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Vertebrates () are 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 ranked grouping in the phylum Chordata. The vertebrates include , , , and various classes of fish and . The fish include the jawless Agnatha, and the jawed Gnathostomata. The jawed fish include both the Chondrichthyes and the Osteichthyes. Bony fish include the Sarcopterygii, which gave rise to the , 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, and appeared, the latter giving rise to 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 , climate change, pollution and the impact of invasive species.
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.
As embryos, vertebrates still have a notochord; as adults, all but the Agnatha have a vertebral column, made of bone or cartilage, instead. Vertebrate embryos have ; in adult fish, these support the , while in adult they develop into other structures.
In the embryo, a neural plate along the back neurulation 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 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 animal 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 . In , the first gill arch pair evolved into the jaws. In 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 have lost the gills of their fish ancestors; they have adapted the swim bladder (that fish use for buoyancy) into to breathe air, and the circulatory system is adapted accordingly. At the same time, they adapted the bony fins of the Sarcopterygii into two pairs of walking , 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 to the blue whale, at up to and weighing some 150 tonnes.
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 labyrinthodontia and reptiles. An example based on work by M.J. Benton in 2004 is given here († = extinct):
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 Osteichthyes have given rise to the tetrapoda; a group of amphibians, the , have given rise to the Reptilia (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.
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 authors of 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.
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.
Molecular
known as conserved signature indels in 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 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 , are known from 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 Gnathostomata may have appeared in the late Ordovician (~445 mya) and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fishes, Actinopterygii and Sarcopterygii, evolved and became common. (2025). 9780534492762, Cengage Learning. ISBN 9780534492762 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 , terrestrial , in the next geological period, the Carboniferous. A group of vertebrates, the , 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 Triassic 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 , , , , and mammals appeared, as well as , 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 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 he was studying. He described them as consisting of four classes, namely fish, reptiles, birds, and mammals, but treated the and as Mollusca. 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 taxon vertebrates into seven classes based on traditional interpretations of gross anatomy and Physiology traits. The commonly held classification lists three classes of fish and four of . This ignores some of the natural relationships between the groupings. For example, the birds derive from a group of reptiles, so "Reptile" excluding "Aves" is not Clade; it is described as paraphyletic.
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.
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 taxa species for each vertebrate class as estimated in the IUCN Red List of Threatened Species, 2014.3.The World Conservation Union. 2014. IUCN Red List of Threatened Species, 2014.3. Summary Statistics for Globally Threatened Species. Table 1: Numbers of threatened species by major groups of organisms (1996–2014). Paraphyletic groups are shown in quotation marks.
Hyperoartia
(lampreys)40 Actinopterygii >32,000 "Sarcopterygii" 8 5,513 bird
(birds) 10,425 Total described species 66,178
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 , climate change, pollution and invasive species.
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