Vertebrate#Evolutionary history

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.{{cite book |last=Freeborn |first=Michelle |title=The fishes of New Zealand |year=2015 |publisher=Te Papa Press |editor-last=Roberts |editor-first=Clive Douglas |volume=2 |editor-last2=Stewart |editor-first2=Andrew L. |editor-last3=Struthers |editor-first3=Carl D. |isbn=978-0-9941041-6-8 |page=6}}

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.{{cite web |last1=Farina |first1=Stacy |title=Vertebrate Phylogeny |url=https://static1.squarespace.com/static/534f1fe6e4b06683005e62e2/t/5b7dfcdd758d4696b0e82644/1534983398205/Biol252_E1_L2_vertebrate_phylogeny_p1_sm.pdf |publisher=Howard University |access-date=7 December 2024 |date=23 August 2018}}

{{see also|Fish anatomy}}

File:Vertebrate body plan.svg

Vertebrates (and other chordates) belong to the Bilateria, a group of animals with mirror symmetrical bodies.{{cite web |title=Trends in evolution |url=https://evolution.berkeley.edu/evolibrary/article/evo_54 |publisher=University of California Museum of Paleontology |access-date=10 January 2019}} They move, typically by swimming, using muscles along the back, supported by a strong but flexible skeletal structure, the spine or vertebral column.{{cite book |last1=Romer |first1=Alfred S. |author1-link=Alfred Romer |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders |pages=161–170 |isbn=0-03-910284-X}} The name 'vertebrate' derives from the Latin {{lang|la|vertebratus}}, 'jointed',{{cite web |title=vertebrate |work=Online Etymology Dictionary |url=http://dictionary.reference.com/browse/vertebrate}} from vertebra, 'joint', in turn from Latin {{lang|la|vertere}}, 'to turn'.{{cite web |title=Definition of Vertebra |url=https://www.merriam-webster.com/dictionary/vertebra |website=Merriam-Webster |date=25 November 2024}}

File:Naturkundemuseum Berlin - Dinosaurierhalle.jpg , 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 {{cvt|26|m|ft}} long.]]

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.{{cite journal |last=Graham |first=A. |title=Development of the pharyngeal arches |journal=American Journal of Medical Genetics Part A |volume=119A |issue=3 |pages=251–256 |year=2003 |pmid=12784288 |doi=10.1002/ajmg.a.10980 |s2cid=28318053 }}{{cite journal |last=Graham |first=A. |title=The development and evolution of the pharyngeal arches |journal=Journal of Anatomy |date=July 2001 |volume=199 |issue=Pt 1-2 |pages=133–141 |doi=10.1046/j.1469-7580.2001.19910133.x |pmid=11523815|pmc=1594982 }}

In the embryo, a layer of cells along the back folds and fuses into a hollow neural tube.{{cite journal |last1=Martín-Durán |first1=José M. |last2=Pang |first2=Kevin |last3=Børve |first3=Aina |last4=Lê |first4=Henrike Semmler |last5=Furu |first5=Anlaug |last6=Cannon |first6=Johanna Taylor |last7=Jondelius |first7=Ulf |last8=Hejnol |first8=Andreas |display-authors=5 |title=Convergent evolution of bilaterian nerve cords |journal=Nature |volume=553 |issue=7686 |date=4 January 2018 |pmid=29236686 |pmc=5756474 |doi=10.1038/nature25030 |doi-access=free |pages=45–50}} 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.{{cite web |title=In brief: How does the nervous system work? |url=https://www.ncbi.nlm.nih.gov/books/NBK279390/ |website=InformedHealth.org |access-date=30 November 2024 |date=4 May 2023}} 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.{{cite book |last=Brusca |first=Richard C. |url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1605353753}}

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.{{cite journal |last=Handrigan |first=Gregory R. |title=Concordia discors: duality in the origin of the vertebrate tail |journal=Journal of Anatomy |volume=202 |issue=Pt 3 |date=2003 |pmid=12713266 |pmc=1571085 |doi=10.1046/j.1469-7580.2003.00163.x |doi-access=free |pages=255–267}}{{cite journal |last=Holland |first=Nicholas D. |last2=Holland |first2=Linda Z. |last3=Holland |first3=Peter W. H. |title=Scenarios for the making of vertebrates |journal=Nature |volume=520 |issue=7548 |date=23 April 2015 |doi=10.1038/nature14433 |pages=450–455}}{{cite journal |last=Hejnol |first=Andreas |last2=Martín-Durán |first2=José M. |title=Getting to the bottom of anal evolution |journal=Zoologischer Anzeiger - A Journal of Comparative Zoology |volume=256 |date=2015 |doi=10.1016/j.jcz.2015.02.006 |doi-access=free |pages=61–74|hdl=1956/10848 |hdl-access=free }}

File:Gills (esox).jpges 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.{{cite book |last=Scott |first=T. |title=Concise encyclopedia biology |year=1996 |publisher=De Gruyter |isbn=978-3-11-010661-9 |page=[https://archive.org/details/conciseencyclope00scot/page/542 542] |url-access=registration |url=https://archive.org/details/conciseencyclope00scot/page/542}} In jawed vertebrates, the first gill arch pair evolved into the jaws.{{cite journal |title=Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle |first1=Martin D. |last1=Brazeau |first2=Marco |last2=Castiello |first3=Amin |last3=El Fassi El Fehri |first4=Louis |last4=Hamilton |first5=Alexander O. |last5=Ivanov |first6=Zerina |last6=Johanson |first7=Matt |last7=Friedman |display-authors=5 |date=20 November 2023 |journal=Nature |volume=623 |issue=7987 |pages=550–554 |doi=10.1038/s41586-023-06702-4 |bibcode=2023Natur.623..550B |doi-access=free |pmid=37914937 |pmc=10651482 |hdl=10044/1/107350 |hdl-access=free}} In amphibians and some primitive bony fishes, the larvae have external gills, branching off from the gill arches.{{cite journal |last=Szarski |first=Henryk |journal=The American Naturalist |year=1957 |volume=91 |issue=860 |pages=283–301 |title=The Origin of the Larva and Metamorphosis in Amphibia |jstor=2458911 |doi=10.1086/281990 |s2cid=85231736 }} 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.{{cite journal |last1=Simões-Costa |first1=Marcos S. |last2=Vasconcelos |first2=Michelle |last3=Sampaio |first3=Allysson C. |last4=Cravo |first4=Roberta M. |last5=Linhares |first5=Vania L. |last6=Hochgreb |first6=Tatiana |last7=Yan |first7=Chao Y.I. |last8=Davidson |first8=Brad |last9=Xavier-Neto |first9=José |display-authors=5 |title=The evolutionary origin of cardiac chambers |year=2005 |journal=Developmental Biology |volume=277 |issue=1 |pages=1–15 |doi=10.1016/j.ydbio.2004.09.026 |pmid=15572135}} 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.{{cite book |last=Clack |first=J. A. |chapter=From Fins to Feet: Transformation and Transition |year=2002 |title=Gaining ground: the origin and evolution of tetrapods |publisher=Indiana University Press |pages=187–260}}

Vertebrates vary in size from the smallest frog species such as Brachycephalus pulex, with a minimum adult snout–vent length of {{convert|6.45|mm|in}}{{Cite journal |last1=Bolaños |first1=Wendy H. |last2=Dias |first2=Iuri Ribeiro |last3=Solé |first3=Mirco |date=2024-02-07 |title=Zooming in on amphibians: Which is the smallest vertebrate in the world? |url=https://onlinelibrary.wiley.com/doi/10.1111/zsc.12654 |journal=Zoologica Scripta |volume=53 |issue=4 |pages=414–418 |doi=10.1111/zsc.12654 |s2cid=267599475}} to the blue whale, at up to {{convert|33|m|ft|abbr=on}} and weighing some 150 tonnes.{{cite web |last1=Chamary |first1=J.V. |title=How large can animals grow? |url=https://www.discoverwildlife.com/animal-facts/how-big-or-small-could-animals-get |website=BBC Discover Wildlife |access-date=29 November 2024 |date=6 June 2024}}

Molecular markers known as conserved signature indels in protein sequences have been identified and provide distinguishing criteria for the vertebrate subphylum.{{Cite journal |last=Gupta |first=Radhey S. |date=January 2016 |title=Molecular signatures that are distinctive characteristics of the vertebrates and chordates and supporting a grouping of vertebrates with the tunicates |url=http://dx.doi.org/10.1016/j.ympev.2015.09.019 |journal=Molecular Phylogenetics and Evolution |volume=94 |issue=Pt A |pages=383–391 |doi=10.1016/j.ympev.2015.09.019 |pmid=26419477 |bibcode=2016MolPE..94..383G}} 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.

{{further|Cambrian explosion}}

File:Haikouichthys cropped.jpg Haikouichthys, 518 mya]]

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{{cite journal |last1=Shu |first1=D-G. |last2=Luo |first2=H-L. |last3=Conway Morris |first3=S. |author3-link=Simon Conway Morris |last4=Zhang |first4=X-L. |last5=Hu |first5=S-X. |last6=Chen |first6=L. |last7=Han |first7=J. |last8=Zhu |first8=M. |last9=Li |first9=Y. |last10=Chen |first10=L-Z. |display-authors=5 |title=Lower Cambrian vertebrates from south China |journal=Nature |volume=402 |issue=6757 |year=1999 |pages=42–46 |doi=10.1038/46965 |bibcode=1999Natur.402...42S |s2cid=4402854 }} and lived about 518 million years ago. These include Haikouichthys, Myllokunmingia, Zhongjianichthys,{{cite journal |last=Shu |first=D. |date=2003 |title=A paleontological perspective of vertebrate origin |journal=Chinese Science Bulletin |volume=48 |issue=8 |pages=725–735 |doi=10.1360/03wd0026}} and probably Yunnanozoon.{{cite journal |last1=Chen |first1=J.-Y. |last2=Huang |first2=D.-Y. |last3=Li |first3=C.-W. |year=1999 |title=An early Cambrian craniate-like chordate |journal=Nature |volume=402 |issue=6761|pages=518–522 |doi=10.1038/990080 |bibcode=1999Natur.402..518C |s2cid=24895681 }} 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.{{cite web |last=Waggoner |first=B. |title=Vertebrates: Fossil Record |url=http://www.ucmp.berkeley.edu/vertebrates/vertfr.html |publisher=University of California Museum of Paleontology |access-date=15 July 2011 |archive-url=https://web.archive.org/web/20110629070158/http://www.ucmp.berkeley.edu/vertebrates/vertfr.html |archive-date=29 June 2011 |url-status=dead}} 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.{{cite journal |doi=10.1111/j.1469-185X.1999.tb00045.x |last1=Donoghue |first1=P. C. J. |last2=Forey |first2=P. L. |last3=Aldridge |first3=R. J. |date=May 2000 |title=Conodont affinity and chordate phylogeny |journal=Biological Reviews |volume=75 |issue=2 |pages=191–251 |pmid=10881388 |s2cid=22803015 }} 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.{{cite journal |last=Murdock |first=Duncan J. E. |last2=Dong |first2=Xi-Ping |last3=Repetski |first3=John E. |last4=Marone |first4=Federica |last5=Stampanoni |first5=Marco |last6=Donoghue |first6=Philip C. J. |display-authors=5 |title=The origin of conodonts and of vertebrate mineralized skeletons |journal=Nature |volume=502 |issue=7472 |date=2013 |doi=10.1038/nature12645 |pages=546–549}}

{{see also|Evolution of fish|Evolution of tetrapods}}

File:Acanthostega BW.jpg, a Devonian labyrinthodont, {{circa}} 365 mya{{cite book |last=Benton |first=Michael J. |title=Vertebrate Palaeontology |date=2019 |publisher=Wiley |section=Acanthostega |page=90 |edition=Kindle}} ]]

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".{{cite book |title=Encyclopædia Britannica |volume=17 |year=1954 |publisher=Encyclopædia Britannica |page=107}} The two groups of bony fishes, Actinopterygii and Sarcopterygii, evolved and became common.{{cite book |last1=Berg |first1=L. R. |last2=Solomon |first2=E. P. |last3=Martin |first3=D. W. |title=Biology |year=2004 |publisher=Cengage Learning |isbn=978-0-534-49276-2 |page=599}} 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.{{cite book |title=Patterns of Evolution, as Illustrated by the Fossil Record |last=Carroll |first=Robert L. |editor-last=Hallam |editor-first=Anthony |editor-link=Anthony Hallam |year=1977 |publisher=Elsevier |isbn=978-0-444-41142-6 |pages=405–420 |url=https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |access-date=October 15, 2020 |archive-date=April 14, 2021 |archive-url=https://web.archive.org/web/20210414082736/https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |url-status=live }} The fins, already possessing bones and joints, evolved into two pairs of walking legs.{{cite journal |last1=Narkiewicz |first1=Katarzyna |last2=Narkiewicz |first2=Marek |title=The age of the oldest tetrapod tracks from Zachełmie, Poland |journal=Lethaia |volume=48 |issue=1 |date=January 2015 |pages=10–12 |doi=10.1111/let.12083 |bibcode=2015Letha..48...10N }} These established themselves as amphibians, terrestrial tetrapods, in the next geological period, the Carboniferous.{{cite book |url=https://books.google.com/books?id=Z0YWn5F9sWkC&pg=PA209 |title=Fins into Limbs: Evolution, Development, and Transformation |isbn=9780226313405 |access-date=2020-04-25 |archive-date=2020-08-09 |archive-url=https://web.archive.org/web/20200809023449/https://books.google.no/books?id=Z0YWn5F9sWkC&pg=PA209 |url-status=live |last=Hall |first=Brian K. |date=15 September 2008 |publisher=University of Chicago Press |page=209}} 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.{{Cite book| edition = 4th| publisher = John Wiley & Sons| isbn = 978-1-118-40764-6| last = Benton| first = Michael| title = Vertebrate Palaeontology| date = 2014 |pages=119, 148}}

File:Hyperodapedon BW2 white background.jpg, a diapsid reptile of the Triassic, {{circa}} 230 mya]]

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.{{cite book |last=Fortey |first=Richard |author-link=Richard Fortey |title=Life: A Natural History of the First Four Billion Years of Life on Earth |publisher=Vintage Books |year=1999 |pages=238–260 |isbn=978-0-375-70261-7}}

File:Fossil bird (Green River Formation, Lower Eocene; Fossil Lake Basin, southwestern Wyoming, USA) (15529177925).jpg, an Eocene bird, {{circa}} 50 mya]]

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.{{cite journal |last1=Pires |first1=Mathias |last2=Mankin |first2=Brian |last3=Silvestro |first3=Daniele |last4=Quental |first4=Tiago |date=26 September 2018 |title=Diversification dynamics of mammalian clades during the K–Pg mass extinction |journal=Biology Letters |volume=14 |issue=9 |doi=10.1098/rsbl.2018.0458 |doi-access=free |pmid=30258031 |pmc=6170748 }}{{cite journal |last1=Lowery |first1=Christopher |last2=Fraass |first2=Andrew |date=8 April 2019 |title=Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction |url=https://www.nature.com/articles/s41559-019-0835-0 |journal=Nature Ecology & Evolution |volume=3 |issue=6 |pages=900–904 |doi=10.1038/s41559-019-0835-0 |pmid=30962557 |bibcode=2019NatEE...3..900L |s2cid=102354122 |via=Nature |hdl=1983/fb08c3c1-c203-4780-bc90-5994ec1030ff |hdl-access=free }}

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,{{cite web |last=Blitz |first=David |author-link=David Blitz |title=Lamarck and Species Evolution |url=https://bertie.ccsu.edu/naturesci/evolution/unit10background/Lamarck.html |publisher=Central Connecticut State University |access-date=7 December 2024}} 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.

File:Fish evolution.png of various groups of vertebrates through the geologic ages. The width of the bubbles signifies the number of families.]]

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.{{cite book |last=Campbell |first=Neil A. |title=Biology |date=1997 |edition=4th |publisher=Benjamin Cummings |isbn=978-0-8053-1940-8 |page=632}} 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.{{cite journal |last=Farris |first=James S. |title=Formal definitions of paraphyly and polyphyly |journal=Systematic Zoology |volume=23 |issue=4 |year=1974 |pages=548-554 |jstor=2412474 |doi=10.2307/2412474}}{{cite journal |last=Rieppel |first=Olivier |title=Monophyly, paraphyly, and natural kinds |journal=Biology and Philosophy |issue=20 |year=2005 |pages=465-487 |doi=10.1007/s10539-004-0679-z |quote=Something had therefore to be done about the term 'Reptilia.' It could no longer be considered to designate a natural (monophyletic) group without including birds, but only to designate an artificial (non-monophyletic) group}}

• Subphylum Vertebrata
• Class Agnatha (jawless fishes, paraphyletic)
• Class Chondrichthyes (cartilaginous fishes)
• Class Osteichthyes (bony fishes, paraphyletic)
• Class Amphibia (traditional amphibians, paraphyletic)
• 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{{cite book |last=Benton |first=M.J. |author-link=Michael Benton |title=Vertebrate Palaeontology |publisher=Blackwell Publishing |date=1 November 2004 |edition=Third |page=33 |url=http://palaeo.gly.bris.ac.uk/benton/vertclass.html |isbn=978-0632056378 |access-date=16 March 2006 |archive-url=https://web.archive.org/web/20081019121413/http://palaeo.gly.bris.ac.uk/benton/vertclass.html |archive-date=19 October 2008 |url-status=dead }} is given here († = extinct, * = paraphyletic group):

• Subphylum Vertebrata
• Infraphylum Agnatha* (lampreys and other jawless fishes)
• Superclass {{extinct}}Anaspidomorphi (anaspids and relatives)
• Class {{extinct}}Anaspida (anaspids)
• Superclass {{extinct}}Cephalaspidomorphi (cephalaspidomorphs)
• Class {{extinct}}Osteostraci (osteostracans)
• Class {{extinct}}Galeaspida (galeaspids)
• Class {{extinct}}Pituriaspida (pituriaspids)
• Superclass Cyclostomata (cyclostomes)
• Class Myxini (hagfish)
• Class Petromyzontida (lampreys)
• Class {{extinct}}Conodonta (conodonts)
• Class {{extinct}}Pteraspidomorpha (pteraspidomorphs)
• Class {{extinct}}Thelodonti (thelodonts)
• Infraphylum Gnathostomata (vertebrates with jaws)
• Class {{extinct}}Placodermi* (extinct armoured fishes)
• Class Chondrichthyes (cartilaginous fishes)
• Class {{extinct}}Acanthodii* (extinct spiny "sharks")
• Superclass Osteichthyes* (bony fishes)
• Class Actinopterygii (ray-finned bony fishes)
• Class Sarcopterygii* (lobe-finned fishes, cladistically 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)
• Incertae sedis
• Genus {{extinct}}Nuucichthys
• Genus {{extinct}}Palaeospondylus

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.{{cite journal |last=Irie |first=Naoki |date=26 December 2018 |title=The phylum Vertebrata: a case for zoological recognition |journal=Zoological Letters |volume=4 Article Number 32 |page=32 |doi=10.1186/s40851-018-0114-y |pmid=30607258 |pmc=6307173 |doi-access=free }}

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.{{cite journal |last=Stach |first=Thomas |title=Chordate phylogeny and evolution: a not so simple three-taxon problem |journal=Journal of Zoology |date=2008 |volume=276 |issue=2 |pages=117–141 |doi=10.1111/j.1469-7998.2008.00497.x |doi-access=free }} 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.{{cite journal |last=Delsuc |first=F. |title=Tunicates and not cephalochordates are the closest living relatives of vertebrates |journal=Nature |date=2006 |volume=439 |issue=7079 |pages=965–968 |bibcode=2006Natur.439..965D |doi=10.1038/nature04336 |pmid=16495997 |s2cid=4382758 |url=https://hal.archives-ouvertes.fr/halsde-00315436/file/Delsuc-Nature06_HAL.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/halsde-00315436/file/Delsuc-Nature06_HAL.pdf |archive-date=9 October 2022 |url-status=live }}{{cite journal |last=Dunn |first=C.W. |title=Broad phylogenetic sampling improves resolution of the animal tree of life |journal=Nature |volume=452 |year=2008 |issue=7188 |doi=10.1038/nature06614 |pages=745–749 |pmid=18322464 |bibcode=2008Natur.452..745D |s2cid=4397099 }}

{{clade|style=font-size:100%

|label1=Chordata

|1={{clade

|label1=Cephalochordata

|1=Leptocardii (lancelets) 140px

|label2=Olfactores

|2={{clade

|1=Tunicata (sea squirts, etc) 90px

|2=Vertebrata 90px

}}

}}

}}

The internal phylogeny of the vertebrates is shown in the below tree.{{cite web |title=Adding time to the tree |website=Understanding Evolution |publisher=University of California Museum of Paleontology |url=https://evolution.berkeley.edu/evolution-101/the-history-of-life-looking-at-the-patterns/adding-time-to-the-tree/ |access-date=8 December 2024}}

{{clade|style=font-size:85%;line-height:85%;

|label1=Vertebrata |sublabel1=(backbone)

|1={{clade

|sublabel1=(jawless)

|1=Agnatha{{efn|Agnatha as traditionally defined is paraphyletic, that is, a taxon including all jawless fishes, but if one only includes living species then the group becomes monophyletic.}} 90 px

|label2=Gnathostomata

|sublabel2=(jawed)

|2={{clade

|label1=Chondrichthyes |sublabel1=(cartilaginous)

|1=75 px

|label2=Osteichthyes |sublabel2=(bony fishes)

|2={{clade

|1={{clade

|label1=Actinopterygii |sublabel1=(ray-fins)

|1=70px

|label2=Sarcopterygii |sublabel2=(lobe-fins)

|2={{clade

|1=Coelacanths 70 px

|label2=Rhipidistia |sublabel2=(lungs)

|2={{clade

|1=Lungfishes 75 px

|label2=Tetrapoda |sublabel2=(4 limbs)

|2={{clade

|1=Amphibians 80 px

|label2=Amniota |sublabel2=(amnion)

|2={{clade

|1=Mammals 55px

|label2=Sauropsida |sublabel2=(scaly skin)

|2={{clade

|1=Lepidosaurs 70px

|2={{clade

|1=Turtles 55px

|2={{clade

|1=Crocodilians 70px

|2=Dinosaurs 60px

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

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.{{Cite journal |last1=Ota |first1=Kinya G. |last2=Fujimoto |first2=Satoko |last3=Oisi |first3=Yasuhiro |last4=Kuratani |first4=Shigeru |date=2017-01-25 |title=Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish |journal=Nature Communications |volume=2 |pages=373 |doi=10.1038/ncomms1355|pmc=3157150 |pmid=21712821 |bibcode=2011NatCo...2..373O}} Molecular data however indicates that they are vertebrates, being most closely related to lampreys.{{cite journal |title=Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes |last1=Kuraku |date=December 1999 |journal=Journal of Molecular Evolution |volume=49 |pages=729–35 |doi=10.1007/PL00006595 |pmid=10594174 |last2=Hoshiyama |first2=D. |last3=Katoh |first3=K. |last4=Suga |first4=H. |last5=Miyata |first5=T. |issue=6 |bibcode=1999JMolE..49..729K |s2cid=5613153 }}{{cite journal |last1=Stock |first1=D. |last2=Whitt |first2=G. S. |title=Evidence from 18S ribosomal RNA sequences that lampreys and hagfish form a natural group |journal=Science |date=7 August 1992 |volume=257 |issue=5071 |doi=10.1126/science.1496398 |pages=787–789 |pmid=1496398 |bibcode=1992Sci...257..787S }} An older view is that they are a sister group of vertebrates in the common taxon of Craniata.{{cite journal |last=Nicholls |first=H. |title=Mouth to Mouth |date=10 September 2009 |journal=Nature |volume=461 |issue=7261 |pages=164–166 |doi=10.1038/461164a |pmid=19741680 |doi-access=free}} In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data.{{Cite journal |last1=Miyashita |first1=Tetsuto |last2=Coates |first2=Michael I. |last3=Farrar |first3=Robert |last4=Larson |first4=Peter |last5=Manning |first5=Phillip L.|last6=Wogelius |first6=Roy A.|last7=Edwards |first7=Nicholas P.|last8=Anné |first8=Jennifer |last9=Bergmann |first9=Uwe |last10=Palmer |first10=A. Richard |last11=Currie |first11=Philip J. |display-authors=5 |date=2019-02-05 |title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=116 |issue=6 |pages=2146–2151 |doi=10.1073/pnas.1814794116 |pmc=6369785 |pmid=30670644 |bibcode=2019PNAS..116.2146M |doi-access=free}}

{{clade|style=font-size:85%;line-height:85%;

|1={{clade

|1=†Myllokunmingiida55px

|label2= Vertebrata |sublabel2=(crown group)

|2={{clade

|1={{clade

|1=†Anaspida 50 px

|2={{clade

|1=†Pipiscius

|2={{clade

|1=†Conodonta90px

|2=Cyclostomata (lampreys and hagfishes) 90 px

}}

}}

}}

|2=Gnathostomata (jawed vertebrates) 70 px

}}

}}

}}

=== 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.The World Conservation Union. 2014. IUCN Red List of Threatened Species, 2014.3. Summary Statistics for Globally Threatened Species. [http://cmsdocs.s3.amazonaws.com/summarystats/2014_3_Summary_Stats_Page_Documents/2014_3_RL_Stats_Table_1.pdf Table 1: Numbers of threatened species by major groups of organisms (1996–2014)]. Paraphyletic groups are shown in quotation marks.

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.{{cite journal |last=Zhang |first=Zhi-Qiang |title=Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) |journal=Zootaxa |volume=3703 |issue=1 |date=30 August 2013 |doi=10.11646/zootaxa.3703.1.3 |url=https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |page=5 |access-date=2 March 2018 |archive-url=https://web.archive.org/web/20190424154926/https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |archive-date=24 April 2019 |url-status=live |doi-access=free }}

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.{{Cite web |title=Living Planet Report 2018 |url=https://wwf.panda.org/knowledge_hub/all_publications/living_planet_report_2018/ |website=wwf.panda.org |access-date=2020-05-21}} Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.{{Cite book |url=https://s3.amazonaws.com/wwfassets/downloads/lpr2018_full_report_spreads.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://s3.amazonaws.com/wwfassets/downloads/lpr2018_full_report_spreads.pdf |archive-date=2022-10-09 |url-status=live |title=Living Planet Report – 2018: Aiming Higher |year=2018 |isbn=978-2-940529-90-2 |last1=Grooten |first1=M. |last2=Almond |first2=R. E. A. |publisher=World Wide Fund for Nature }} 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.{{Cite magazine |title=WWF Finds Human Activity Is Decimating Wildlife Populations |url=https://time.com/5438605/human-activity-wildlife-populations-wwf-report/ |magazine=Time |access-date=2020-05-21}} The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species.{{Cite book |author=IPBES |date=2019-11-25 |title=Summary for policymakers of the global assessment report on biodiversity and ecosystem services |editor1=S. Diaz |editor2=J. Settele |editor3=E.S. Brondízio |editor4=H.T. Ngo |editor5=M. Guèze |editor6=J. Agard |editor7=A. Arneth |editor8=P. Balvanera |editor9=K.A. Brauman |editor10=S.H.M. Butchart |editor11=K.M.A. Chan |editor12=L.A. Garibaldi |editor13=K. Ichii |editor14=J. Liu |editor15=S.M. Subramanian |editor16=G.F. Midgley |editor17=P. Miloslavich |editor18=Z. Molnár |editor19=D. Obura |editor20=A. Pfaff |editor21=S. Polasky |editor22=A. Purvis |editor23=J. Razzaque |editor24=B. Reyers |editor25=R. Roy Chowdhury |editor26=Y.J. Shin |editor27=I.J. Visseren-Hamakers |editor28=K.J. Willis |editor29=C.N. Zay |display-editors=5 |publisher=IPBES Secretariat |location=Bonn |pages=1–56 |url=https://zenodo.org/record/3553579 |doi=10.5281/zenodo.3553579}}

{{notelist}}

• {{annotated link|Marine vertebrate}}
• {{annotated link|Taxonomy of the vertebrates (Young, 1962)}}

{{reflist}}

• {{cite book |last=Kardong |first=Kenneth V. |title=Vertebrates: Comparative Anatomy, Function, Evolution |edition=second |publisher=McGraw-Hill |year=1998 |location=USA |pages=747 pp

|isbn=978-0-697-28654-3 |ref=none}}

• {{ITIS |id=331030 |taxon=Vertebrata |access-date=6 August 2007}}

{{Wikispecies|Vertebrata}}

• [http://tolweb.org/Vertebrata/14829 Tree of Life]
• [http://www.nature.com/nature/journal/v439/n7079/abs/nature04336.html Tunicates and not cephalochordates are the closest living relatives of vertebrates]
• [http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Vertebrate Pests] chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
• [https://web.archive.org/web/20180421094527/http://logic-law.com/index.php?title=Subphylum_Vertebrata The Vertebrates]
• [https://web.archive.org/web/20131103084123/http://www.ibiology.org/ibioseminars/evolution-ecology/marc-w-kirschner-part-1.html The Origin of Vertebrates] Marc W. Kirschner, iBioSeminars, 2008.

{{Animalia}}

{{Chordata}}

{{Portal bar|Animals}}

{{Taxonbar|from=Q25241}}

{{Authority control}}

*

Category:Terreneuvian first appearances

Category:Extant Cambrian first appearances