# Vertebrate

Vertebrates (/ˈvɜːrtəbrɪts, -ˌbreɪts/) are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebrata with some 65,000 species, by far the largest grouping in the phylum Chordata. The vertebrates include mammals, birds, amphibians, and various classes of fish and reptiles. The fish include the jawless Agnatha, and the jawed Gnathostomata. The jawed fish include both the cartilaginous fish and the bony fish. Bony fish include the lobe-finned fish, which gave rise to the tetrapods, the animals with four limbs. Despite their success, vertebrates still only make up less than five percent of all described animal species.
The first vertebrates appeared in the Cambrian explosion some 518 million years ago. Jawed vertebrates evolved in the Ordovician, followed by bony fishes in the Devonian. The first amphibians appeared on land in the Carboniferous. During the Triassic, mammals and dinosaurs appeared, the latter giving rise to birds in the Jurassic. Extant species are roughly equally divided between fishes of all kinds, and tetrapods. Populations of many species have been in steep decline since 1970 because of land-use change, overexploitation of natural resources, climate change, pollution and the impact of invasive species.

## Characteristics
### Unique features

Vertebrates belong to Chordata, a phylum characterised by five synapomorphies (unique characteristics): namely a notochord, a hollow nerve cord along the back, an endostyle (often as a thyroid gland), and pharyngeal gills arranged in pairs. Vertebrates share these characteristics with other chordates.
Vertebrates are distinguished from all other animals, including other chordates, by multiple synapomorphies: namely the vertebral column, skull of bone or cartilage, large brain divided into 3 or more sections, a muscular heart with multiple chambers; an inner ear with semicircular canals; sense organs including eyes, ears, and nose; and digestive organs including intestine, liver, pancreas, and stomach.

### Physical

Vertebrates (and other chordates) belong to the Bilateria, a group of animals with mirror symmetrical bodies. They move, typically by swimming, using muscles along the back, supported by a strong but flexible skeletal structure, the spine or vertebral column. The name 'vertebrate' derives from the Latin vertebratus, 'jointed', from vertebra, 'joint', in turn from Latin vertere, 'to turn'.
As embryos, vertebrates still have a notochord; as adults, all but the jawless fishes have a vertebral column, made of bone or cartilage, instead. Vertebrate embryos have pharyngeal arches; in adult fish, these support the gills, while in adult tetrapods they develop into other structures.
In the embryo, a layer of cells along the back folds and fuses into a hollow neural tube. This develops into the spinal cord, and at its front end, the brain. The brain receives information about the world through nerves which carry signals from sense organs in the skin and body. Because the ancestors of vertebrates usually moved forwards, the front of the body encountered stimuli before the rest of the body, favouring cephalisation, the evolution of a head containing sense organs and a brain to process the sensory information.
Vertebrates have a tubular gut that extends from the mouth to the anus. The vertebral column typically continues beyond the anus to form an elongated tail.
The ancestral vertebrates, and most extant species, are aquatic and carry out gas 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 bony branchial arches. In jawed vertebrates, the first gill arch pair evolved into the jaws. In amphibians and some primitive bony fishes, the larvae have external gills, branching off from the gill arches. Oxygen is carried from the gills to the body in the blood, and carbon dioxide is returned to the gills, in a closed circulatory system driven by a chambered heart. The tetrapods have lost the gills of their fish ancestors; they have adapted the swim bladder (that fish use for buoyancy) into lungs to breathe air, and the circulatory system is adapted accordingly. At the same time, they adapted the bony fins of the lobe-finned fishes into two pairs of walking legs, carrying the weight of the body via the shoulder and pelvic girdles.
Vertebrates vary in size from the smallest frog species such as Brachycephalus pulex, with a minimum adult snout–vent length of 6.45 millimetres (0.254 in) to the blue whale, at up to 33 m (108 ft) and weighing some 150 tonnes.

### Molecular

Molecular markers known as conserved signature indels in protein sequences have been identified and provide distinguishing criteria for the vertebrate subphylum. Five molecular markers are exclusively shared by all vertebrates and reliably distinguish them from all other animals; these include protein synthesis elongation factor-2, eukaryotic translation initiation factor 3, adenosine kinase and a protein related to ubiquitin carboxyl-terminal hydrolase). A specific relationship between vertebrates and tunicates is supported by two molecular markers, the proteins Rrp44 (associated with the exosome complex) and serine C-palmitoyltransferase. These are exclusively shared by species from these two subphyla, but not by cephalochordates.

## Evolutionary history
### Cambrian explosion: first vertebrates

Vertebrates originated during the Cambrian explosion at the start of the Paleozoic, which saw a rise in animal diversity. The earliest known vertebrates belong to the Chengjiang biota and lived about 518 million years ago. These include Haikouichthys, Myllokunmingia, Zhongjianichthys, and probably Yunnanozoon. 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. 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. Zoologists have debated whether teeth mineralized 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.

### Paleozoic: from fish to amphibians

The first jawed vertebrates may have appeared in the late Ordovician (~445 mya) and became common in the Devonian period, often known as the "Age of Fishes". The two groups of bony fishes, Actinopterygii and Sarcopterygii, evolved and became common. By the middle of the Devonian, a lineage of sarcopterygii with both gills and air-breathing lungs adapted to life in swampy pools used their muscular paired fins to propel themselves on land. The fins, already possessing bones and joints, evolved into two pairs of walking legs. These established themselves as amphibians, terrestrial tetrapods, in the next geological period, the Carboniferous. A group of vertebrates, the amniotes, with membranes around the embryo allowing it to survive on dry land, branched from amphibious tetrapods in the Carboniferous.

### Mesozoic: from reptiles to mammals and birds

At the onset of the Mesozoic, all larger vertebrate groups were devastated after the largest mass extinction in earth history. The following recovery 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 modern lissamphibians, turtles, crocodilians, lizards, and mammals appeared, as well as dinosaurs, which gave rise to birds later in the Mesozoic. In the seas, various groups of marine reptiles evolved, as did new groups of fish. At the end of the Mesozoic, another extinction event extirpated dinosaurs (other than birds) and many other vertebrate groups.

### Cenozoic: Age of Mammals

The Cenozoic, 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, with marsupial mammals in the Southern Hemisphere.

## Approaches to classification
### Taxonomic history

In 1811, Jean-Baptiste Lamarck defined the vertebrates as a taxonomic group, a phylum distinct from the invertebrates he was studying. He described them as consisting of four classes, namely fish, reptiles, birds, and mammals, but treated the cephalochordates and tunicates as molluscs. In 1866, Ernst Haeckel called both his "Craniata" (vertebrates) and his "Acrania" (cephalochordates) "Vertebrata". In 1877, Ray Lankester grouped the Craniates, cephalochordates, and "Urochordates (tunicates) as "Vertebrata". In 1880–1881, Francis Maitland Balfour placed the Vertebrata as a subphylum within the Chordates. In 2018, Naoki Irie and colleagues proposed making Vertebrata a full phylum.

### Traditional taxonomy

Conventional evolutionary taxonomy groups extant vertebrates into seven classes based on traditional interpretations of gross anatomical and physiological traits. The commonly held classification lists three classes of fish and four of tetrapods. This ignores some of the natural relationships between the groupings. For example, the birds derive from a group of reptiles, so "Reptilia" excluding "Aves" is not a natural grouping; it is described as paraphyletic.
* Subphylum Vertebrata
** Class Agnatha (jawless fishes, paraphyletic)
** Class Chondrichthyes (cartilaginous fishes)
** Class Osteichthyes (bony fishes, paraphyletic)
** Class Amphibia (amphibians)
** Class Reptilia (reptiles, paraphyletic)
** Class Aves (birds)
** Class Mammalia (mammals)
In addition to these, there are two classes of extinct armoured fishes, Placodermi and Acanthodii, both paraphyletic.
Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. An example based on work by M.J. Benton in 2004 is given here († = extinct):
* Subphylum Vertebrata
** †Palaeospondylus
** Infraphylum Agnatha or Cephalaspidomorphi (lampreys and other jawless fishes)
*** Superclass †Anaspidomorphi (anaspids and relatives)
** Infraphylum Gnathostomata (vertebrates with jaws)
*** Class †Placodermi (extinct armoured fishes)
*** Class Chondrichthyes (cartilaginous fishes)
*** Class †Acanthodii (extinct spiny "sharks")
*** Superclass Osteichthyes (bony fishes)
**** Class Actinopterygii (ray-finned bony fishes)
**** Class Sarcopterygii (lobe-finned fishes, including the tetrapods)
*** Superclass Tetrapoda (four-limbed vertebrates)
**** Class Amphibia (amphibians, some ancestral to the amniotes)—now a paraphyletic group
**** Class Synapsida (mammals and their extinct relatives)
**** Class Sauropsida (reptiles and birds)
While this traditional taxonomy is orderly, most of the groups are paraphyletic, meaning that the structure does not accurately reflect the natural evolved grouping. For instance, descendants of the first reptiles include modern reptiles, mammals and birds; the agnathans have given rise to the jawed vertebrates; the bony fishes have given rise to the land vertebrates; a group of amphibians, the labyrinthodonts, have given rise to the reptiles (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.

### External phylogeny

The closest relatives of vertebrates have been debated over the years. It was once thought that the Cephalochordata was the sister taxon to Vertebrata. This group, Notochordata, was taken to be sister to the Tunicata. Since 2006, analysis has shown that the tunicates + vertebrates form a clade, the Olfactores, with Cephalochordata as its sister (the Olfactores hypothesis), as shown in the following phylogenetic tree.

### Internal phylogeny

The internal phylogeny of the vertebrates is shown in the below tree.
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 phylogenetic analyses based on morphology to place them outside Vertebrata. Molecular data however indicates that they are vertebrates, being most closely related to lampreys. An older view is that they are a sister group of vertebrates in the common taxon of Craniata. In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data.

## Diversity
### Species by group

Described and extant vertebrate species are split roughly evenly but non-phylogenetically between non-tetrapod "fish" and tetrapods. 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. Paraphyletic groups are shown in quotation marks.
| Vertebrate groups | Vertebrate groups | Vertebrate groups | Image | Class | Estimated number of described species | Group totals |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawless | "Fish" | | Myxini (hagfish) | 78 | >32,900 |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawless | "Fish" | | Hyperoartia (lampreys) | 40 | >32,900 |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawed | "Fish" | | Chondrichthyes | >1,100 | >32,900 |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawed | "Fish" | | Actinopterygii | >32,000 | >32,900 |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawed | "Fish" | | "Sarcopterygii" | 8 | >32,900 |
| Anamniote lack amniotic membrane so need to reproduce in water | Jawed | Tetrapods | | Amphibia | 7,302 | 33,278 |
| Amniote have amniotic membrane adapted to reproducing on land | Jawed | Tetrapods | | "Reptilia" | 10,711 | 33,278 |
| Amniote have amniotic membrane adapted to reproducing on land | Jawed | Tetrapods | | Mammalia | 5,513 | 33,278 |
| Amniote have amniotic membrane adapted to reproducing on land | Jawed | Tetrapods | | Aves (birds) | 10,425 | 33,278 |
| Total described species | Total described species | Total described species | Total described species | Total described species | Total described species | 66,178 |
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.

### Population trends

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." According to WWF, this could lead to a sixth major extinction event. The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species.