Fire salamander (Amphibia), saltwater crocodile (Reptilia), southern
cassowary (Aves), black-and-rufous giant elephant shrew (Mammalia),
ocean sunfish (Osteichthyes)
J-B. Lamarck, 1801
Simplified grouping (see text)
Fishes (cladistically including the Tetrapods)
Ossea Batsch, 1788
Vertebrates /ˈvɜːrtɪbrɪts/ comprise all species of animals within
the subphylum Vertebrata /-eɪ/ (chordates with backbones).
Vertebrates represent the overwhelming majority of the phylum
Chordata, with currently about 66,000 species described.
Vertebrates include the jawless fish and the jawed vertebrates, which
include the cartilaginous fish (sharks, rays, and ratfish) and the
A bony fish clade known as the lobe-finned fishes is included with
tetrapods, which are further divided into amphibians, reptiles,
mammals, and birds. Extant vertebrates range in size from the frog
species Paedophryne amauensis, at as little as 7.7 mm
(0.30 in), to the blue whale, at up to 33 m (108 ft).
Vertebrates make up less than five percent of all described animal
species; the rest are invertebrates, which lack vertebral columns.
The vertebrates traditionally include the hagfish, which do not have
proper vertebrae due to their loss in evolution, though their
closest living relatives, the lampreys, do.
Hagfish do, however,
possess a cranium. For this reason, the vertebrate subphylum is
sometimes referred to as "Craniata" when discussing morphology.
Molecular analysis since 1992 has suggested that hagfish are most
closely related to lampreys, and so also are vertebrates in a
monophyletic sense. Others consider them a sister group of vertebrates
in the common taxon of craniata.
Anatomy and morphology
2.1 Vertebral column
2.3 Central nervous system
3 Evolutionary history
3.1 First vertebrates
3.2 From fish to amphibians
3.4 After the Mesozoic
4.1 Traditional classification
4.2 Phylogenetic relationships
5 Number of extant species
Vertebrate species databases
6 Reproductive systems
7 See also
10 External links
The word origin of vertebrate derives from the
Latin word vertebratus
(Pliny), meaning joint of the spine. The Proto-Indo-European
language origins are still unclear.
Vertebrate is derived from the word vertebra, which refers to any of
the bones or segments of the spinal column.
Anatomy and morphology
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), with a hollow tube of nervous tissue (the spinal cord)
above it and the gastrointestinal tract below.
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
The defining characteristic of a vertebrate is the vertebral column,
in which the notochord (a stiff rod of uniform composition) found in
all 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).
However, a few vertebrates have secondarily lost this anatomy,
retaining the notochord into adulthood, such as the sturgeon and
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.
Fossilized skeleton of Diplodocus carnegii, showing an extreme example
of the backbone that characterizes the vertebrates.
Gill arches bearing gills in a pike
All 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. 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 chordate relatives have
more than 50 pairs of gills.
In amphibians and some primitive bony fishes, the larvae bear external
gills, branching off from the gill arches. 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
While the more derived vertebrates lack gills, the gill arches form
during fetal development, and form the basis of essential structures
such as jaws, the thyroid gland, the larynx, the columella
(corresponding to the stapes in mammals) and, in mammals, the malleus
Central nervous system
The central nervous system of vertebrates is based on a hollow 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 cells, and critical to coordinating the
functions of cellular components.
Neural crest cells migrate
through the body from the nerve cord during development, and initiate
the formation of neural ganglia and structures such as the jaws and
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 organs
comparable to those of vertebrates. Other chordates do not show any
trends towards cephalisation.
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. Two laterally placed eyes form around
outgrowths from the midbrain, except in hagfish, though this may be a
secondary loss. The forebrain is well developed and subdivided
in most tetrapods, while the midbrain dominates in many fish and some
salamanders. Vesicles of the forebrain are usually paired, giving rise
to hemispheres like the cerebral hemispheres in mammals.
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 squids, have a ventral rather than dorsal
system of ganglions, with a split brain stem running on each side of
the mouth or gut.
Evolution of fish
Evolution of fish and
Evolution of tetrapods
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. 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. All of these early vertebrates lacked
jaws in the common sense and relied on filter feeding close to the
seabed. 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.
From fish to amphibians
Acanthostega, a fish-like early labyrinthodont.
The first jawed vertebrates appeared in the latest
became common in the Devonian, often known as the "Age of Fishes".
The two groups of bony fishes, the actinopterygii and sarcopterygii,
evolved and became common. 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 labyrinthodonts, which was a transitional form
between fishes and amphibians.
Amniotes branched from labyrinthodonts in the subsequent Carboniferous
Parareptilia and synapsid amniotes were common during the
late Paleozoic, while diapsids became dominant during the Mesozoic. In
the sea, the bony fishes became dominant; the birds, a derived form of
dinosaurs, evolved in the Jurassic. The demise of the non-avian
dinosaurs at the end of the
Cretaceous allowed for the expansion of
mammals, which had evolved from the therapsids, a group of synapsid
amniotes, during the late
After the Mesozoic
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 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).
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).
There are several ways of classifying animals. Evolutionary
systematics relies on anatomy, physiology and evolutionary history,
which is determined through similarities in anatomy and, if possible,
the genetics of organisms. Phylogenetic classification is based
solely on phylogeny. Evolutionary systematics gives an overview;
phylogenetic systematics gives detail. The two systems are thus
complementary rather than opposed.
Traditional spindle diagram of the evolution of the vertebrates at
Conventional classification has living vertebrates grouped into seven
classes based on traditional interpretations of gross anatomical and
physiological traits. This classification is the one most commonly
encountered in school textbooks, overviews, non-specialist, and
popular works. The extant vertebrates are:
Agnatha (jawless fishes)
Chondrichthyes (cartilaginous fishes)
Osteichthyes (bony fishes)
In addition to these, there are two classes of extinct armoured
Placodermi and the Acanthodii.
Other ways of classifying the vertebrates have been devised,
particularly with emphasis on the phylogeny of early amphibians and
reptiles. An example based on Janvier (1981, 1997), Shu et al. (2003),
and Benton (2004) is given here:
Cephalaspidomorphi (lampreys and other jawless
Gnathostomata (vertebrates with jaws)
Placodermi (extinct armoured fishes)
Chondrichthyes (cartilaginous fishes)
Acanthodii (extinct spiny "sharks")
Osteichthyes (bony vertebrates)
Actinopterygii (ray-finned bony fishes)
Sarcopterygii (lobe-finned fishes, tetrapods are inside this
Amphibia (amphibians, some ancestral to the amniotes)- now a
Synapsida (mammals are placed inside this thought to be extinct
taxon; this group includes mammals and the extinct mammal-like
Sauropsida (reptiles, birds are inside this group in a
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. 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)
taxa, meaning they do not include all the descendants of the first
representative of the group. For example, the agnathans have given
rise to the jawed vertebrates; the bony fishes have given rise to the
land vertebrates; the traditional "amphibians" have given rise to the
reptiles (traditionally including the synapsids, 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, organized by their known
evolutionary history and sometimes disregarding the conventional
interpretations of their anatomy and physiology.
In phylogenetic taxonomy, the relationships between animals are not
typically divided into ranks, but illustrated as a nested "family
tree" known as a cladogram. Phylogenetic groups are given definitions
based on their relationship to one another, rather than purely on
physical traits, such as the presence of a backbone. This nesting
pattern is often combined with traditional taxonomy (as above), in a
practice known as evolutionary taxonomy.
The cladogram presented below is based on studies compiled by Philippe
Janvier and others for the Tree of Life Web Project.
Placodermi (armoured fishes)
Chondrichthyes (cartilaginous fishes)
Actinopterygii (ray-finned fishes)
Sarcopterygii (lobe-finned fish)
Number of extant species
The number of described vertebrate species are split evenly between
tetrapods and fish. The following table lists the number of described
extant species for each vertebrate class as estimated in the IUCN Red
List of Threatened Species, 2014.3.
Estimated number of
so need to
Total described species
The IUCN estimates that 1,305,075 extant invertebrate species have
been described, 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
Mammal species of the World
Nearly all vertebrates undergo sexual reproduction. They produce
haploid gametes by meiosis. The smaller, motile gametes are
spermatozoa and the larger, non-motile gametes are ova. These fuse by
the process of fertilisation to form 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. The effects of inbreeding have been studied in many
In several species of fish, inbreeding was found to decrease
Inbreeding was observed to increase juvenile mortality in 11 small
A common breeding practice for pet dogs is mating between close
relatives (e.g. between half- and full siblings). This practice
generally has a negative effect on measures of reproductive success,
including decreased litter size and puppy survival.
Incestuous matings in birds result in severe fitness costs due to
inbreeding depression (e.g. reduction in hatchability of eggs and
reduced progeny survival).
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 breeding site fidelity.
Individuals that return to natal ponds to breed will likely encounter
siblings as potential mates. Although incest is possible, Bufo
americanus siblings rarely mate. 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
Inbreeding avoidance mechanisms can also operate subsequent to
copulation. In guppies, a post-copulatory mechanism of inbreeding
avoidance occurs based on competition between sperm of rival males for
achieving fertilization. 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.
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. 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. 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 (and see Heterosis). Vertebrates
have evolved numerous diverse mechanisms for avoiding close inbreeding
and promoting outcrossing (and see
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
The purple-crowned fairywren females paired with related males may
undertake 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.
Southern pied babblers (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. 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.
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. 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 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 Colombian Rainbow boa
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. The WW females are likely
produced by terminal automixis.
Mole salamanders are an ancient (2.4-3.8 million year-old) unisexual
vertebrate lineage. 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
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. 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.
The mangrove killifish (Kryptolebias marmoratus) produces both eggs
and sperm by meiosis and routinely reproduces by self-fertilisation.
The capacity for selfing in these fishes has apparently persisted for
at least several hundred thousand years. 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. 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. 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.
Skeletal system of the horse
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Wikispecies has information related to Vertebrata
Tree of Life
Tunicates and not cephalochordates are the closest living relatives of
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
The Origin of
Vertebrates Marc W. Kirschner, iBioSeminars, 2008.
Ctenophora (comb jellies)
Cnidaria (jellyfish and relatives)
craniates / vertebrates
Echinodermata (starfish and relatives)
Kinorhyncha (mud dragons)
Priapulida (penis worms)
Nematomorpha (horsehair worms)
Onychophora (velvet worms)
Chaetognatha (arrow worms)
Gnathostomulida (jaw worms)
Dicyemida or Rhombozoa
Annelida (ringed worms)
Nemertea (ribbon worms)
Entoprocta or Kamptozoa
Ectoprocta (moss animals)
Brachiopoda (lamp shells)
Phoronida (horseshoe worms)
Anthozoa inc. corals
Medusozoa inc. jellyfish
Asterozoa inc. starfish
Phyla with ≥5000 extant species bolded
Monoblastozoa (nomen dubium)
Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class
³traditionally placed in Anapsida
italic are paraphyletic groups
Fauna Europaea: 16752
BNF: cb11974791b (d