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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, and a cranium, or skull. The vertebrae are irregular bones, and the intervertebral discs are of fibrocartilage. 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 in the phylum Chordata. They evolved in the Cambrian period, over 500 million years ago. 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, the animals with four limbs. Vertebrates make up less than five percent of all described animal species.

Etymology

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The word 'vertebrate' derives from the Latin vertebratus ("jointed"),[4] from vertebra, "joint", in turn from Latin vertere to turn.[5]

Characteristics

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See also: Fish anatomy
Idealised vertebrate body plan, showing key characteristics[6]

Vertebrates belong to the Bilateria, a group of animals with mirror symmetrical bodies.[7] They move, typically by swimming, using muscles along the back, supported by a strong but flexible skeletal structure, the spine or vertebral column that gives the group their name.[6]

Fossilized skeleton (cast) of Diplodocus carnegii, showing an extreme example of the vertebral column that gives the vertebrates their name. The species is a tetrapod, its four legs adapting the fish-like body plan for walking on land. The specimen is 26 m (85 ft) long.

Vertebrates belong to the chordates, a phylum characterised by a notochord, a hollow nerve cord along the back, and pharyngeal gills arranged in pairs. As embryos, vertebrates still have a notochord; as adults, all but the jawless fishes have a vertebral column, made of bone or cartilage, instead.[6] Vertebrate embryos have pharyngeal arches; in adult fish, these support the gills, while in adult tetrapods they develop into other structures.[8][9]

In the embryo, a layer of cells along the back folds and fuses into a hollow neural tube.[10] This develops into the spinal cord, and at its front end, the brain.[6] The brain receives information about the world through nerves which carry signals from sense organs in the skin and body.[11] 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.[12]

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.[13] This means that (unlike in some invertebrates like annelid worms) the anus is not usually at the end of the body.[6]

Branchial arches bearing gills in a pike

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.[14][6] In jawed vertebrates, the first gill arch pair evolved into the jaws.[15] In amphibians and some primitive bony fishes, the larvae have external gills, branching off from the gill arches.[16] 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.[17] 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.[17]

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)[18] to the blue whale, at up to 33 m (108 ft) and weighing some 150 tonnes.[19]

Evolutionary history

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See also: Evolution of fish and Evolution of tetrapods

External relationships

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It was once thought that the Cephalochordata was the sister taxon to Vertebrata. This group, Notochordata, was taken to be sister to the Tunicata (the Notochordata hypothesis).[20] 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.[21][22][23]

 Chordata 
 Cephalochordata 

 Amphioxiformes (lancelets)

Olfactores

 Tunicata (sea squirts, etc)

 Craniata 

 Vertebrata

Cambrian explosion: first vertebrates

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The Cambrian Haikouichthys, 518 mya[24]

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[25] and lived about 518 million years ago.[1] These include Haikouichthys, Myllokunmingia,[25] Zhongjianichthys,[24] and probably Yunnanozoon.[26] 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.[27] 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.[28]

Paleozoic: from fish to amphibians

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Acanthostega, a Devonian labyrinthodont, c. 365 mya[29]

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".[30] The two groups of bony fishes, Actinopterygii and Sarcopterygii, evolved and became common.[31] By the middle of the Devonian, a lineage of sarcopterygii left the water,[32] establishing themselves as amphibians, terrestrial tetrapod vertebrates, in the next geological period, the Carboniferous.[33]

Mesozoic: from reptiles to mammals and birds

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Hyperodapedon, a diapsid reptile of the Triassic, c. 230 mya

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.[34]

Cenozoic: diversification

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Nahmavis, an Eocene bird, c. 50 mya

The Cenozoic world saw great diversification of ray-finned fishes, amphibians, reptiles, birds and mammals.[35][36]

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 vertebrate subphylum.[23] Specifically, five CSIs in the following proteins: protein synthesis elongation factor-2, eukaryotic translation initiation factor 3, adenosine kinase and a protein related to ubiquitin carboxyl-terminal hydrolase are exclusively shared by all vertebrates and reliably distinguish them from all other animals.[23] A specific relationship between vertebrates and tunicates is strongly supported by two CSIs found in 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.[23]

Classification

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Traditional classification

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Diversity of various groups of vertebrates through the geologic ages. The width of the bubbles signifies the number of families.

Conventional classification 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.[37]

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 Janvier (1981, 1997), Shu et al. (2003), and Benton (2004)[38] is given here († = extinct):

While this traditional classification is orderly, most of the groups are paraphyletic, meaning that the classification does not accurately reflect the natural evolved grouping.[38] 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 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 and sometimes disregarding the conventional interpretations of their anatomy and physiology.[39]

Phylogenetic relationships

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The phylogenetic tree below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al.,[40][41] and complemented (based on,[42][43] and [44]). A dagger (†) denotes an extinct clade, whereas all other clades have living descendants.

Vertebrata/
Craniata

As shown, †"Ostracodermi" (armoured jawless fishes) and †"Placodermi" (armoured jawed fishes) are paraphylectic groups, separated from gnathostomes and eugnathostomes respectively.[45][46]

The placement of hagfishes 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.[47] Molecular data, however, indicates they are vertebrates closely related to lampreys.[48][49] An older view is that they are a sister group of vertebrates in the common taxon of Craniata.[50] A study by Miyashita et al. (2019), reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data.[51]

†Myllokunmingiida

Vertebrata
(crown group)

Diversity

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Species by group

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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.[52] Paraphyletic groups are shown in quotation marks.

Vertebrate groups Image Class Estimated number of
described species[52][53] 		Group
totals[52]
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,[52] which means that less than 5% of the described animal species in the world are vertebrates.[54]

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The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.[55] Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.[56] The authors note that, "An average trend in population change is not an average of total numbers of animals lost."[56] According to WWF, this could lead to a sixth major extinction event.[57] The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species.[58]

See also

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References

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Bibliography

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