Vertebrates are distinguished from all other animals, including other chordates, by multiple synapomorphies: namely a vertebral column; a skull of bone or cartilage; a large brain divided into 3 or more sections, a muscular heart with multiple chambers; an inner ear withsemicircular canals; sense organs including the eyes, ears, and nose; and digestive organs including the intestines, liver, pancreas, and stomach.[5]
Idealised vertebrate body plan, showing key characteristics
Vertebrates (and other chordates) belong to theBilateria, a group of animals with mirror symmetrical bodies.[6] They move, typically by swimming, usingmuscles along the back, supported by a strong but flexibleskeletal structure, the spine orvertebral column.[7] The name 'vertebrate' derives from theLatinvertebratus, 'jointed',[8] fromvertebra, 'joint', in turn from Latinvertere, 'to turn'.[9]
Fossilized skeleton (cast) ofDiplodocus carnegii, showing an extreme example of thevertebral column that gives the vertebrates their name. The species is atetrapod, its four legs adapting the fish-like body plan for walking on land. The specimen is 26 m (85 ft) long.
As embryos, vertebrates still have a notochord. In all but thejawless fishes, it is replaced with a vertebral column (made ofbone orcartilage) during development.[7] Vertebrate embryos havepharyngeal arches; in adultfish, these support thegills, while in adultTetrapods they develop into other structures.[10][11]
In the embryo, alayer of cells along the backfolds and fuses into a hollowneural tube.[12] This develops into thespinal cord, and at its front end, thebrain.[12] The brain receives information about the world through nerves which carry signals fromsense organs in the skin and body.[13] Because the ancestors of vertebrates usually moved forwards, the front of the body encountered stimuli before the rest of the body, favouringcephalisation, the evolution of a head containing sense organs and a brain to process the sensory information.[14]
Vertebrates have a tubulargut that extends from themouth to theanus. The vertebral column typically continues beyond the anus to form an elongatedtail.[15][16][17]
The ancestral vertebrates, and most extant species, areaquatic and carry outgas exchange in their gills. The gills are finely-branched structures which bring the blood close to the water. They are positioned just behind the head, supported by cartilaginous or bonybranchial arches.[18] Injawed vertebrates, the first gill arch pair evolved into the jaws.[19] Inamphibians and some primitive bony fishes, the larvae haveexternal gills, branching off from the gill arches.[20]Oxygen is carried from the gills to the body in theblood, andcarbon dioxide is returned to the gills, in a closedcirculatory system driven by a chamberedheart.[21] TheTetrapods have lost the gills of their fish ancestors; they have adapted theswim bladder (that fish use for buoyancy) intolungs to breathe air, and the circulatory system is adapted accordingly.[22] At the same time, they adapted the bony fins of thelobe-finned fishes into two pairs of walkinglegs, carrying the weight of the body via theshoulder and pelvic girdles.[22]
Vertebrates originated during theCambrian explosion at the start of the Paleozoic, which saw a rise in animal diversity. The earliest known vertebrates belong to theChengjiang biota[27] and lived about 518 million years ago.[1] These includeHaikouichthys,Myllokunmingia,[27]Zhongjianichthys,[26] and probablyYunnanozoon.[28] Unlike other Cambrian animals, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail, but lacked jaws.[29] As such, one perspective is thatHaikouichthys and otherMyllokunmingiidae probably represent basalstem group craniates rather than actual vertebrates.[30]
A vertebrate group of uncertain phylogeny, small eel-likeconodonts, are known frommicrofossils of their paired tooth segments from the late Cambrian to the end of the Triassic.[31] Zoologists have debated whether teethmineralized first, given the hard teeth of the soft-bodied conodonts, and then bones, or vice versa, but it seems that the mineralized skeleton came first.[32]
The firstjawed vertebrates may have appeared in the lateOrdovician (~445 mya) orSilurian, and became common in theDevonian period, often known as the "Age of Fishes".[34] Thebony fishes appeared in the Silurian; they became common in the Devonian.[35] By the middle of the Devonian, a lineage of bony fishes, thesarcopterygii, with both gills and air-breathing lungs adapted to life in swampy pools, used their muscular paired fins to propel themselves on land.[36] The fins, already possessing bones and joints, evolved into the two pairs of walking legs of the firsttetrapods[37] in theFamennian stage of theDevonian.[38] These tetrapods established themselves on land asamphibians in the next geological period, theCarboniferous.[39] A group of vertebrates, theamniotes, with membranes around theembryo allowing it to survive on dry land, branched from amphibious tetrapods in the Carboniferous.[40]
At the onset of the Mesozoic, all larger vertebrate groups were devastated after thelargest mass extinction in earth history. The followingrecovery phase saw the emergence of many new vertebrate groups that are still around today, and this time has been described as the origin of modern ecosystems. On the continents, the ancestors of modernlissamphibians,turtles,crocodilians,lizards, and mammals appeared, as well asdinosaurs, which gave rise to birds later in the Mesozoic. In the seas, various groups of marine reptiles evolved, as did new groups of fish.[40] At the end of the Mesozoic,another extinction event extirpated dinosaurs (other than birds) and many other vertebrate groups.[41]
TheCenozoic, the current era, is sometimes called the "Age of Mammals", because of the dominance of the terrestrial environment by that group.Placental mammals have predominantly occupied the Northern Hemisphere, withmarsupial mammals in the Southern Hemisphere.[42][43]
In 1801,Jean-Baptiste Lamarck defined the vertebrates as a taxonomic group,[2] aphylum distinct from theinvertebrates he was studying.[44] He described them as consisting of four classes, namely fish, reptiles, birds, and mammals,[45] but treated thecephalochordates andtunicates asmolluscs.[44] In 1866,Ernst Haeckel called both his Craniata (vertebrates) and his Acrania (cephalochordates) Vertebrata.[44] In 1877,Ray Lankester grouped the craniates, cephalochordates, and urochordates (tunicates) as Vertebrata.[44] In 1880–1881,Francis Maitland Balfour placed the Vertebrata as a subphylum within the chordates.[44] In 2018, Naoki Irie and colleagues proposed making Vertebrata a full phylum.[44]
In 1758,Linnaeus classified hagfishes asVermes, not vertebrates.[46]In 1806,André Marie Constant Duméril grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills (Marsupibranchii). The cyclostomes were seen as either degenerate cartilaginous fishes or primitive vertebrates.[47] In 1889,Edward Drinker Cope coined the nameAgnatha ("jawless") for a group that included the cyclostomes and fossil groups in which jaws could not be observed.[47] Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata (jawed vertebrates). In 1927,Erik Stensiö suggested that the two groups of living agnathans (i.e. the cyclostomes) arose independently from fossil agnathans.[47] In 1977,Søren Løvtrup argued that lampreys are more closely related to gnathostomes, based on characters such as radial muscles in the fins, true lymphocytes, neuromasts in the inner ear, and a cerebellum. This implied that Vertebrata and Craniata were distinct taxa.[47] The validity of the taxon "Craniata" was examined in 2002 by Delarbre et al. usingmtDNA sequencing, concluding that Myxini is more closely related toHyperoartia than to Gnathostomata - i.e., that modern jawless fishes form a clade calledCyclostomata. This implies that Vertebrata should return to its old content (Gnathostomata +Cyclostomata) and the name Craniata is a junior synonym of Vertebrata.[48] In 2010, the debate concluded when the French paleontologistPhilippe Janvier stated that he accepted that both vertebrates and cyclostomes were monophyletic, and that "the intuitions of 19th century zoologists were correct in assuming that [cyclostomes] (notably, hagfishes) are strongly degenerate and have lost many characters over time."[49]
Diversity of various groups of vertebrates through thegeologic ages. The width of the bubbles signifies the number offamilies.
Conventionalevolutionary taxonomy groupsextant vertebrates into seven classes based on traditional interpretations of grossanatomical andphysiological traits. The commonly held classification lists three classes of fish and four ofTetrapods.[50] This ignores some of the natural relationships between the groupings. For example, the birds derive from a group of reptiles, so "Reptilia" excludingAves is nota natural grouping; it is described asparaphyletic and shown in quotation marks.[51][52]
In addition to these, there are two classes of extinct armoured fishes,Placodermi andAcanthodii (paraphyly of both groups is suspected).
Other ways of classifying the vertebrates have been devised, particularly with emphasis on thephylogeny ofearly amphibians and reptiles. An example based on work by M.J. Benton in 2004[53] is given here († =extinct, "" = paraphyletic):
While this traditional taxonomy is orderly, most of the groups are paraphyletic, meaning that the structure does not accurately reflect the natural evolved grouping.[53] For instance, descendants of the first reptiles include modern reptiles, mammals and birds; the agnathans have given rise to the jawed vertebrates; thebony fishes have given rise to theland vertebrates; a group of amphibians, thelabyrinthodonts, have given rise to thereptiles (traditionally including the mammal-like synapsids), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on phylogeny, organized by their known evolutionary history.[44]
The closest relatives of vertebrates have been debated over the years. It was once thought that theCephalochordata was thesister taxon to Vertebrata. This group, Notochordata, was taken to be sister to theTunicata.[54] Since 2006, analysis has shown that the tunicates + vertebrates form a clade, the Olfactores, with Cephalochordata as its sister (theOlfactores hypothesis), as shown in the followingphylogenetic tree.[55][56][25]
The placement of hagfishes within the vertebrates has been controversial. Their lack of proper vertebrae (among other characteristics of jawless lampreys and jawed vertebrates) led authors of phylogenetic analyses based onmorphology to place them outside Vertebrata.[58]Molecular data however indicates that they are vertebrates, being most closely related to lampreys.[59][60] An older view is that they are a sister group of vertebrates in the common taxon of Craniata.[61] In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting theCyclostomata hypothesis using only morphological data.[62]
A wider issue is the position of fossil agnathans, such as the Myllokunmingiida. Tetsuto Miyashita and colleagues in 2019 place them tentatively as part of the Vertebrata total group, outside the Vertebrata crown group that led to all extant vertebrates. These fossils have a cranium (a skull of bone or cartilage) but at most a rudimentary vertebral column, so they can be viewed as part of a craniate clade that also includes the crown group vertebrates which possess a full vertebral column.[63]
Described and extant vertebrate species are split roughly evenly but non-phylogenetically between non-tetrapod "fish" andTetrapods. The following table lists the number of describedextant species for each vertebrateclass as estimated in theIUCN Red List of Threatened Species, 2014.3.[64] Paraphyletic groups are shown in quotation marks.
^Agnatha as traditionally defined is paraphyletic, that is, a taxon including all jawless fishes, but if one only includes living species then the group becomes monophyletic.
^Freeborn, Michelle (2015). Roberts, Clive Douglas; Stewart, Andrew L.; Struthers, Carl D. (eds.).The fishes of New Zealand. Vol. 2. Te Papa Press. p. 6.ISBN978-0-9941041-6-8.
^Farina, Stacy (23 August 2018)."Vertebrate Phylogeny"(PDF). Howard University. Retrieved7 December 2024.
^Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation".Invertebrates(PDF).Sinauer Associates. pp. 345–372.ISBN978-1605353753.
^abClack, J. A. (2002). "From Fins to Feet: Transformation and Transition".Gaining ground: the origin and evolution of tetrapods.Indiana University Press. pp. 187–260.
^Murdock, Duncan J. E.; Dong, Xi-Ping; Repetski, John E.; Marone, Federica; Stampanoni, Marco; et al. (2013). "The origin of conodonts and of vertebrate mineralized skeletons".Nature.502 (7472):546–549.Bibcode:2013Natur.502..546M.doi:10.1038/nature12645.PMID24132236.
^Benton, Michael J. (2019). "Acanthostega".Vertebrate Palaeontology (Kindle ed.).Wiley. p. 90.
^Encyclopædia Britannica. Vol. 17. Encyclopædia Britannica. 1954. p. 107.
^Berg, L. R.; Solomon, E. P.; Martin, D. W. (2004).Biology. Cengage Learning. p. 599.ISBN978-0-534-49276-2.
^Delarbre, Christiane; Gallut, C.; Barriel, V.; Janvier, P.; Gachelin, G.; et al. (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly".Molecular Phylogenetics and Evolution.22 (2):184–192.Bibcode:2002MolPE..22..184D.doi:10.1006/mpev.2001.1045.PMID11820840.
^Farris, James S. (1974). "Formal definitions of paraphyly and polyphyly".Systematic Zoology.23 (4):548–554.doi:10.2307/2412474.JSTOR2412474.
^Rieppel, Olivier (2005). "Monophyly, paraphyly, and natural kinds".Biology and Philosophy.20 (2–3):465–487.doi:10.1007/s10539-004-0679-z.Something had therefore to be done about the term 'Reptilia.' It could no longer be considered to designate a natural (monophyletic) group without including birds, but only to designate an artificial (non-monophyletic) group
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