels/collegiate/article/salamander/65027">Salamanders]] are seemingly the least modified in body form. They do not actively pursue prey and at best are only marginal swimmers. In swimming or crawling, the salamander’s body and tail undulate. Frogs and toads hop using hind-limb propulsion and the forelimbs as body props. This dominance of the hind limb in locomotion is best seen in swimming when the forelimbs are drawn back against the body. In contrast to the salamanders and frogs, the burrowing, wormlike apodans are without limbs.</p><p>Amphibians usually trap food using a tongue that can be shot out of the mouth, or they use the mouth itself to grasp and ingest food. There is great variation in foods; only the larvae of frogs and toads appear to be plant feeders, a specialization that is reflected in the highly modified jaws and guts of the tadpoles.</p><p>Amphibians have retained a simple egg cell with a gelatinous cover. The eggs are laid in ponds, streams, or even in damp places high in trees, usually in great numbers. Fertilized eggs develop into free-swimming larvae, which then metamorphose to adults, but in highly specialized forms.</p><p>The class Reptilia retains many of the structural characteristics of the ancestral amphibian. While most <a class="reflink" href="/levels/collegiate/article/reptile/110251">reptiles]] are carnivorous, feeding on other organisms, a few are herbivorous (<em>e.g.,</em> tortoises). As cold-blooded animals, reptiles tend to be limited to temperate and tropical areas, but, where found, they are relatively common, although not as large or conspicuous as birds or mammals. Most reptiles are terrestrial, but a few are aquatic. As basic tetrapods, reptiles move about by creeping or swimming in a fashion similar to amphibians. Some reptiles, however, can lift the body from the ground and run rapidly either in a quadrupedal or bipedal fashion. Reptiles lay relatively large, shelled eggs. In a few instances, the eggs and young are cared for by the female; in others, the young are born alive (ovovivipary).</p><p><a class="reflink" href="/levels/collegiate/article/bird/105921">Birds]] are warm-blooded, and, although most are capable of flight, others are sedentary and some are flightless. Like their relatives the reptiles, birds lay shelled eggs that differ largely in the amount of calcification (hardening) of the shell. The young are usually cared for in a nest until they are capable of flight and self-feeding, but some birds hatch in a well-developed state that allows them to begin feeding immediately or even take flight. The megapods lay their eggs in mounds of rotting vegetation, which supplies the heat for incubation. (Nesting activities similar to those of some birds are seen in the crocodilians.)</p><p>The <a class="reflink" href="/levels/collegiate/article/mammal/105972">mammals]] range in size from tiny shrews or small bats weighing only a few grams to the largest known animals, the whales. Most mammals are terrestrial, feeding on both animal and vegetable matter, but a few are partially aquatic or entirely so, as in the case of the whales or porpoises. Mammals move about in a great variety of ways: burrowing, bipedal or tetrapedal running, flying, or swimming. Reproduction in mammals is usually viviparous, the young developing in the uterus, where nutritive materials are made available through an allantoic placenta or, in a few cases, a yolk sac. The fertilized egg develops directly into the adult. The <a class="reflink" href="/levels/collegiate/article/monotreme/105973">monotremes]] (platypus and echidna) differ from other mammals in that they lay eggs which hatch, and the relatively undeveloped young are carried in a pouch or kept in a nest; the growing young lap up a milk nutrient fluid exuded from the belly of the mother.</p></section></section>
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<section class="toc-anchor" id="49524.toc" style="padding-top: 46px; margin-top: -46px;"><header><h1>Form and function<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"><i class="fontello-icon-list"></i></button></h1></header><section class="toc-anchor" id="49525.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>External features</h2></header><p>The evolution of the notochord, dorsal nerve tube, and pharyngeal slits in chordate structure suggests improved swimming capability and probably greater ability to capture prey. Specialization in the vertebrate for the active capture of larger prey is evident both in the structure of the mouth and in the relatively simple structure of the pharynx, with its strong gill development. Specialization for feeding is again seen in the two basic groups of vertebrates, the agnathans and gnathostomes. Swimming adaptations are also numerous and involve variations both in body form and in medial fins and the two pairs of lateral fins.</p></section><section class="toc-anchor" id="49526.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>Internal features</h2></header><section class="toc-anchor" id="49527.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>The <a class="reflink" href="/levels/collegiate/article/skeleton/110160">skeletal system]]</h3></header><p>Support and protection are provided by the exoskeletal and endoskeletal divisions of the skeletal system. The <a class="reflink" href="/levels/collegiate/article/exoskeleton/33440">exoskeleton]], when present, is basically protective but functions in tooth support in the mouth region. The endoskeleton protects the brain and spinal cord and assists primarily with locomotion in the trunk and tail regions. The endoskeleton begins as <a class="reflink" href="/levels/collegiate/article/cartilage/20556">cartilage]] and may remain so or may develop into <a class="reflink" href="/levels/collegiate/article/bone/110163">bone]]. The cartilaginous endoskeleton, found in the shark or chimaerid, is usually calcified so as to be stiffer and stronger. Bone is distinctive but highly variable; some types of bone contain cells, others do not, or the bone may be laminar, spongy, or arranged in sheathing layers around blood channels.</p></section><section class="toc-anchor" id="49528.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3><a class="reflink" href="/levels/collegiate/article/tissue/72629">Tissues]] and muscles</h3></header><p>Tissue development in the vertebrate is unique in its complexity; tissues in the strict sense (defined as a mass or sheet of similar cells with a similar function), however, do not exist. The simplest situation is seen in the epidermis, but even here there is a layered system in which different cell types provide different functions (such as protection and secretion). The stratified epithelium of the vertebrate is highly characteristic of that group (a similar one is seen in only one invertebrate group, the class Chaetognatha).</p><p>Other tissues of the vertebrate are more complex than the epithelium. For example, skeletal <a class="reflink" href="/levels/collegiate/article/muscle/110701">muscle]] consists not only of striated muscle fibres but also of connective tissue, which binds it together and attaches it by way of tendons. This contractible tissue includes nerves and blood vessels and their contained blood. Skeletal muscles thus appear as simple organs, just as do the smooth muscles in the wall of the gut or the iris muscles of the eye. Such unique histological complexity runs through the entire body of the vertebrate.</p></section><section class="toc-anchor" id="49529.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3><a class="reflink" href="/levels/collegiate/article/nervous-system/110703">Nervous system]] and organs of sensation</h3></header><p>The dorsal position, tubular structure, and epidermal origin of the central nervous system are definitive of the chordates, although some may see similarities with the hemichordates. The sensory structures are distinctive of the chordates and include the paired nasal, optic, and otic organs (along with the strongly differentiated head).</p><p>The nasal vesicle is variously open to the environment, and its sensory cells, as chemical receptors, are not unlike those in the taste buds of the mouth. The eye is the most complex organ of the head and is a lateral outpocketing of the anterior end of the brain tube. Later it acquires a lens of epidermal origin. The act of focusing the eye (accommodation) shows extensive adaptive variation among the different groups of vertebrates.</p><p>The otic vesicle starts from a simple sac formed by the invagination of an ectodermal placode. These developmental changes also include the changes of innervation. Whereas the original structure was basically an equilibrium adaptation, other functions, such as an awareness of movement or the sensation of the proximity of prey, developed.</p><p>The lateral-line system of canals and sensory organs is a unique vertebrate feature. The elements of this system are found on the head as well as the body. This system is related to the ear and presumably at its origin served a similar function. This system is lost in terrestrial vertebrate forms.</p></section><section class="toc-anchor" id="49530.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>The digestive system</h3></header><p>The digestive system of the vertebrate is distinctive in its structure but not in its function. The mouth and pharynx can be considered as parts of this system; the latter as an expanded cavity in the head is unmatched in any other group. The stomach and gut have been discussed above.</p><p>Presumably the original condition of the digestive glands was that of a ventral diverticulum which may have received the food mass into its cavity. This diverticulum, matched by the diverticulum seen in the amphioxus or the “intestine” of the tunicate, produced the secretions (bilelike and enzymes) of both liver and pancreas. Through time, the liver gradually differentiated from the pancreas. The size and separation of the liver from the gut suggest its separate blood and metabolic activities. The most obvious by-product of the liver, bile, necessitated the formation of a gall bladder and a duct connection with the gut. The pancreas, in contrast, continued to produce digestive enzymes, but its secretory cells were no longer in direct contact with the food mass. Because the pancreas was only a partial source of intestinal digestive enzymes, it was sometimes reduced in size and enclosed in the gut wall itself (agnaths) or dispersed as tiny bits of tissue in the mesentary supporting the gut (actinopterygians).</p></section><section class="toc-anchor" id="49531.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>The excretory system</h3></header><p>The excretory system is unique in its nephrons, which filter the blood in the glomeruli and remove a variety of wastes from the body through selective secretion and reabsorption. In the shark or the coelacanth <em>Latimeria</em>, urea is used to raise the osmotic pressure of the blood to that of the marine habitat, thus saving these organisms considerable metabolic energy. The large intestine (sometimes centred in a rectal gland) acts as an auxiliary excretory organ, as do also the gills of fishes or the sweat glands of mammals.</p></section><section class="toc-anchor" id="49532.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>Respiration and gas exchange</h3></header><p>Respiration, like excretion, involves specialized body structures, such as lungs or gills, but also can involve other areas, such as the skin itself. Respiration involves exchange of gases both between the body of the organism and the environment and between the blood system and the body tissues. It also involves cellular respiration where oxygen is used and carbon dioxide is produced. There is nothing characteristic of the vertebrate in this functional area; even the hemoglobin of the blood is suggested in the respiratory pigments of other animals.</p></section><section class="toc-anchor" id="49533.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>The circulatory system</h3></header><p>The circulatory system of vertebrates is closed in that fluids course through vessels, but there is free movement of cells in and out of blood. Some leukocyte (white blood cell) movement out of the capillaries and fluid leakage are observed in all tissues. Blood tissues are distinctive in the range of specialized cells, although these vary in detail among animals. The immune function of the blood is best developed in the vertebrate.</p></section><section class="toc-anchor" id="49534.toc" style="padding-top: 46px; margin-top: -46px;"><header><h3>The endocrine system</h3></header><p>The endocrine system is characterized by its separate organs. The occurrence of a pituitary or a thyroid gland is suggestive of the evolutionary change and specialization that took place within this group. The relatively unspecialized nature of some parts of this system is seen in certain scattered cells in the gut wall or even the clumps of islet cells of the pancreas.</p></section></section></section>
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[[File:Naturkundemuseum Berlin - Dinosaurierhalle.jpg|upright=3.65|thumb|centre|Fossilized skeleton of ''[[Diplodocus carnegii]]'', showing an extreme example of the [[Vertebral column|backbone]] that characterizes the vertebrates.]]
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<section class="toc-anchor" id="49535.toc" style="padding-top: 46px; margin-top: -46px;"><header><h1>Evolution and paleontology<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"><i class="fontello-icon-list"></i></button></h1></header><p>The knowledge of vertebrates as revealed by fossils has grown rapidly during the past few decades, but there is much still to be discovered. The ancestral vertebrate (protovertebrate) has been sought for more than 100 years, and the likelihood of finding it today is not much greater than in the past. It can be assumed that the protovertebrate was small and soft-bodied, two factors that suggest the improbability of finding a fossilized form in a recognizable condition. There are Cambrian fossils that have been suggested to be fossil cephalochordates and there are scales of agnath fishes, but the first type of fossil is too simple and the second already too complex to explain the transition.</p><p>Malcolm T. Jollie</p></section>
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<section class="toc-anchor" id="49536.toc" style="padding-top: 46px; margin-top: -46px;"><header><h1>Classification<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"><i class="fontello-icon-list"></i></button></h1></header><section class="toc-anchor" id="49537.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>Annotated classification</h2></header><dl class="md-classification-tax1"><dt class="md-classification-class"><span class="md-classification-label">Subphylum Vertebrata (or Craniata)</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Bilaterally symmetrical; internal skeletal support with skull enclosing a highly developed brain and a vertebral column and nerve cord; paired, jointed appendages; skin; advanced organ systems; sense organs concentrated in head.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/agnathan/111050">Agnatha]] </span><span class="md-classification-common-name">(hagfishes, lampreys)</span></dt><dd class="md-classification-description">Primitive; jawless; paired fins are poorly developed or lacking; rasping tongue; notochord without bone; skin is soft, glandular, and slimy; true gill arches absent; marine habitat.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/placoderm/60275">Placodermi]] </span><span class="md-classification-common-name">(placoderms)</span></dt><dd class="md-classification-description">†Extinct; fishlike; jaws supported by both cranium and hyoid arch (amphistylic); partly ossified cranium; primitive; head and trunk have armour that is jointed at the neck; pelvic fins present or absent; pectoral fins or finlike structures often present; gill arches.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/chondrichthian/111051">Chondrichthyes]] </span><span class="md-classification-common-name">(sharks, rays, and skates)</span></dt><dd class="md-classification-description">Cartilaginous fishes; jaws; paired fins; no swim bladder; pelvic fins in males often modified to form claspers; gill arches internal to gills; reduced notochord; lateral-line system; paired nostrils; internal nares absent; separate sexes; internal fertilization and direct development; oviparous, ovoviviparous, or viviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Elasmobranchii </span><span class="md-classification-common-name">(sharks and rays)</span></dt><dd class="md-classification-description">Numerous teeth derived of placoid scales; 5 to 7 gill clefts; operculum absent; cloaca; upper jaw not fused with braincase; dorsal fin nonerectile; with spiracles; worldwide distribution.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass <a class="reflink" href="/levels/collegiate/article/chimaera/24102">Holocephali]] </span><span class="md-classification-common-name">(chimeras)</span></dt><dd class="md-classification-description">Teeth fused to bony plates; no scales; 4 gill pairs under 1 gill opening on each side; no cloaca; no spiracles; operculum present; upper jaw fused to braincase; dorsal fin erectile; whiplike tail; claspers present in males; temperate marine freshwater.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class Osteichthyes </span><span class="md-classification-common-name">(<a class="reflink" href="/levels/collegiate/article/bony-fish/80646">bony fishes]])</span></dt><dd class="md-classification-description">Jaws; partly or fully ossified skeleton; usually a swim bladder; paired fins; gills covered by a bony operculum; scales; paired nostrils with or without internal nares; lateral-line system; mostly oviparous with external fertilization; some ovoviviparous or viviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Actinopterygii </span><span class="md-classification-common-name">(ray-finned fishes)</span></dt><dd class="md-classification-description">Generally lack choanae; no fleshy base to paired fins; no internal nares; air sacs usually function as swim bladder; skeleton usually well ossified.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Sarcopterygii </span><span class="md-classification-common-name">(lobe-finned fishes)</span></dt><dd class="md-classification-description">Usually possess a choana; paired fins with a fleshy base over a bony skeleton; persisting notochord; 2 dorsal fins; nares are internal.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/amphibian/110233">Amphibia]]</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Cold-blooded; respire by lungs, gills, skin, or mouth lining; larval stage in water or in egg; skin is usually moist with mucous glands and without scales; tetrapods; freshwater and terrestrial; paired appendages are legs; 10 pairs of cranial nerves; separate sexes; external fertilization with development into tadpole larvae; some have internal development, ovoviviparous or viviparous.</dd></dl><dl class="md-classification-tax4"><dt class="md-classification-class"><span class="md-classification-label">Order Aponda (or Gymnophiona; </span><span class="md-classification-common-name"><a class="reflink" href="/levels/collegiate/article/Gymnophiona/110235">caecilians]])</span></dt><dd class="md-classification-description">Wormlike; no limbs or girdles; compact skull; lidless, minute eyes; persistent notochord; tail; scales present in some species.</dd></dl><dl class="md-classification-tax4"><dt class="md-classification-class"><span class="md-classification-label">Order <a class="reflink" href="/levels/collegiate/article/Anura/110236">Anura]] (or Salientia; </span><span class="md-classification-common-name">frogs and toads)</span></dt><dd class="md-classification-description">Tailless; elongated hind limbs modified for jumping; larvae lack true teeth and external gills.</dd></dl><dl class="md-classification-tax4"><dt class="md-classification-class"><span class="md-classification-label">Order Caudata (or Urodela; </span><span class="md-classification-common-name"><a class="reflink" href="/levels/collegiate/article/salamander/65027">salamanders]])</span></dt><dd class="md-classification-description">Tail; limbs normal; many skeletal elements cartilaginous; larvae with true teeth and external gills.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/reptile/110251">Reptilia]]</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Cold-blooded; no larval stage; breathing by lungs; well-ossified skull; dry skin; scales; no glands; 5-toed limbs; claws; 3- or 4-chambered heart with incomplete ventricle separation; 12 pairs of cranial nerves; internal fertilization, direct development; oviparous and ovoviviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Anapsida </span><span class="md-classification-common-name">(turtles, tortoises, terrapins)</span></dt><dd class="md-classification-description">No temporal skull openings; body encased in bony shell; no teeth in living members; oviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Lepidosauria</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">No bipedal specializations; 2 complete temporal openings; complete palate; oviparous; male is without penis.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass <a class="reflink" href="/levels/collegiate/article/archosaur/9300">Archosauria]] </span><span class="md-classification-common-name">(ruling reptiles)</span></dt><dd class="md-classification-description">Some ancient forms had bipedal locomotion; longer hind legs; semiaquatic; webbed feet; teeth in sockets; single penis; oviparous; includes extinct dinosaurs.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Synaptosauria</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">†Extinct; single temporal opening on area of cheek.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Ichthyopterygia</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">†Extinct; temporal openings high up on skull; fishlike; spindle-shaped body; high tail fin; triangular dorsal fin; paddlelike legs; marine.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Synapsida</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">†Extinct; mammallike; lateral temporal opening.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/bird/105921">Aves]]</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Warm-blooded; skull has only 1 condyle; front limbs primarily modified for flight; hind limbs are legs with 4 or fewer toes; body covered with feathers; scales on feet; 4-chambered heart; no teeth; horny beak; lungs with extended air sacs; 12 pairs of cranial nerves; internal fertilization; oviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Archaeornithes</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">†Extinct; teeth in both jaws; long, feathered tail; less specialized for flight; body elongated and reptilelike; forelimb had 3 clawed digits; small brain and eyes; nonpneumatic bones.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Neornithes </span><span class="md-classification-common-name">(true birds)</span></dt><dd class="md-classification-description">Well-developed sternum; tail is not long; no teeth; forelimbs modified to wings; teeth replaced by horny rhamphoteca over bill.</dd></dl><dl class="md-classification-tax2"><dt class="md-classification-class"><span class="md-classification-label">Class <a class="reflink" href="/levels/collegiate/article/mammal/105972">Mammalia]]</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Warm-blooded; mammary glands; lower jaw is composed of 1 bone; hair; advanced brain; skin with different glands and hair; ears with 3 middle-ear bones; 12 pairs of cranial nerves; 4-chambered heart; young nourished by milk from mammary gland; internal fertilization; mostly viviparous, some oviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Prototheria</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Primitive; egg-laying; hair; mammary glands without nipples; pectoral girdle; separate oviducts that open into cloacal chamber that is shared with excretory ducts; oviparous.</dd></dl><dl class="md-classification-tax3"><dt class="md-classification-class"><span class="md-classification-label">Subclass Theria</span><span class="md-classification-common-name"></span></dt><dd class="md-classification-description">Mammary glands with nipples; functional teeth; oviducts partly fused; with or without a cloaca; uterus and vagina; viviparous.(Ed.)</dd></dl></section><section class="toc-anchor" id="49538.toc" style="padding-top: 46px; margin-top: -46px;"><header><h2>Critical appraisal</h2></header><p>The classification of animals is presently in a state of flux. The classification presented here is traditional and conservative. Because traditional theories of taxonomy tend to be nonquantitative, various interpretations of relationships or patterns can be presented and defended.</p><p>The alternative cladistic style of taxonomy is an attempt to force taxonomy into a testable, highly objective operation. One tentative classification based in cladistics separates the vertebrates into two superclasses (Agnatha and Gnathostomata). Agnathans are jawless, while the gnathostomates encompass the remainder of the jawed vertebrates. Living agnathans are placed in the class Cyclostomata. Gnathostomates can be further divided into the epiclasses Elasmobranchiomorphi (sharks and rays) and Teleostomi (bony fishes and tetrapods). The former group are identified primarily by a cartilaginous skeleton, while the latter group have developed a bony skeleton. Two subepiclasses of the teleostomes are Ichthyopterygii (or Osteichthyes; bony fishes) and Cheiropterygii (tetrapods), the latter being further divided into the classes Amphibia, Reptilia, Aves, and Mammalia.</p><p>Although this classification includes and uses traditional taxonomic categories, their position in the hierarchy may be changed. Separation of agnath and gnathostome is opposed by those cladists who chart the origin of gnathostomes from the agnath, believing that the differences in mouth and tooth structure are a result of modification. The Gnathostomata is subdivided into the Elasmobranchiomorphi and the Teleostomi largely on the basis of mouth and tooth structure. The creation of epiclasses and subepiclasses in the alternative classification is not important in itself; the creation of a dichotomy between Ichthyopterygii and Cheiropterygii, however, is important, although from the evolutionary view it is evident that the one evolved from the other.</p><p>Malcolm T. Jollie</p></section></section>
−
=== Gills ===
+
<section class="toc-anchor" id="biblio" style="padding-top: 46px; margin-top: -46px;"><header><h1 data-bind="string: 'xsl.article.additional.reading'">Additional Reading</h1></header>
−
[[File:Gills (esox).jpg|thumb|right|Gill arches bearing gills in a [[northern pike|pike]]]]
+
<p> <span class="smallcaps">Charles G. Crispens, Jr.</span>, <em>The Vertebrates, Their Forms and Functions</em> (1978); <span class="smallcaps">J.Z. Young</span>, <em>The Life of Vertebrates</em>, 3rd ed. (1981); <span class="smallcaps">Libbie H. Hyman</span>, <em>Hyman’s Comparative Vertebrate Anatomy</em>, 3rd ed., edited by <span class="smallcaps">Marvalee H. Wake</span> (1979); <span class="smallcaps">Edwin H. Colbert,</span> <em>Evolution of the Vertebrates: A History of the Backboned Animals Through Time</em>, 3rd ed. (1980); <span class="smallcaps">Alfred Sherwood Romer</span> and <span class="smallcaps">Thomas S. Parsons</span>, <em>The Vertebrate Body</em>, 6th ed. (1986);<span class="smallcaps"> Leonard B. Radinsky</span>, <em>The Evolution of Vertebrate Design</em> (1987); <span class="smallcaps">Robert L. Carroll</span>, <em>Vertebrate Paleontology and Evolution</em> (1988); and <span class="smallcaps">F. Harvey Pough</span>, <span class="smallcaps">John B. Heiser</span>, and <span class="smallcaps">William N. McFarland</span>, <em>Vertebrate Life</em>, 3rd ed. (1989).</p>
−
 
+
<p>Malcolm T. Jollie</p></section>
−
All [[Basal (phylogenetics)|basal]] vertebrates breathe with [[gills]]. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the [[pharynx]] to the exterior. Each [[gill]] is supported by a cartilagenous or bony [[gill arch]].<ref>{{cite book|last=Scott|first=T.|title=Concise encyclopedia biology|year=1996|publisher=Walter de Gruyter|isbn=978-3-11-010661-9|page=542}}</ref> The [[bony fish]] have three pairs of arches, [[cartilaginous fish]] have five to seven pairs, while the primitive [[jawless fish]] have seven. The vertebrate ancestor no doubt had more arches than this, as some of their [[Chordata|chordate]] relatives have more than 50 pairs of gills.<ref name="Romer" />
+
</article>
−
 
 
−
In [[amphibians]] and some primitive bony [[fishes]], the [[larva]]e bear [[external gills]], branching off from the gill arches.<ref>{{cite journal|journal=The American Naturalist|year=1957|volume=91|page=287|publisher=Essex Institute|jstor = 2458911|title=The Origin of the Larva and Metamorphosis in Amphibia | doi = 10.1086/281990 }}</ref> These are reduced in adulthood, their function taken over by the gills proper in fishes and by [[lung]]s in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal [[gill]] system as seen in fish apparently being irrevocably lost very early in the evolution of [[tetrapod]]s.<ref name=Gaining_ground>Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. ''Indiana University Press'', Bloomington, Indiana. 369 pp</ref>
 
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−
While the more derived [[vertebrates]] lack gills, the gill arches form during [[Prenatal development|fetal development]], and form the basis of essential structures such as [[jaw]]s, the [[thyroid gland]], the [[larynx]], the ''columella'' (corresponding to the [[stapes]] in [[mammals]]) and, in mammals, the [[Ossicles|malleus and incus]].<ref name="Romer" />
 
−
 
 
−
=== Central nervous system ===
 
−
The [[central nervous system]] of vertebrates is based on a hollow [[Dorsal nerve cord|nerve cord]] running along the length of the animal. Of particular importance and unique to vertebrates is the presence of [[neural crest]] cells. These are progenitors of [[stem cell]]s, and critical to coordinating the functions of cellular components.<ref name=Teng2006>Teng, L.; Labosky, P. A. (2006). [https://www.ncbi.nlm.nih.gov/pubmed/17076284 "Neural crest stem cells"] In: Jean-Pierre Saint-Jeannet, ''Neural Crest Induction and Differentiation'', pp. 206-212, Springer Science & Business Media. {{ISBN|9780387469546}}.</ref> Neural crest cells migrate through the body from the [[Dorsal nerve cord|nerve cord]] during development, and initiate the formation of [[Ganglion|neural ganglia]] and structures such as the jaws and skull.<ref name=Gans1983>{{cite journal | last1 = Gans | first1 = C. | last2 = Northcutt | first2 = R. G. | year = 1983 | title = Neural crest and the origin of vertebrates: a new head | url = | journal = Science | volume = 220 | issue = 4594| pages = 268–273 | doi = 10.1126/science.220.4594.268 | bibcode = 1983Sci...220..268G }}</ref><ref name=Bronner2012>{{cite journal | last1 = Bronner | first1 = M. E. | last2 = LeDouarin | first2 = N. M. | date = 1 June 2012 | title = Evolution and development of the neural crest: An overview | url = | journal = Developmental Biology | volume = 366 | issue = 1| pages = 2–9 | doi = 10.1016/j.ydbio.2011.12.042 | pmc = 3351559 | pmid=22230617}}</ref><ref>Dupin, E.; Creuzet, S.; Le Douarin, N.M. (2007) "The Contribution of the Neural Crest to the Vertebrate Body".
 
−
In: Jean-Pierre Saint-Jeannet, ''Neural Crest Induction and Differentiation'', pp. 96–119, Springer Science & Business Media. {{ISBN|9780387469546}}. {{doi|10.1007/978-0-387-46954-6_6}}. [https://www.ncbi.nlm.nih.gov/books/NBK6098/ Full text]</ref>
 
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The vertebrates are the only [[chordate]] group to exhibit [[cephalisation]], the concentration of [[brain]] functions in the head. A slight swelling of the anterior end of the nerve cord is found in the [[lancelet]], a chordate, though it lacks the eyes and other complex sense [[Organ (anatomy)|organs]] comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.<ref name="Romer"/>
 
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−
A [[peripheral nervous system]] branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the [[central canal of spinal cord]] into three primary brain vesicles: The [[prosencephalon]] (forebrain), [[mesencephalon]] (midbrain) and [[rhombencephalon]] (hindbrain), further differentiated in the various vertebrate groups.<ref name=Hildebrand>Hildebrand, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''. [[New York City|New York]]</ref> Two laterally placed [[eye]]s form around outgrowths from the midbrain, except in [[hagfish]], though this may be a secondary loss.<ref>{{cite web|url=http://www.physorg.com/news115919015.html|title=Keeping an eye on evolution|date=3 December 2007|accessdate=4 December 2007|work=PhysOrg.com}}</ref><ref>[http://tolweb.org/Hyperotreti Hyperotreti - Hagfishes]</ref> The forebrain is well developed and subdivided in most [[tetrapod]]s, while the midbrain dominates in many [[fish]] and some [[salamander]]s. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the [[cerebral hemisphere]]s in [[mammals]].<ref name=Hildebrand/>
 
−
 
 
−
The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates. All [[invertebrates]] with well-developed brains, such as [[insects]], [[spiders]] and [[squid]]s, have a ventral rather than dorsal system of [[ganglion]]s, with a split [[brain stem]] running on each side of the mouth or gut.<ref name="Romer"/>
 
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−
== Evolutionary history ==
 
−
{{see also|Evolution of fish|Evolution of tetrapods}}
 
−
 
 
−
=== First vertebrates ===
 
−
[[File:Haikouichthys cropped.jpg|thumb|right|The early vertebrate ''[[Haikouichthys]]'']]
 
−
 
 
−
Vertebrates originated about 525 million years ago during the [[Cambrian explosion]], which saw the rise in organism diversity. The earliest known vertebrate is believed to be the ''[[Myllokunmingia]]''.<ref name = "Shu et al. 1999">{{cite journal|title=Lower Cambrian vertebrates from south China | author=Shu |date=4 November 1999|journal=Nature| volume=402|pages=42–46|doi= 10.1038/46965|last2=Luo|first2=H-L.|last3=Conway Morris|first3=S.|last4=Zhang|first4=X-L.|last5=Hu|first5=S-X.|last6=Chen|first6=L.|last7=Han|first7=J.|last8=Zhu|first8=M.|last9=Li|first9=Y.|issue=6757|bibcode=1999Natur.402...42S|first10=L-Z.|displayauthors=1 |last10=Chen }}</ref> Another early vertebrate is ''[[Haikouichthys ercaicunensis]]''. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.<ref>{{cite web|last=Waggoner|first=B.|title=Vertebrates: Fossil Record|url=http://www.ucmp.berkeley.edu/vertebrates/vertfr.html|publisher=UCMP|accessdate=15 July 2011}}</ref> All of these early vertebrates lacked [[jaw]]s in the common sense and relied on filter feeding close to the seabed.<ref>{{cite book|title=The Complete Guide to Prehistoric Life|year=2005|publisher=Firefly Books|author1=Tim Haines, T.|author2=Chambers, P.}}</ref> A vertebrate group of uncertain phylogeny, small-eel-like [[conodont]]s, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.<ref>{{Cite journal | doi = 10.1111/j.1469-185X.1999.tb00045.x| last1 = Donoghue | first1 = P. C. J. | last2 = Forey | first2 = P. L. | last3 = Aldridge | first3 = R. J. | date = May 2000 | title = Conodont affinity and chordate phylogeny | url = | journal = Biological Reviews | volume = 75 | issue = 2| pages = 191–251 | pmid = 10881388 }}</ref>
 
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−
=== From fish to amphibians ===
 
−
[[File:Acanthostega BW.jpg|thumb|''[[Acanthostega]]'', a fish-like early [[Labyrinthodontia|labyrinthodont]].]]
 
−
 
 
−
The first [[Gnathostomata|jawed vertebrates]] appeared in the latest [[Ordovician]] and became common in the [[Devonian]], often known as the "Age of Fishes".<ref name=britannica1954>{{cite book|title=Encyclopædia Britannica: a new survey of universal knowledge, Volume 17|year=1954|publisher=Encyclopædia Britannica|page=107}}</ref> The two groups of [[bony fishes]], the [[actinopterygii]] and [[sarcopterygii]], evolved and became common.<ref>{{cite book|title=Biology|year=2004|publisher=Cengage Learning|isbn=978-0-534-49276-2|author=Berg, L.R.|author2=Solomon, E.P. |author3=Martin, D.W. |page=599}}</ref> The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the [[Placodermi]], a group of armoured fish that dominated the entirety of that period since the late [[Silurian]]. The Devonian also saw the rise of the first [[Labyrinthodontia|labyrinthodonts]], which was a transitional form between fishes and [[amphibians]].
 
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−
=== Mesozoic vertebrates ===
 
−
[[Amniotes]] branched from labyrinthodonts in the subsequent [[Carboniferous]] period. The [[Parareptilia]] and [[synapsid]] amniotes were common during the late [[Paleozoic]], while [[diapsid]]s became dominant during the [[Mesozoic]]. In the sea, the [[bony fishes]] became dominant; the [[birds]], a derived form of [[dinosaurs]], evolved in the [[Jurassic]].<ref>{{cite book|last=Cloudsley-Thompson|first=J. L.|title=Ecology and behaviour of Mesozoic reptiles|year=2005|publisher=Springer|location=9783540224211|page=6}}</ref> The demise of the non-avian dinosaurs at the end of the [[Cretaceous]] allowed for the expansion of [[mammals]], which had evolved from the [[therapsid]]s, a group of synapsid amniotes, during the late [[Triassic]] Period.
 
−
 
 
−
=== After the Mesozoic ===
 
−
The [[Cenozoic]] world has seen great diversification of bony fishes, frogs, birds and mammals.
 
−
 
 
−
Over half of all living vertebrate species (about 32,000 species) are fish (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over {{convert|4,600|m|ft|abbr=off}} to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about {{convert|11,000|m|ft|abbr=off}}. Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (with roughly 7,000 species); mammals (with approximately 5,500 species); and reptiles and birds (with about 20,000 species divided evenly between the two classes). Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea [[snakes]], penguins, cetaceans).
 
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−
== Classification ==
 
−
There are several ways of classifying animals. [[Evolutionary taxonomy|Evolutionary systematics]] relies on [[anatomy]], [[physiology]] and [[evolution]]ary history, which is determined through similarities in anatomy and, if possible, the [[genetics]] of organisms.<ref>Chalopin, D.; Tomaszkiewicz, M.; Galiana, D.; Volff, J.N. (2012). LTR Retroelement-Derived Protein-Coding Genes and Vertebrate Evolution. In: Witzany G (ed). Viruses: Essential Agents of Life, Springer, Dordrecht, pp 269-282.</ref> [[Cladistics|Phylogenetic classification]] is based solely on [[phylogeny]].<ref>{{cite book|title=Australian water bugs: their biology and identification (Hemiptera-Heteroptera, Gerromorpha & Nepomorpha)|year=2004|publisher=Apollo Books|isbn=978-87-88757-78-1|author=Andersen, N.M.|author2=Weir, T.A. |page=38}}</ref> Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.<ref>Hildebran, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''. [[New York (state)|New York]], page 33: ''Comment: The problem of naming sister groups''</ref>
 
−
 
 
−
=== Traditional classification ===
 
−
[[File:Spindle diagram.jpg|thumb|right|Traditional spindle diagram of the evolution of the vertebrates at class level]]
 
−
 
 
−
Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The [[extant taxon|extant]] vertebrates are:<ref name="Romer"/>
 
−
 
 
−
* '''Subphylum Vertebrata'''
 
−
** Class [[Agnatha]] (jawless fishes)
 
−
** Class [[Chondrichthyes]] (cartilaginous fishes)
 
−
** Class [[Osteichthyes]] (bony fishes)
 
−
** Class [[Amphibia]] (amphibians)
 
−
** Class [[Reptilia]] (reptiles)
 
−
** Class [[Aves]] (birds)
 
−
** Class [[Mammalia]] (mammals)
 
−
 
 
−
In addition to these, there are two classes of extinct armoured fishes, the [[Placodermi]] and the [[Acanthodii]].
 
−
 
 
−
Other ways of classifying the vertebrates have been devised, particularly with emphasis on the [[phylogeny]] of [[labyrinthodontia|early amphibians]] and reptiles. An example based on Janvier (1981, 1997), Shu ''et al.'' (2003), and Benton (2004)<ref name="Benton2004">{{cite book | last =Benton | first =M.J. | authorlink =Michael Benton | title =Vertebrate Palaeontology | publisher =[[Blackwell Publishing]] | date= 1 November 2004 |edition=Third | location = | pages =33, 455 pp. | url =http://palaeo.gly.bris.ac.uk/benton/vertclass.html | doi = | id = | isbn =978-0632056378}}</ref> is given here:
 
−
 
 
−
* '''Subphylum Vertebrata'''
 
−
**''[[Palaeospondylus]]''
 
−
** '''Superclass [[Agnatha]]''' or [[Cephalaspidomorphi]] ([[lamprey]]s and other jawless fishes)
 
−
** '''Infraphylum [[Gnathostomata]]''' (vertebrates with jaws)
 
−
*** Class †[[Placodermi]] (extinct armoured fishes)
 
−
*** Class [[Chondrichthyes]] (cartilaginous fishes)
 
−
*** Class †[[Acanthodii]] (extinct spiny "sharks")
 
−
*** '''Superclass [[Osteichthyes]]''' (bony vertebrates)
 
−
**** Class [[Actinopterygii]] (ray-finned bony fishes)
 
−
**** Class [[Sarcopterygii]] (lobe-finned fishes, including the tetrapods)
 
−
**** Class [[Amphibia]] (amphibians, some ancestral to the [[amniotes]])- now a paraphyletic group
 
−
**** Class [[Synapsida]] (mammals and the extinct mammal-like reptiles)
 
−
**** Class [[Sauropsida]] (reptiles and birds)
 
−
 
 
−
While this traditional classification is orderly, most of the groups are [[paraphyletic]], i.e. do not contain all descendants of the class's common ancestor.<ref name=Benton2004 /> For instance, descendants of the first reptiles include modern reptiles, as well as mammals and birds. Most of the classes listed are not "complete" (and are therefore paraphyletic) [[taxon|taxa]], meaning they do not include all the descendants of the first representative of the group. For example, the [[agnatha]]ns have given rise to the [[Gnathostomata|jawed vertebrates]]; the [[Osteichthyes|bony fishes]] have given rise to the [[tetrapoda|land vertebrates]]; the traditional "[[Labyrinthodont|amphibians]]" have given rise to the [[Reptilia|reptiles]] (traditionally including the [[synapsid]]s, or mammal-like "reptiles"), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on [[phylogeny]]{{Citation needed|date=October 2016}}, organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.
 
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−
=== Phylogenetic relationships ===
 
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In [[Phylogenetics|phylogenetic taxonomy]], the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a [[phylogenetic tree]]. The one below is based on studies compiled by Philippe Janvier and others for the ''Tree of Life Web Project'' and Delsuc et al.<ref name=tol>Janvier, P. 1997. Vertebrata. Animals with backbones. Version 1 January 1997 (under construction). http://tolweb.org/Vertebrata/14829/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/</ref><ref>{{cite journal|vauthors=Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ|date=April 2018|title=A phylogenomic framework and timescale for comparative studies of tunicates|url=https://doi.org/10.1186/s12915-018-0499-2|journal=BMC Biology|volume=16|issue=1|pages=39|doi=10.1186/s12915-018-0499-2|pmc=5899321|pmid=29653534}}</ref><!--getting a bit old now, and what about recent edits to the tree?-->
 
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{{clade| style=font-size:85%;line-height:70%
 
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|label1='''Vertebrata'''/
 
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|1={{clade
 
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|label1=[[Agnatha]]/
 
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  |sublabel1=[[Cyclostomes]]
 
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  |1={{clade
 
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|1=[[Hyperoartia]] (lampreys)[[File:Nejonöga, Iduns kokbok.jpg|70 px]]
 
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  |2=[[Myxini]]
 
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}}
 
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  |2=?†[[Euconodonta]]
 
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  |label3=<span style="color:white;">unnamed</span>
 
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  |3={{clade
 
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      |1=†[[Pteraspidomorphi]][[File:Astraspis desiderata.png|50 px]]
 
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      |2=?†[[Thelodonti]][[File:Sphenonectris turnernae white background.jpg|50 px]]
 
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      |label3=<span style="color:white;">unnamed</span>
 
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      |3={{clade
 
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        |1=?†[[Anaspida]][[File:Pharyngolepis2.png|50 px]]
 
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        |label2=<span style="color:white;">unnamed</span>
 
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        |2={{clade
 
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            |1=†[[Galeaspida]]
 
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            |label2=<span style="color:white;">unnamed</span>
 
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            |2={{clade
 
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              |1=?†[[Pituriaspida]]
 
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              |2=†[[Osteostraci]]
 
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              |label3=[[Gnathostomata]]
 
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              |3={{clade
 
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                  |1=†[[Placodermi]] (armoured fishes)[[File:Dunkleosteus intermedius.jpg|70 px]]
 
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                  |label2= <span style="color:white;">unnamed</span>
 
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                  |2={{clade
 
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                    |1=[[Acanthodians]], incl. [[Chondrichthyes]] (cartilaginous fishes)[[File:Acanthodes BW.jpg|70px]][[File:Carcharodon carcharias drawing.jpg|70 px]]
 
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                    |label2=[[Euteleostomi]]
 
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                        |2={{clade
 
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                          |1=[[Actinopterygii]] (ray-finned fishes)[[File:Cyprinus carpio3.jpg|70px]]
 
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                          |label2=[[Sarcopterygii]] (lobe-finned fish)
 
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                          |2={{clade
 
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                              |1=?†[[Onychodontiformes]][[File:OnychodusDB15 flipped.jpg|70 px]]
 
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                              |2=[[Actinistia]] (coelacanths)[[File:Coelacanth flipped.png|70 px]]
 
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                              |label3= <span style="color:white;">unnamed</span>
 
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                              |3={{clade
 
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                                |1=†[[Porolepiformes]][[File:Reconstruction of Porolepis sp flipped.jpg|70 px]]
 
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                                |2=[[Dipnoi]] (lungfishes)[[File:Barramunda coloured.jpg|70 px]]
 
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                                }}
 
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                              |label4= <span style="color:white;">unnamed</span>
 
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                              |4={{clade
 
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                                |1=†[[Rhizodontidae|Rhizodontimorpha]][[File:Gooloogongia loomesi reconstruction.jpg|70 px]]
 
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                                |2={{clade
 
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                                    |1=†[[Tristichopteridae]][[File:Eusthenodon DB15 flipped.jpg|70 px]]
 
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                                    |2=[[Tetrapoda]][[File:Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg|70 px]]
 
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                                    }}
 
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                                }}
 
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                              }}
 
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                          }}
 
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                        }}
 
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                    }}
 
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                  }}
 
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              }}
 
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            }}
 
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        }}
 
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      }}
 
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  |sublabel1=[[Chordata]]}}
 
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== Number of extant species ==
 
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The number of described vertebrate species are split evenly between [[tetrapod]]s and [[fish]]. The following table lists the number of described [[Extant taxa|extant]] species for each vertebrate [[Class (biology)|class]] as estimated in the [[IUCN Red List of Threatened Species]]'', 2014.3.<ref name=IUCN1014/>
 
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{| class="wikitable"
 
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|-
 
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! colspan=3 | Vertebrate groups
 
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! Image
 
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! Class
 
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! Estimated number of<br />described species<ref name=IUCN1014 />
 
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! Group<br />totals<ref name=IUCN1014 />
 
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|-
 
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! rowspan=6 style="background:rgb(120,120,185)" | [[Anamniote|<span style="color:white;">Anamniote</span>]]<br /><br /><span style="color:white;"><small>lack<br />[[amniotic membrane|<span style="color:white;">amniotic<br />membrane</span>]]<br />so need to<br />reproduce<br />in water</small></span>
 
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! rowspan=2 style="background:rgb(120,120,185)" | [[Jawless vertebrates|<span style="color:white;">Jawless</span>]]
 
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! rowspan=5 style="background:rgb(130,130,130)" | [[Fish|<span style="color:white;">Fish</span>]]
 
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| [[File:Eptatretus polytrema.jpg|140px]]
 
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| align=center | [[Myxini]]<br /><small>([[hagfish]])</small>
 
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| align=center |
 
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| align=center rowspan=5 | 32,900
 
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|-
 
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| [[File:Eudontomyzon danfordi Tiszai ingola.jpg|140px]]
 
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| align=center | [[Hyperoartia]]<br /><small>([[lamprey]])</small>
 
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| align=center |
 
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|-
 
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! rowspan=7 style="background:rgb(120,120,185)" | [[Jawed vertebrates|<span style="color:white;">Jawed</span>]]
 
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| [[File:Shark fish chondrichthyes.jpg|140px]]
 
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| align=center | [[cartilaginous fish|cartilaginous<br />fish]]
 
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| align=center |
 
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|-
 
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| [[File:Carassius wild golden fish 2013 G1.jpg|140px]]
 
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| align=center | [[ray-finned fish|ray-finned<br />fish]]
 
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|
 
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|-
 
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| [[File:Coelacanth-bgiu.png|140px]]
 
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| align=center | [[lobe-finned fish|lobe-finned<br />fish]]
 
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| align=center |
 
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|-
 
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! rowspan=4 style="background:rgb(130,130,130)" | [[Tetrapod|<span style="color:white;">Tetrapods</span>]]
 
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| [[File:Lithobates pipiens.jpg|140px]]
 
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| align=center | [[amphibian]]s
 
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| align=center | 7,302
 
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| rowspan=4 align=center | 33,278
 
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|-
 
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! rowspan=3 style="background:rgb(120,120,185)" | [[Amniote|<span style="color:white;">Amniote</span>]]<br /><br /><span style="color:white;"><small>have<br />[[amniotic membrane|<span style="color:white;">amniotic<br />membrane</span>]]<br />adapted to<br />reproducing<br />on land</small></span>
 
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| [[File:Florida Box Turtle Digon3.jpg|140px]]
 
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| align=center | [[reptile]]s
 
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| align=center | 10,711
 
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|-
 
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| [[File:Secretary bird (Sagittarius serpentarius) 2.jpg|140px]]
 
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| align=center | [[bird]]s
 
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| align=center | 10,425
 
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|-
 
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| [[File:Squirrel (PSF).png|140px]]
 
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| align=center | [[mammal]]s
 
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| align=center | 5,513
 
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|-
 
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| colspan=6 align=right | '''Total described species'''
 
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| align=center | '''66,178'''
 
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|}
 
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The IUCN estimates that 1,305,075 [[Invertebrate#Number of extant species|extant invertebrate species]] have been described,<ref name=IUCN1014 /> which means that less than 5% of the [[Animal#Number of extant species|described animal species]] in the world are vertebrates.
 
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=== Vertebrate species databases ===
 
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The following databases maintain (more or less) up-to-date lists of vertebrate species:
 
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* Fish: [[FishBase|Fishbase]]
 
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* Amphibians: [http://www.amphibiaweb.org Amphibiaweb]
 
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* Reptiles: [[Reptile Database]]
 
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* Birds: [https://avibase.bsc-eoc.org/avibase.jsp?lang=EN Avibase]
 
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* Mammals: [https://www.departments.bucknell.edu/biology/resources/msw3/ Mammal species of the World]
 
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==Reproductive systems==
 
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Nearly all vertebrates undergo [[sexual reproduction]]. They produce [[Ploidy#Haploid and monoploid|haploid]] gametes by [[meiosis]]. The smaller, motile gametes are [[Spermatozoon|spermatozoa]] and the larger, non-motile gametes are [[Egg cell|ova]]. These fuse by the process of [[fertilisation]] to form diploid [[zygote]]s, which develop into new individuals.
 
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===Inbreeding===
 
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During sexual reproduction, mating with a close relative ([[inbreeding]]) often leads to [[inbreeding depression]]. Inbreeding depression is considered to be largely due to expression of deleterious [[Dominance (genetics)|recessive]] [[mutation]]s.<ref name="pmid19834483">{{cite journal |author1=Charlesworth, D. |author2=Willis, J.H. |title=The genetics of inbreeding depression |journal=Nat. Rev. Genet. |volume=10 |issue=11 |pages=783–796 |date=November 2009 |pmid=19834483 |doi=10.1038/nrg2664 |url=}}</ref> The effects of inbreeding have been studied in many vertebrate species.
 
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In several species of fish, inbreeding was found to decrease reproductive success.<ref name="pmid16189545">{{cite journal |author1=Gallardo, J.A. |author2=Neira, R. |title=Environmental dependence of inbreeding depression in cultured Coho salmon (Oncorhynchus kisutch): aggressiveness, dominance and intraspecific competition |journal=Heredity (Edinb) |volume=95 |issue=6 |pages=449–56 |date=July 2005 |pmid=16189545 |doi=10.1038/sj.hdy.6800741 |url=}}</ref><ref name="pmid19486236">{{cite journal |author1=Ala-Honkola, O.| author2=Uddström, A. |author3=Pauli, B.D. |author4=Lindström, K. |title=Strong inbreeding depression in male mating behaviour in a poeciliid fish |journal=J. Evol. Biol. |volume=22 |issue=7 |pages=1396–1406 |year=2009 |pmid=19486236 |doi=10.1111/j.1420-9101.2009.01765.x |url=}}</ref><ref name="pmid23798977">{{cite journal |author1=Bickley, L.K. |author2=Brown, A.R. |author3=Hosken, D.J. |author4=Hamilton, P.B. |author5=Le Page, G. |author6=Paull, G.C. |author7=Owen, S.F. |author8=Tyler, C.R. |title=Interactive effects of inbreeding and endocrine disruption on reproduction in a model laboratory fish |journal=Evol Appl |volume=6 |issue=2 |pages=279–289 |date=February 2013 |pmid=23798977 |pmc=3689353 |doi=10.1111/j.1752-4571.2012.00288.x |url=}}</ref>
 
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Inbreeding was observed to increase juvenile mortality in 11 small animal species.<ref name="pmid7043080">{{cite journal |author1=Ralls, K. |author2=Ballou, J. |title=Effect of inbreeding on juvenile mortality in some small mammal species |journal=Lab. Anim. |volume=16 |issue=2 |pages=159–66 |year=1982 |pmid=7043080 |doi= 10.1258/002367782781110151|url=}}</ref>
 
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A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full [[sibling]]s).<ref name="pmid21737321">{{cite journal |author=Leroy, G. |title=Genetic diversity, inbreeding and breeding practices in dogs: results from pedigree analyses |journal=Vet. J. |volume=189 |issue=2 |pages=177–182 |date=August 2011 |pmid=21737321 |doi=10.1016/j.tvjl.2011.06.016 |url=}}</ref> This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival.<ref name="pmid10490080">{{cite journal |author1=van der Beek, S. |author2=Nielen, A.L.|author3=Schukken, Y.H. |author4=Brascamp, E.W. |title=Evaluation of genetic, common-litter, and within-litter effects on preweaning mortality in a birth cohort of puppies |journal=Am. J. Vet. Res. |volume=60 |issue=9 |pages=1106–10 |year=1999 |pmid=10490080 |doi= |url=}}</ref><ref name="pmid15803761">{{cite journal |author1=Gresky, C. |author2=Hamann, H. |author3=Distl, O. |title=[Influence of inbreeding on litter size and the proportion of stillborn puppies in dachshunds] |language=German |journal=Berl. Munch. Tierarztl. Wochenschr. |volume=118 |issue=3-4 |pages=134–9 |year=2005 |pmid=15803761 |doi= |url=}}</ref><ref name="pmid25475165">{{cite journal |author1=Leroy, G. |author2=Phocas, F. |author3=Hedan, B. |author4=Verrier, E. |author5=Rognon, X. |title=Inbreeding impact on litter size and survival in selected canine breeds |journal=Vet. J. |volume=203 |issue=1 |pages=74–8 |year=2015 |pmid=25475165 |doi=10.1016/j.tvjl.2014.11.008 |url=}}</ref>
 
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[[Incest]]uous matings in birds result in severe [[Fitness (biology)|fitness]] costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).<ref name="pmid12144022">{{cite journal |author1=Keller, L.F. |author2=Grant, P.R. |author3=Grant, B.R.|author4=Petren, K. |title=Environmental conditions affect the magnitude of inbreeding depression in survival of Darwin's finches |journal=Evolution |volume=56 |issue=6 |pages=1229–39 |year=2002 |pmid=12144022 |doi= 10.1111/j.0014-3820.2002.tb01434.x|url=}}</ref><ref name="pmid22643890">{{cite journal |author1=Hemmings, N.L. |author2=Slate, J. |author3=Birkhead, T.R. |title=Inbreeding causes early death in a passerine bird |journal=Nat Commun |volume=3 |issue= |pages=863 |year=2012 |pmid=22643890 |doi=10.1038/ncomms1870 |url=|bibcode=2012NatCo...3E.863H }}</ref><ref name=Kingma>{{cite journal | author1=Kingma, S.A. |author2=Hall, M.L. |author3=Peters, A. | year = 2013 | title = Breeding synchronization facilitates extrapair mating for inbreeding avoidance | url = | journal = Behavioral Ecology | volume = 24 | issue = 6| pages = 1390–1397 | doi = 10.1093/beheco/art078 }}</ref>
 
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====Inbreeding avoidance====
 
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As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding. Numerous inbreeding avoidance mechanisms operating prior to mating have been described.
 
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Toads and many other amphibians display [[Philopatry|breeding site fidelity]]. Individuals that return to natal ponds to breed will likely encounter [[sibling]]s as potential mates. Although [[incest]] is possible, ''Bufo americanus'' siblings rarely mate.<ref name=Waldman>{{cite journal |author1=Waldman, B. |author2=Rice, J.E. |author3=Honeycutt, R.L. | year = 1992 | title = Kin recognition and incest avoidance in toads | url = | journal = Am. Zool. | volume = 32 | issue = | pages = 18–30 | doi = 10.1093/icb/32.1.18 }}</ref> These toads likely recognize and actively avoid close kins as mates. Advertisement vocalizations by males appear to serve as cues by which females recognize their kin.<ref name=Waldman />
 
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Inbreeding avoidance mechanisms can also operate subsequent to [[Sexual intercourse|copulation]]. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between [[sperm]] of rival males for achieving [[fertilization]].<ref name=Fitzpatrick>{{cite journal |author1=Fitzpatrick, J.L. |author2=Evans, J.P. |title=Postcopulatory inbreeding avoidance in guppies |journal=J. Evol. Biol. |volume=27 |issue=12 |pages=2585–94 |year=2014 |pmid=25387854 |doi=10.1111/jeb.12545 |url=}}</ref> In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed.<ref name=Fitzpatrick />
 
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When female [[sand lizard]]s mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.<ref name=Olsson>{{cite journal |author1=Olsson, M. |author2=Shine, R. |author3=Madsen, T. |author4=Gullberg, A. Tegelström H |title=Sperm choice by females |journal=Trends Ecol. Evol. |volume=12 |issue=11 |pages=445–6 |year=1997 |pmid=21238151 |doi= 10.1016/s0169-5347(97)85751-5|url=}}</ref> On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives.<ref name=Olsson /> This preference may enhance the fitness of progeny by reducing [[inbreeding depression]].
 
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===Outcrossing===
 
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Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny<ref name="pmid3898363">{{cite journal |author1=Bernstein, H. |author2=Byerly, H.C. |author3=Hopf, F.A. |author4=Michod, R.E. |title=Genetic damage, mutation, and the evolution of sex |journal=Science |volume=229 |issue=4719 |pages=1277–81 |year=1985 |pmid=3898363 |doi= 10.1126/science.3898363|url=|bibcode=1985Sci...229.1277B }}</ref> (and see [[Heterosis]]). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing<ref name="pmid21237809">{{cite journal |author1=Pusey, A. |author2=Wolf, M. |title=Inbreeding avoidance in animals |journal=Trends Ecol. Evol. |volume=11 |issue=5 |pages=201–6 |year=1996 |pmid=21237809 |doi= 10.1016/0169-5347(96)10028-8|url=}}</ref> (and see [[Inbreeding avoidance]]).
 
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Outcrossing as a way of avoiding inbreeding depression, has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit when the offspring are produced as a result of a mating between close relatives. In natural populations of the [[great tit]] (''Parus major''), inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.<ref name="pmid18211876">{{cite journal |author1=Szulkin, M. |author2=Sheldon, B.C. |title=Dispersal as a means of inbreeding avoidance in a wild bird population |journal=Proc. Biol. Sci. |volume=275 |issue=1635 |pages=703–11 |year=2008 |pmid=18211876 |pmc=2596843 |doi=10.1098/rspb.2007.0989 |url=}}</ref>
 
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The [[purple-crowned fairywren]] females paired with related males may undertake [[Extra-pair copulation|extra-pair matings]] that can reduce the negative effects of inbreeding. However, there are ecological and demographic constraints on extra pair matings. Nevertheless, 46% of broods produced by incestuously paired females contained extra-pair young.<ref name=Kingma />
 
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[[Southern pied babbler]]s (''Turdoides bicolor'') appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.<ref name="pmid22471769">{{cite journal |author1=Nelson-Flower, M.J. |author2=Hockey, P.A. |author3=O'Ryan, C. |author4=Ridley, A.R. |title=Inbreeding avoidance mechanisms: dispersal dynamics in cooperatively breeding southern pied babblers |journal=J Anim Ecol |volume=81 |issue=4 |pages=876–83 |year=2012 |pmid=22471769 |doi=10.1111/j.1365-2656.2012.01983.x |url=}}</ref> Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated.
 
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Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.<ref name="pmid26577076">{{cite journal |author1=Riehl, C. |author2=Stern, C.A. |title=How cooperatively breeding birds identify relatives and avoid incest: New insights into dispersal and kin recognition |journal=BioEssays |volume=37 |issue=12 |pages=1303–8 |year=2015 |pmid=26577076 |doi=10.1002/bies.201500120 |url=}}</ref> Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups.
 
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===Parthenogenesis===
 
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Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization.
 
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Reproduction in [[Squamata|squamate]] reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. However, various species, including the [[Epicrates maurus|Colombian Rainbow boa]] (''Epicrates maurus''), ''[[Agkistrodon contortrix]]'' (copperhead snake) and ''[[Agkistrodon piscivorus]]'' (cotton mouth snake) can also reproduce by facultative parthenogenesis -that is, they are capable of switching from a sexual mode of reproduction to an [[Asexual reproduction|asexual]] mode- resulting in production of WW female progeny.<ref name="pmid22977071">{{cite journal |author1=Booth, W. |author2=Smith, C.F. |author3=Eskridge, P.H. |author4=Hoss, S.K. |author5=Mendelson, J.R. |author6=Schuett, G.W. |title=Facultative parthenogenesis discovered in wild vertebrates |journal=Biol. Lett. |volume=8 |issue=6 |pages=983–5 |year=2012 |pmid=22977071 |pmc=3497136 |doi=10.1098/rsbl.2012.0666 |url=}}</ref><ref name="pmid21868391">{{cite journal |author1=Booth, W. |author2=Million, L. |author3=Reynolds, R.G. |author4=Burghardt, G.M. |author5=Vargo, E.L. |author6=Schal, C. |author7=Tzika, A.C. |author8=Schuett, G.W. |title=Consecutive virgin births in the new world boid snake, the Colombian rainbow Boa, Epicrates maurus |journal=J. Hered. |volume=102 |issue=6 |pages=759–63 |year=2011 |pmid=21868391 |doi=10.1093/jhered/esr080 |url=}}</ref> The WW females are likely produced by [[Thelytoky#Automixis with terminal fusion|terminal automixis]].
 
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[[Mole salamander]]s are an ancient (2.4-3.8 million year-old) unisexual vertebrate lineage.<ref name="pmid17546077">{{cite journal |author1=Bogart, J.P. |author2=Bi, K. |author3=Fu, J. |author4=Noble, D.W. |author5=Niedzwiecki, J. |title=Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes |journal=Genome |volume=50 |issue=2 |pages=119–36 |date=February 2007 |pmid=17546077 |doi=10.1139/g06-152 |url=}}</ref> In the [[polyploid]] unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander. [[Synapsis]] and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between [[homologous chromosome]]s. Thus little, if any, genetic variation is produced. Recombination between [[Ploidy#Homoeologous|homeologous]] chromosomes occurs only rarely, if at all.<ref name="pmid20358399">{{cite journal |author1=Bi, K, |author2=Bogart, J.P. |title=Probing the meiotic mechanism of intergenomic exchanges by genomic in situ hybridization on lampbrush chromosomes of unisexual Ambystoma (Amphibia: Caudata) |journal=Chromosome Res. |volume=18 |issue=3 |pages=371–82 |date=April 2010 |pmid=20358399 |doi=10.1007/s10577-010-9121-3 |url=}}</ref> Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms. However, meiosis may have been maintained during evolution by the efficient recombinational repair of DNA damages that meiosis provides, an advantage that could be realized at each generation.<ref>Bernstein, H.; Bernstein, C.; Michod, R.W. (2011). Meiosis as an Evolutionary Adaptation for DNA Repair. Chapter 19 pages 357-382 in "DNA Repair" (Inna Kruman editor). InTech Open Publisher. DOI: 10.5772/25117 {{ISBN|978-953-307-697-3}} http://www.intechopen.com/books/dna-repair/meiosis-as-an-evolutionary-adaptation-for-dna-repair</ref>
 
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===Self-fertilization===
 
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The [[mangrove killifish]] (''Kryptolebias marmoratus'') produces both eggs and sperm by meiosis and routinely reproduces by [[Reproduction#Autogamy|self-fertilisation]]. The capacity for [[selfing]] in these fishes has apparently persisted for at least several hundred thousand years.<ref name="pmid19706532">{{cite journal |author1=Tatarenkov, A. |author2=Lima, S.M. |author3=Taylor, D.S. |author4=Avise, J.C. |title=Long-term retention of self-fertilization in a fish clade |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=106 |issue=34 |pages=14456–9 |date=25 August 2009 |pmid=19706532 |pmc=2732792 |doi=10.1073/pnas.0907852106 |url=|bibcode=2009PNAS..10614456T }}</ref> Each individual [[hermaphrodite]] normally fertilizes itself when an egg and sperm that it has produced by an internal organ unite inside the fish's body.<ref name="urlGonadal morphology in the self-fertilizing mangrove killifish, Kryptolebias marmoratus - Springer">{{cite journal |url=https://link.springer.com/article/10.1007/s10228-006-0362-2#/page-1 |title=Gonadal morphology in the self-fertilizing mangrove killifish, Kryptolebias marmoratus |format= |work= |accessdate= |doi=10.1007/s10228-006-0362-2 |volume=53 |journal=Ichthyological Research |pages=427–430}}</ref> In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another.<ref name=Avise>{{cite journal |author1=Avise, J.C. |author2=Tatarenkov, A. |title=Allard's argument versus Baker's contention for the adaptive significance of selfing in a hermaphroditic fish |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=109 |issue=46 |pages=18862–7 |date=13 November 2012 |pmid=23112206 |pmc=3503157 |doi=10.1073/pnas.1217202109 |url=|bibcode=2012PNAS..10918862A }}</ref><ref name="pmid22990587">{{cite journal |author1=Earley, R.L. |author2=Hanninen, A.F. |author3=Fuller, A. |author4=Garcia, M.J. |author5=Lee, E.A. |title=Phenotypic plasticity and integration in the mangrove rivulus (Kryptolebias marmoratus): a prospectus |journal=Integr. Comp. Biol. |volume=52 |issue=6 |pages=814–27 |year=2012 |pmid=22990587 |pmc=3501102 |doi=10.1093/icb/ics118 |url=}}</ref> Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of ''fertilization assurance'' ([[Fertilisation#Self-Pollination|reproductive assurance]]) at each generation.<ref name=Avise />
 
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== See also ==
 
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* [[Marine vertebrate]]
 
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* [[Skeletal system of the horse]]
 
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== References ==
 
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{{Reflist|30em}}
 
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== Bibliography ==
 
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{{refbegin}}
 
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* {{cite book | last =Kardong | first =Kenneth V. | authorlink =Kenneth Kardong | title =Vertebrates: Comparative Anatomy, Function, Evolution | edition = second | publisher =[[McGraw-Hill]] | year= 1998 | location =USA | pages =747 pp. | doi = | id =
 
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| isbn =0-697-28654-1 }}
 
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* {{ITIS |id=331030 |taxon=Vertebrata |accessdate=6 August 2007}}
 
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{{refend}}
 
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== External links ==
 
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{{Wikispecies|Vertebrata}}
 
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* [http://tolweb.org/Vertebrata/14829 Tree of Life]
 
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* [http://www.nature.com/nature/journal/v439/n7079/abs/nature04336.html Tunicates and not cephalochordates are the closest living relatives of vertebrates]
 
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*[http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Vertebrate Pests] chapter in [[United States Environmental Protection Agency]] and [[University of Florida]]/[[Institute of Food and Agricultural Sciences]] National Public Health Pesticide Applicator Training Manual
 
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* [http://logic-law.com/index.php?title=Subphylum_Vertebrata The Vertebrates]
 
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* [https://web.archive.org/web/20131103084123/http://www.ibiology.org/ibioseminars/evolution-ecology/marc-w-kirschner-part-1.html The Origin of Vertebrates] [[Marc W. Kirschner]], ''iBioSeminars'', 2008.
 
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{{Animalia}}
 
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{{Chordata}}
 
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{{Portal bar|Animals}}
 
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{{Taxonbar|from=Q25241}}
 
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{{Authority control}}
 
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[[Category:Vertebrates|*]]
 
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[[Category:Terreneuvian first appearances]]
 
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[[Category:Extant Cambrian first appearances]]
 

2018/8/19/ (日) 13:46時点における版

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<section class="toc-anchor" id="intro" style="padding-top: 46px; margin-top: -46px;"><header>

Contents

Introduction

</header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=121619" data-article-id="110433" data-assembly-id="121619" data-show-google-classroom="false" data-credit="Encyclopædia Britannica, Inc." data-large-path="93546-050-597FCB04.jpg" title="vertebrate: major groups of vertebrates" data-caption="The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals. " data-media-type="IMAGE">

<img class="lazy" width="220" height="110" data-original="93546-004-F9C81CA9.jpg" alt="vertebrate: major groups of vertebrates" data-generated-alt="true" src="93546-004-F9C81CA9.jpg" style="display: block;">Encyclopædia Britannica, Inc.]]<figcaption>Encyclopædia Britannica, Inc.</figcaption></figure>

vertebrate, 

           also called Craniata,
any animal of the subphylum Vertebrata, the predominant subphylum of the phylum Chordata. They have backbones, from which they derive their name. The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone.

</section>

The subphylum is one of the best known of all groups of animals. Its members include the classes Agnatha, Chondrichthyes, and Osteichthyes (all fishes); Amphibia (amphibians); Reptilia (reptiles); Aves (birds); and Mammalia (mammals).

<section class="toc-anchor" id="49518.toc" style="padding-top: 46px; margin-top: -46px;"><header>

General features<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"></button>

</header>

Although the vertebral column is perhaps the most obvious vertebrate feature, it was not present in the first vertebrates, which probably had only a notochord. The vertebrate has a distinct head, with a differentiated tubular brain and three pairs of sense organs (nasal, optic, and otic). The body is divided into trunk and tail regions. The presence of pharyngeal slits with gills indicates a relatively high metabolic rate. A well-developed notochord enclosed in perichordal connective tissue, with a tubular spinal cord in a connective tissue canal above it, is flanked by a number of segmented muscle masses. A sensory ganglion develops on the dorsal root of the spinal nerve, and segmental autonomic ganglia grow below the notochord. The trunk region is filled with a large, bilateral body cavity (coelom) with contained viscera, and this coelom extends anteriorly into the visceral arches. A digestive system consists of an esophagus extending from the pharynx to the stomach and a gut from the stomach to the anus. A distinct heart, anteroventral to the liver, is enclosed in a pericardial sac. A basic pattern of closed circulatory vessels is largely preserved in most living forms. Unique, bilateral kidneys lie retroperitoneally (dorsal to the main body cavity) and serve blood maintenance and excretory functions. Reproductive organs are formed from tissue adjacent to the kidneys; this original close association is attested by the tubular connections seen in males of living forms. The ducts of the excretory organs open through the body wall into a cloacal chamber, as does the anus of the digestive tract. Reproductive cells are shed through nearby abdominal pores or through special ducts. A muscular tail continues the axial musculature of the trunk.

Approximately 45,000 living species constitute the vertebrates. Species of several classes are found from the high Arctic or Antarctic to the tropics around the Earth; they are missing only from interior Antarctica and Greenland and from the North Polar ice pack. In size, vertebrates range from minute fishes to elephants and whales (of up to 100 tons), the largest animals ever to have existed. Vertebrates are adapted to life underground, on the surface, and in the air. They feed upon plants, invertebrate animals, and one another. Vertebrate faunas are important to humans for food and recreation.

</section> <section class="toc-anchor" id="49519.toc" style="padding-top: 46px; margin-top: -46px;"><header>

Natural history<button class="btn toc-invoker" rel="tooltip" aria-label="Table of Contents" data-original-title="Table of Contents"></button>

</header>

In order to give a broad and comparative view of their life histories, the vertebrates are subdivided here into major groups based on morphology: the cyclostomes (jawless fishes), the chondrichthyes (cartilaginous fishes), the teleostomes (bony fishes), and the tetrapods.

<section class="toc-anchor" id="49520.toc" style="padding-top: 46px; margin-top: -46px;"><header>

The <a class="reflink" href="/levels/collegiate/article/cyclostome/28385">cyclostomes]]

</header><figure><a href="/levels/collegiate/article/vertebrate/110433/media?assemblyId=70896" data-article-id="110433" data-assembly-id="70896" data-show-google-classroom="false" data-credit="Oxford Scientific Films/Bruce Coleman Ltd." title="lamprey" data-caption="Lamprey (Lampetra) on rainbow trout." data-media-type="IMAGE">
<img class="lazy" width="220" height="132" data-original="37317-004-B45DFB50.jpg" title="lamprey" alt="lamprey" data-generated-alt="true" src="37317-004-B45DFB50.jpg" style="display: block;">Oxford Scientific Films/Bruce Coleman Ltd.


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