Editing Vertebrate (section)

== Approaches to classification ==

=== Taxonomic history ===

In 1811, [[Jean-Baptiste Lamarck]] defined the vertebrates as a taxonomic group,<ref name="Nielsen2012"/> a [[phylum]] distinct from the [[invertebrate]]s he was studying.<ref name="Irie 2018"/> He described them as consisting of four classes, namely fish, reptiles, birds, and mammals,<ref>{{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}}</ref> but treated the [[cephalochordate]]s and [[tunicate]]s as [[Mollusca|mollusc]]s.<ref name="Irie 2018"/> In 1866, [[Ernst Haeckel]] called both his "Craniata" (vertebrates) and his "Acrania" (cephalochordates) "Vertebrata".<ref name="Irie 2018"/> In 1877, [[Ray Lankester]] grouped the Craniates, cephalochordates, and "Urochordates (tunicates) as "Vertebrata".<ref name="Irie 2018"/> In 1880–1881, [[Francis Maitland Balfour]] placed the Vertebrata as a subphylum within the Chordates.<ref name="Irie 2018"/> In 2018, Naoki Irie and colleagues proposed making Vertebrata a full phylum.<ref name="Irie 2018"/>

=== Traditional taxonomy ===

[[File:Fish evolution.png|thumb|upright=1.35|[[Biodiversity|Diversity]] of various groups of vertebrates through the [[geologic ages]]. The width of the bubbles signifies the number of [[family (biology)|families]].]]

Conventional [[evolutionary taxonomy]] groups [[extant taxon|extant]] vertebrates into seven classes based on traditional interpretations of gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits. The commonly held classification lists three classes of fish and four of [[tetrapod]]s.<ref name="Campbell">{{cite book |last=Campbell |first=Neil A. |title=Biology |date=1997 |edition=4th |publisher=[[Benjamin Cummings]] |isbn=978-0-8053-1940-8 |page=632}}</ref> This ignores some of the natural relationships between the groupings. For example, the birds derive from a group of reptiles, so "[[Reptile|Reptilia]]" excluding "[[Aves]]" is not [[Clade|a natural grouping]]; it is described as [[paraphyletic]].<ref>{{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}}</ref><ref>{{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}}</ref>

* '''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<!--intentional overlink-->]] and [[Acanthodii]], both paraphyletic.

Other ways of classifying the vertebrates have been devised, particularly with emphasis on the [[phylogeny]] of [[labyrinthodontia|early amphibians]] and reptiles. An example based on work by M.J. Benton in 2004<ref name="Benton2004">{{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 }}</ref> is given here († = [[extinct]]):

* '''Subphylum Vertebrata'''
** '''Infraphylum "[[Agnatha]]"''' ([[lamprey]]s and other jawless fishes)
***'''Superclass {{extinct}}[[Anaspidomorphi]]''' (anaspids and relatives)
****Class {{extinct}}[[Anaspida]] (anaspids)
***'''Superclass [[Cyclostomi|Cyclostomata]]''' (cyclostomes)
****Class [[Hagfish|Myxini]] (hagfish)
****Class [[Lamprey|Petromyzontida]] (lampreys)
***Class {{extinct}}[[Cephalaspidomorphi]] (cephalaspidomorphs)
***Class {{extinct}}[[Conodont|Conodonta]] (conodonts)
***Class {{extinct}}[[Pteraspidomorphi|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, [[Cladistics|cladistically]] including the tetrapods)
*** '''Superclass [[Tetrapoda]]''' (four-limbed vertebrates)
**** Class "[[Amphibia]]" (amphibians, some ancestral to the [[amniote]]s)—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.<ref name=Benton2004 /> For instance, descendants of the first reptiles include modern reptiles, mammals and birds; the agnathans have given rise to the jawed vertebrates; the [[Osteichthyes|bony fishes]] have given rise to the [[tetrapoda|land vertebrates]]; a group of amphibians, the [[labyrinthodont]]s, have given rise to the [[Reptilia|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.<ref name="Irie 2018">{{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 }}</ref>

=== 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]].<ref>{{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 }}</ref> Since 2006, analysis has shown that the tunicates + vertebrates form a clade, the Olfactores, with Cephalochordata as its sister (the [[Olfactores#Olfactores hypothesis|Olfactores hypothesis]]), as shown in the following [[phylogenetic tree]].<ref name="Delsuc 2006">{{cite journal |last=Delsuc |first=F. |title=Tunicates and not cephalochordates are the closest living relatives of vertebrates |journal=[[Nature (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 }}</ref><ref name="Dunn 2008">{{cite journal |last=Dunn |first=C.W. |title=Broad phylogenetic sampling improves resolution of the animal tree of life |journal=[[Nature (journal)|Nature]] |volume=452 |year=2008 |issue=7188 |doi=10.1038/nature06614 |pages=745–749 |pmid=18322464 |bibcode=2008Natur.452..745D |s2cid=4397099 }}</ref><ref name="Gupta 2016" />

{{clade|style=font-size:100%
|label1=[[Chordata]]
|1={{clade
  |label1=[[Cephalochordata]]
  |1=[[Leptocardii]] (lancelets) <span style="{{MirrorH}}">[[File:Branchiostoma lanceolatum (Amphioxus lanceolatus).png|140px]]</span>
  |label2=[[Olfactores]]
  |2={{clade
    |1=[[Tunicata]] ([[sea squirt]]s, etc) [[File:Salpa scheme.png|90px]]
    |2='''Vertebrata''' [[File:Haikouichthys cropped.jpg|90px]]
     }}
  }}
}}

=== Internal phylogeny ===

The internal phylogeny of the vertebrates is shown in the below tree.<ref name="UCMP 2024">{{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}}</ref>

{{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.}} [[File:Nejonöga, Iduns kokbok.jpg|90 px]]
   |label2=[[Gnathostomata]]
   |sublabel2=(jawed)
   |2={{clade
      |label1=[[Chondrichthyes]] |sublabel1=(cartilaginous)
      |1=<span style="{{MirrorH}}">[[File:White shark (Duane Raver).png|75 px]]</span>
      |label2=[[Osteichthyes]] |sublabel2=(bony fishes)
      |2={{clade
         |1={{clade
            |label1=[[Actinopterygii]] |sublabel1=(ray-fins)
            |1=[[File:Common carp (white background).jpg|70px]]
            |label2=[[Sarcopterygii]] |sublabel2=(lobe-fins)
            |2={{clade
               <!--|label1=[[Actinistia]]-->
               |1=[[Coelacanth]]s [[File:Coelacanth flipped.png|70 px]]
               |label2=[[Rhipidistia]] |sublabel2=(lungs)
               |2={{clade
                  |1=[[Lungfish]]es [[File:Barramunda coloured.jpg|75 px]]
                  |label2=[[Tetrapod]]a |sublabel2=(4 limbs)
                  |2={{clade
                     |1=[[Amphibian]]s [[File:Salamandra_salamandra_(white_background).jpg|80 px]] 
                     |label2=[[Amniote|Amniota]] |sublabel2=(amnion)
                     |2={{clade
                        |1=[[Mammal]]s [[File:Rattus norvegicus (white background).png|55px]]
                        |label2=[[Sauropsida]] |sublabel2=(scaly skin)
                        |2={{clade
                           |1=[[Lepidosauria|Lepidosaurs]] [[File:Python natalensis Smith 1840 white background.jpg|70px]] 
                           |2={{clade
                              |1=[[Turtle]]s [[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Morenia ocellata).jpg|55px]]
                              |2={{clade
                                 |1=[[Crocodilia|Crocodilians]] [[File:Bristol.zoo.westafrican.dwarf.croc.arp. white background.jpg|70px]] 
                                 |2=[[Dinosaur]]s [[File:Spot-billed pelican takeoff white background.jpg|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 (biology)|morphology]] to place them outside Vertebrata.<ref>{{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}}</ref> [[Molecular data]] however indicates that they are vertebrates, being most closely related to lampreys.<ref name=hagfish>{{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 }}</ref><ref>{{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 (journal)|Science]] |date=7 August 1992 |volume=257 |issue=5071 |doi=10.1126/science.1496398 |pages=787–789 |pmid=1496398 |bibcode=1992Sci...257..787S }}</ref> An older view is that they are a sister group of vertebrates in the common taxon of Craniata.<ref>{{cite journal |last=Nicholls |first=H. |title=Mouth to Mouth |date=10 September 2009 |journal=[[Nature (journal)|Nature]] |volume=461 |issue=7261 |pages=164–166 |doi=10.1038/461164a |pmid=19741680 |doi-access=free}}</ref> In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the [[Cyclostomata]] hypothesis using only morphological data.<ref>{{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}}</ref>

{{clade|style=font-size:85%;line-height:85%;
|1={{clade
   |1=†[[Myllokunmingiidae|Myllokunmingiida]]<span style="{{MirrorH}}">[[File:Myllokunmingia.png|55px]]</span>
   |label2= '''Vertebrata''' |sublabel2=(crown group)
   |2={{clade
      |1={{clade
         |1=†[[Anaspida]] [[File:Anaspida.png|50 px]]
         |2={{clade
            |1=†''[[Pipiscius]]''
            |2={{clade
               |1=†[[Conodont]]a<span style="{{MirrorH}}">[[File:ConodontZICA.png|90px]]</span>
               |2=[[Cyclostomata]] (lampreys and hagfishes) [[File:Nejonöga, Iduns kokbok.jpg|90 px]] 
               }} 
            }}
         }}
      |2=[[Gnathostomata]] (jawed vertebrates)<span style="{{MirrorH}}"><span style="{{MirrorH}}"> [[File:Dunkleosteus terrelli 2024 reconstruction.jpg|70 px]]</span></span>
      }}
   }}
}}