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Vertebrate

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For an explanation of similar terms, see Craniate.

Vertebrate
Temporal range:
Cambrian Stage 3–Present,
518 –0 Ma[1]
Diversity of vertebrates: Acipenser oxyrinchus (Actinopterygii), an African bush elephant (Tetrapoda), a tiger shark (Chondrichthyes) and a river lamprey (Agnatha).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Olfactores
Subphylum: Vertebrata
J-B. Lamarck, 1801[2]
Infraphyla
Synonyms

Ossea Batsch, 1788[2]

Vertebrates (/ˈvɜːrtəbrɪts, -ˌbreɪts/)[3] are animals with a backbone or spine, consisting of vertebrae and intervertebral discs. The vertebrae are irregular bones, and the intervertebral discs are of fibrocartilage. The vertebral column surrounds and protects the spinal cord. The other feature unique to vertebrates is the presence of a cranium, the bony dome of the skull.

The vertebrates make up the subphylum Vertebrata with some 65,000 species in the phylum Chordata. The vertebrates include mammals, birds, amphibians, and various classes of reptiles and fish. Classes of 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 (four limbed vertebrates).

Vertebrates make up less than five percent of all described animal species; the rest are all invertebrates, that lack a backbone.

Etymology

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The word vertebrate derives from the Latin word vertebratus, meaning jointed,[4] from vertebra meaning joint from Latin vertere to turn.[5]

Anatomy and morphology

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All vertebrates are built along the basic chordate body plan of five synapomorphies. These are a rigid axial skeleton that includes a vertebral column developed around an elastic notochord. The notochord becomes the intervertebral discs, and runs dorsally to the gut tube along the length of an animal, hence the common name of backbone. [6] The axial endoskeleton typically continues beyond the anus/cloaca to form an elongated tail.[7] Some vertebrates evolved to become tailless with only a vestigial coccyx.[citation needed] A dorsal nerve cord, which folds and fuses into a hollow neural tube during embryonic development and eventually gives rise to the brain and spinal cord, runs more dorsally to the axial endoskeleton (enclosed by protective skeletal extensions known as neural arches), with a fore-end enlargement that is contained within a distinct skeletonized braincase (hence the alternative name for vertebrates, the craniates).[citation needed] All vertebrate embryos develop transient pharyngeal arches, which in fish develop into the branchial arches that support the gills. Other vertebrate features are a jaw, hyoid and/or the middle ear ossicles.[citation needed] An iodine-concentrating organ called the endostyle, which functions as a filter feeding organ in aquatic animals has evolved into the thyroid in most vertebrates.[citation needed]

Vertebrates vary in body length ranging from the frog species Brachycephalus pulex, a Brazilian flea toad, with a minimum adult snout–vent length of 6.45 millimetres (0.254 in)[8] to the blue whale, at up to 33 m (108 ft).

Vertebral column

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Fossilized skeleton (cast) of Diplodocus carnegii, showing an extreme example of the backbone that characterizes the vertebrates.

With only one exception, the defining characteristic of all vertebrates is the vertebral column, in which the embryonic notochord found in all chordates is replaced by a segmented series of mineralized elements called vertebrae separated by fibrocartilaginous intervertebral discs, which are embryonic and evolutionary remnants of 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.[9]

Gills

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Gill arches bearing gills in a pike

Most vertebrates are aquatic and carry out gas exchange via gills. The gills are carried right behind the head, bordering the posterior margins of a series of crescentic openings from the pharynx to the outside. Each gill is supported by a cartilaginous or bony gill arch,[10] which develop embryonically from pharyngeal arches. Bony fish have three pairs of gill arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven pairs. The ancestral vertebrates likely had more arches than seven, as some of their chordate relatives have more than 50 pairs of gill opens,[7] although most, if not all, of these openings are actually involved in filter feeding rather than respiration. In jawed vertebrates, the first gill arch pair evolved into the jointed jaws and form an additional 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 as a distinct part of the body.[11]

In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.[12] These are reduced in adulthood, their respiratory function taken over by the internal gills proper in fishes and by cutaneous respiration in most amphibians. While some amphibians such as axolotl retain the external gills into adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods, who evolved lungs (which are homologous to swim bladders) to breathe air.[13]

While the more specialized terrestrial 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.[7]

Central nervous system

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The central nervous system of vertebrates is based on the embryonic dorsal nerve cord (which then flattens into a neural plate before folding and fusing over into a hollow neural tube) running along the dorsal aspect of the notochord. Of particular importance and unique to vertebrates is the presence of neural crest cells, which are progenitor cells critical to coordinating the functions of cellular components.[14] Neural crest cells migrate through the body from the dorsal nerve cord during development, initiate the formation of neuronal ganglia and various special sense organs.[15][16][17] 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.[citation needed]

The vertebrates are the only chordate group with neural cephalization, and their neural functions are centralized towards a series of enlarged clusters in the head, which give rise to a brain. A slight swelling of the anterior end of the nerve cord is found in invertebrate chordates such as lancelets (a sister subphylum known as the cephalochordates), though it lacks eyes and other complex special sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalization.[7]

The rostral end of the neural 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), which are further differentiated in the various vertebrate groups.[18] Two laterally placed retinas and optical nerves form around outgrowths from the midbrain, except in hagfish which may have secondarily lost the structures.[19][20] The forebrain is more well-developed in most tetrapods and subdivided into the telencephalon and diencephalon, while the midbrain dominates in fish and some salamanders. In vertebrates with paired appendages, especially tetrapods, a pair of secondary enlargements of the hindbrain become the cerebella, which modulate complex motor coordinations. The brain vesicles are usually bilaterally symmetrical, giving rise to the paired cerebral hemispheres in mammals.[18]

The resultant anatomy of a central nervous system arising from a single nerve cord dorsal to the gut tube, headed by a series of (typically paired) brain vesicles, is unique to vertebrates. This is in stark contrast to invertebrates with well-developed central nervous systems such as arthropods and cephalopods, which have an often ladder-like ventral nerve cord made of paired segmental ganglia on the opposite (ventral) side of the gut tube, with a split brain stem circumventing the foregut around each side to form a brain on the dorsal side of the mouth.[7] The higher functions of the vertebrate CNS are highly centralized towards the brain (particularly the forebrain), while the invertebrate CNS is significantly more decentralized with the segmental ganglia having substantial neural autonomy independent of the brain (which itself is a fused cluster of segmental ganglia from the rostral metameres).[citation needed]

Molecular signatures

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Molecular markers known as conserved signature indels (CSIs) in protein sequences have been identified and provide distinguishing criteria for the subphylum Vertebrata.[21] Specifically, 5 CSIs in the following proteins: protein synthesis elongation factor-2 (EF-2), eukaryotic translation initiation factor 3 (eIF3), adenosine kinase (AdK) and a protein related to ubiquitin carboxyl-terminal hydrolase are exclusively shared by all vertebrates and reliably distinguish them from all other metazoan.[21] A specific relationship between vertebrates and tunicates is strongly supported by two CSIs found in the proteins Rrp44 (associated with exosome complex) and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not cephalochordates, indicating vertebrates are more closely related to tunicates than cephalochordates.[21]

Evolutionary history

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See also: Evolution of fish and Evolution of tetrapods

External relationships

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Originally, the "Notochordata hypothesis" suggested that the Cephalochordata is the sister taxon to Craniata (Vertebrata). This group, called the Notochordata, was placed as sister group to the Tunicata (Urochordata). Although this was once the leading hypothesis,[22] studies since 2006 analyzing large sequencing datasets strongly support Olfactores (tunicates + vertebrates) as a monophyletic clade,[23][24][21] and the placement of Cephalochordata as sister-group to Olfactores (known as the "Olfactores hypothesis"). As chordates, they all share the presence of a notochord, at least during a stage of their life cycle.[citation needed]

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

 Chordata 
 Cephalochordata 

 Amphioxiformes (lancelets) 

Olfactores

 Tunicata/Urochordata (sea squirts, salps, larvaceans) 

 Craniata 

 Vertebrata 

First vertebrates

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The early vertebrate Haikouichthys

Vertebrates originated during the Cambrian explosion, which saw a rise in organism diversity. The earliest known vertebrates belongs to the Chengjiang biota[25] and lived about 518 million years ago.[1] These include Haikouichthys, Myllokunmingia,[25] Zhongjianichthys,[26] and probably Haikouella.[27] 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.[28] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.[29][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.[30]

From fish to amphibians

[edit]
Acanthostega, a fish-like early labyrinthodont.

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".[31] The two groups of bony fishes, the Actinopterygii and Sarcopterygii, evolved and became common.[32] The Devonian also saw the demise of virtually all jawless fishes save for lampreys and hagfishes, as well as the Placodermi, a group of armoured fish that dominated the entirety of that period since the late Silurian as well as the eurypterids, dominant animals of the preceding Silurian, and the anomalocarids. By the middle of the Devonian, several droughts, anoxic events and oceanic competition led a lineage of sarcopterygii to leave water, eventually establishing themselves as terrestrial tetrapods in the succeeding Carboniferous.[citation needed]

Mesozoic

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Amniotes branched from amphibious tetrapods early in the Carboniferous period. The synapsid amniotes were dominant during the late Paleozoic, the Permian, while diapsid amniotes became dominant during the Mesozoic. In the sea, the teleosts and sharks became dominant. Mesothermic synapsids called cynodonts gave rise to endothermic mammals and diapsids called dinosaurs eventually gave rise to endothermic birds, both in the Jurassic.[33]

After the Mesozoic

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The Cenozoic world saw great diversification of bony fishes, amphibians, reptiles, birds and mammals.[34][35]

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 the Tibetan stone loach (Triplophysa stolickai) in western Tibetan hot springs near Longmu Lake at an elevation of 5,200 metres (17,100 feet) to an unknown species of snailfish (genus Pseudoliparis) in the Izu–Ogasawara Trench at a depth of 8,336 metres (27,349 feet).[36][37] 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 snakes, penguins, cetaceans).[citation needed]

Classification

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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.[38] Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.[39]

Traditional classification

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Traditional spindle diagram of the evolution of the vertebrates at class level

Conventional classification has extant vertebrates grouped 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.[40]

In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii, both considered 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 Janvier (1981, 1997), Shu et al. (2003), and Benton (2004)[41] is given here († = extinct):

Diversity of various groups of vertebrates through 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 paraphyletic, i.e. do not contain all descendants of the class's common ancestor.[41] 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; 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,[42] organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.

Phylogenetic relationships

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In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks but illustrated as a nested "family tree" known as a phylogenetic tree. The cladogram below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al.,[43][44] and complemented (based on,[45][46] and [47]). A dagger (†) denotes an extinct clade, whereas all other clades have living descendants.

Vertebrata/
Craniata

As shown in this cladogram, the †"Ostracodermi" (armoured jawless fishes) and †"Placodermi" (armoured jawed fishes) are paraphylectic groups, separated from gnathostomes and eugnathostomes respectively.[48][49]

Teleostei (Neopterygii) and Tetrapoda (amphibians, mammals, reptiles, birds) each make up about 50% of today's vertebrate diversity, while all other groups are either extinct or rare. The next cladogram shows the extant clades of tetrapods (the four-limbed vertebrates), and a selection of extinct (†) groups:

Tetrapoda
four‑limbed vertebrates

The placement of hagfish on the vertebrate tree of life has been controversial. Their lack of proper vertebrae (among with other characteristics of lampreys and jawed vertebrates) led phylogenetic analyses based on morphology to place them outside Vertebrata.[50] Molecular data, however, indicates they are vertebrates closely related to lampreys.[51][52] An older view is that they are a sister group of vertebrates in the common taxon of Craniata.[53] A study by Miyashita et al. (2019), reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data.[54]

Number of extant species

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The number of described vertebrate species are split 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.[55]

Vertebrate groups Image Class Estimated number of
described species[55][56] 		Group
totals[55]
Anamniote

lack
amniotic
membrane
so need to
reproduce
in water 		Jawless Fish Myxini
(hagfish) 		78 >32,900
Hyperoartia
(lamprey) 		40
Jawed cartilaginous
fish 		>1,100
ray-finned
fish 		>32,000
lobe-finned
fish 		8
Tetrapods amphibians 7,302 33,278
Amniote

have
amniotic
membrane
adapted to
reproducing
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,[55] which means that less than 5% of the described animal species in the world are vertebrates.

Species databases

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

[edit]

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.[57] Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.[58] The authors note that, "An average trend in population change is not an average of total numbers of animals lost."[58] According to WWF, this could lead to a sixth major extinction event.[59] The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species.[60]

See also

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Bibliography

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