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.

[6] They move, typically by swimming, using muscles along the back, supported by a strong but flexible skeletal structure, the spine or vertebral column.

[12] The brain receives information about the world through nerves which carry signals from sense organs in the skin and body.

[22] 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.

[22] 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)[23] to the blue whale, at up to 33 m (108 ft) and weighing some 150 tonnes.

[24] Molecular markers known as conserved signature indels in protein sequences have been identified and provide distinguishing criteria for the vertebrate subphylum.

[25] 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.

[28] 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.

[29] 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.

[31] 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".

[34] 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.

[37] 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.

[38] At the onset of the Mesozoic, all larger vertebrate groups were devastated after the largest mass extinction in earth history.

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.

[42] Conventional evolutionary taxonomy groups extant vertebrates into seven classes based on traditional interpretations of gross anatomical and physiological traits.

[49][50][25] Amphioxiformes (lancelets) Tunicata (sea squirts, etc) Vertebrata The internal phylogeny of the vertebrates is shown in the below tree.

Coelacanths Lungfishes Amphibians Mammals Lepidosauria Turtles Crocodilia Dinosaurs 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.

[55] In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data.