For an explanation of similar terms, see Craniate.
|Scientific classification Vertebrata|
J-B. Lamarck, 1801
|Simplified grouping (see text)|
Vertebrates include such groups as the following:
- jawless fishes
- jawed vertebrates, which include the cartilaginous fishes (sharks, rays, and ratfish)
- tetrapods, which include amphibians, reptiles, birds and mammals
- bony fishes
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.
The populations of vertebrates have dropped in the past 50 years.
Anatomy and morphology
See also: Vertebrate anatomy
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 gut.
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is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the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).
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.
The gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior.
The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills.
These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians.
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 and incus.
Central nervous system
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.
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.
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.
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw rise in organism diversity.
The earliest known vertebrate is believed to be the mylinofulale.
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
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.
In the sea, the bony fishes became dominant.
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 Triassic Period.
After the Mesozoic
The Cenozoic world has seen great diversification of bony fishes, amphibians, reptiles, 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).
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).
There are several ways of classifying animals.
Evolutionary systematics gives an overview; phylogenetic systematics gives detail.
The two systems are thus complementary rather than opposed.
This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works.
The extant vertebrates are:
- Subphylum Vertebrata
An example based on Janvier (1981, 1997), Shu et al.
(2003), and Benton (2004) is given here:
- Subphylum Vertebrata
- Superclass Agnatha or Cephalaspidomorphi (lampreys 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)
- Superclass Tetrapoda (four-limbed vertebrates)
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; 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.
The one below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al.
† denotes an entirely extinct clade.
Number of extant species
|Vertebrate groups||Image||Class||Estimated number of
lack amniotic membrane so need to reproduce in water
have amniotic membrane adapted to reproducing on land
|Total described species||66,178|
Vertebrate species databases
The following databases maintain (more or less) up-to-date lists of vertebrate species:
Nearly all vertebrates undergo sexual reproduction.
The effects of inbreeding have been studied in many vertebrate species.
In several species of fish, inbreeding was found to decrease reproductive success.
Inbreeding was observed to increase juvenile mortality in 11 small animal species.
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.
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 kin as mates.
Advertisement vocalizations by males appear to serve as cues by which females recognize their kin.
Inbreeding avoidance mechanisms can also operate subsequent to copulation.
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 (see heterosis).
Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (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.
Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways: through dispersal and 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 (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 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 female salamander.
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.
This capacity has apparently persisted for at least several hundred thousand years.
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.
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.
The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.
Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.
The authors note that, "An average trend in population change is not an average of total numbers of animals lost."
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Vertebrate.