Source: Wikipedia, the free encyclopedia.

Temporal range:
Diversity of vertebrates: ).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
J-B. Lamarck, 1801[2]

Ossea Batsch, 1788[2]

Vertebrates (

Chordata, with currently about 69,963 species described.[5]

Vertebrates comprise groups such as the following


polyphyletic group comprising all that lack vertebral columns, which include non-vertebrate chordates such as lancelets

The vertebrates traditionally include the

monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata.[10]


The word vertebrate derives from the

Anatomy and morphology

All vertebrates are built along the basic chordate

gut tube, with a hollow dorsal nerve cord (the neural tube, which develops into the brain and spinal cord) running more dorsal to it. The endoskeleton continues beyond the anus and often forms an elongated tail (post-anal tail).[14] All vertebrates also possess pharyngeal arches, as well as an iodine-concentrating organ called the endostyle, which develops into the thyroid
in adults.

Vertebral column

Diplodocus carnegii, showing an extreme example of the backbone
that characterizes the vertebrates.

With only one exception, the defining characteristic of all vertebrate is the

fibrocartilaginous intervertebral discs, which are derived embryonically and evolutionarily from the notochord. Hagfish
are the only extant vertebrate whose notochord persists and is not integrated/ replaced by the vertebral column.

A few vertebrates have secondarily lost this feature and retain the notochord into adulthood, such as the sturgeon[15] and coelacanth. Jawed vertebrates are typified by paired appendages (fins or limbs, which may be secondarily lost), but this trait is not required to qualify an animal as a vertebrate.


Gill arches bearing gills in a pike


oral cavity ahead of the pharynx. Research also suggests that the sixth branchial arch contributed to the formation of the vertebrate shoulder, which separated the head from the body.[17]


fishes, the larvae bear external gills, branching off from the gill arches.[18] These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs 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 tetrapods.[19]

While the more derived vertebrates lack gills, the gill arches form during

thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and, in mammals, the malleus and incus.[14]

Central nervous system


ganglia and structures such as the jaws and skull.[21][22][23] The peripheral nervous system forms when neural crest cells branch out laterally from the dorsal nerve cord and migrate together with the mesodermal somites
to innervate the various different structures that develop in the body.

The vertebrates are the only

organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalization.[14]

The rostral end of the neural tube is expanded by a thickening of the walls and expansion of the

The resulting anatomy of the central nervous system with a single hollow nerve cord dorsal to the

brain stem circumventing the foregut around each side to form a brain on the dorsal side of the mouth.[14]

Another distinct neural feature of vertebrates is the

encephalization have given vertebrates a unique advantage in developing higher neural functions such as complex motor coordination and cognition
. It also allows vertebrates to
sensorily slower and motorically clumsier with larger sizes), which are crucial for the eventual adaptive success of vertebrates in seizing dominant niches of higher trophic levels in both terrestrial and aquatic ecosystems

Molecular signatures

In addition to the morphological characteristics used to define vertebrates (i.e. the presence of a notochord, the development of a vertebral column from the notochord, a dorsal nerve cord, pharyngeal gills, a post-anal tail, etc.), molecular markers known as

The CSIs in these protein sequences are predicted to have important functionality in vertebrates.

A specific relationship between Vertebrates and

Evolutionary history

External relationships

Originally, the "Notochordata hypothesis" suggested that the

chordates, they all share the presence of a notochord
, at least during a stage of their life cycle.

The following cladogram summarizes the systematic relationships between the Olfactores (vertebrates and tunicates) and the Cephalochordata.



Amphioxiformes (lancelets) 






First vertebrates

The early vertebrate Haikouichthys

Vertebrates originated during the

Haikouella.[33] 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.[34] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.[35][page needed] A vertebrate group of uncertain phylogeny, small eel-like conodonts, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.[36]

From fish to amphibians

Acanthostega, a fish-like early labyrinthodont.

The first

tetrapods in the succeeding Carboniferous

Mesozoic vertebrates

birds, both in the Jurassic.[39] After all dinosaurs except birds went extinct by the end of the Cretaceous
, birds and mammals diversified and filled their niches.

After the Mesozoic

The Cenozoic world saw great diversification of bony fishes, amphibians, reptiles, birds and mammals.[40][41]

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 4,600 metres (15,100 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 metres (36,000 feet). Many fish varieties are the main predators in most of the world's freshwater and marine water bodies . 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

, cetaceans).


There are several ways of classifying animals.

phylogeny.[42] Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.[43]

Traditional classification

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

extant vertebrates are:[14]

In addition to these, there are two classes of extinct armoured fishes, the


Other ways of classifying the vertebrates have been devised, particularly with emphasis on the


geologic ages. The width of the bubbles signifies the diversity (number of families

While this traditional classification is orderly, most of the groups are

organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.

Phylogenetic relationships





Myxini (hagfish)


















rat fish)


(cartilaginous fishes)



bichirs, reedfish)




Actinistia (coelacanths)


Dipnoi (lungfishes)




Tetrapoda (see below)

(jawed vertebrates)

Note that, as shown in the cladogram above, the †"

Placodermi" (armoured jawed fishes) are shown to be paraphylectic groups, separated from Gnathostomes and Eugnathostomes respectively.[51][52]

Also note that

(†) groups:


salamanders, caecilians)






placental mammals)




Scaled reptiles (lizards, snakes)


crocodiles, alligatorids, gavialids)





four‑limbed vertebrates

Note that


The placement of hagfish on the vertebrate tree of life has been controversial. Their lack of proper

Cyclostomata hypothesis using only morphological data.[53]


(crown group)

Number of extant species

The number of described vertebrate species are split between

Vertebrate groups Image Class Estimated number of
described species[54][55]

so need to
in water
78 >32,900
Tetrapods amphibians 7,302 33,278

adapted to
on land
reptiles 10,711
mammals 5,513
birds 10,425
Total described species 66,178

The IUCN estimates that 1,305,075 extant invertebrate species have been described,[54] which means that less than 5% of the described animal species in the world are vertebrates.

Vertebrate species databases

The following databases maintain (more or less) up-to-date lists of vertebrate species:

Reproductive systems

Nearly all vertebrates undergo

diploid zygotes
, which develop into new individuals.


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 recessive mutations.[56] The effects of inbreeding have been studied in many vertebrate species.

In several species of fish, inbreeding was found to decrease reproductive success.[57][58][59]

Inbreeding was observed to increase juvenile mortality in 11 small animal species.[60]

A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings).[61] This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival.[62][63][64]

Incestuous matings in birds result in severe fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).[65][66][67]

Inbreeding avoidance

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. Toads and many other amphibians display

Advertisement vocalizations by males appear to serve as cues by which females recognize their kin.[68]

fertilization.[69] 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.[69]

When female sand lizards 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.[70] 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.[70] This preference may enhance the fitness of progeny by reducing inbreeding depression.


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[71] (see heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing[72] (see inbreeding avoidance).

Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit (Parus major) when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.[73]

Purple-crowned fairywren females paired with related males may undertake extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints.[67]

Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates.[74] Although both genders 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.

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.[75] Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups.


Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization.

Reproduction in

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 mode—resulting in production of WW female progeny.[76][77] The WW females are likely produced by terminal automixis.[citation needed

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 chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs only rarely, if at all.[79] 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.[citation needed



mangrove killifish (Kryptolebias marmoratus) and Kryptolebias hermaphroditus, are the only known vertebrates to self-fertilize.[80] They produce eggs and sperm by meiosis and routinely reproduce by self-fertilisation. This capacity has apparently persisted for at least several hundred thousand years.[81] Each individual hermaphrodite normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ.[82] 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.[83][84] 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 (reproductive assurance) at each generation.[83]

Population trends


See also


  1. cladistically included within Sarcopterygii.[6]


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External links