Editing Tetrapod (section)

==Definitions==
The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with the earliest members of the group.<ref name=":0">{{Cite journal |last=Laurin |first=Michel |date=2002-03-01 |title=Tetrapod Phylogeny, Amphibian Origins, and the Definition of the Name Tetrapoda |url=http://dx.doi.org/10.1080/10635150252899815 |journal=Systematic Biology |volume=51 |issue=2 |pages=364–369 |doi=10.1080/10635150252899815 |pmid=12028737 |issn=1076-836X}}</ref><ref name=":2">{{Cite journal |last=Anderson |first=Jason S. |date=2002-09-01 |title=Use of Well-Known Names in Phylogenetic Nomenclature: A Reply to Laurin |journal=Systematic Biology |volume=51 |issue=5 |pages=822–827 |doi=10.1080/10635150290102447 |pmid=12396594 |issn=1076-836X|doi-access=free }}</ref><ref name="RCQ03">{{cite journal |last=Ruta |first=M. |author2=Coates, M.I. |author3=Quicke, D.L.J. |year=2003 |title=Early tetrapod relationships revisited |url=http://www.amphibiatree.org/sites/amphibiatree.org/files/RutaETAL2003Tetrapod.pdf |journal=Biological Reviews |volume=78 |issue=2 |pages=251–345 |doi=10.1017/S1464793102006103 |pmid=12803423|s2cid=31298396 }}</ref><ref name=":3">{{Cite journal |last1=Laurin |first1=Michel |last2=Anderson |first2=Jason S. |date=2004-02-01 |editor-last=Simon |editor-first=Chris |title=Meaning of the Name Tetrapoda in the Scientific Literature: An Exchange |journal=Systematic Biology |language=en |volume=53 |issue=1 |pages=68–80 |doi=10.1080/10635150490264716 |pmid=14965901 |s2cid=15922260 |issn=1076-836X|doi-access=free }}</ref>

=== Apomorphy-based definitions ===
{{See also|Stegocephali}}
A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct [[Digit (anatomy)|digits]] (fingers and toes), as well as legless vertebrates with limbed ancestors.<ref name=":2" /><ref name="RCQ03" /> Limbs and digits are major [[apomorphies]] (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. The first vertebrates with limbs and digits evolved in the [[Devonian]], including the [[Late Devonian]]-age ''[[Ichthyostega]]'' and ''[[Acanthostega]]'', as well as the trackmakers of the [[Middle Devonian]]-age [[Zachelmie trackways]].<ref name="NarkiewiczNarkiewicz2015" />

Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through [[convergent evolution]]. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalent [[Cladistics|cladistic]] definition. Cladistics is a modern branch of [[Taxonomy (biology)|taxonomy]] which classifies organisms through evolutionary relationships, as reconstructed by [[phylogenetic analyses]]. A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a [[Monophyly|monophyletic]] [[clade]], a group with all of its component taxa sharing a single common ancestor.<ref name="RCQ03" /> In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates.<ref name=":3" />

=== Crown group tetrapods ===
[[File:Tetrapoda PhyloCode (en).svg|left|thumb|307x307px|A simplified cladogram demonstrating differing definitions of Tetrapoda:

<br>* Under the [[Apomorphy and synapomorphy|apomorphy]]-based definition used by many paleontologists, tetrapods originate at the orange star ("First vertebrates with tetrapod limb")
<br>* When restricted to the [[crown group]], tetrapods originate at the "last common ancestor of recent tetrapods"

]]
A portion of tetrapod workers, led by French paleontologist [[Michel Laurin]], prefer to restrict the definition of tetrapod to the [[crown group]].<ref name=":0" /><ref name=":1">{{Cite book |last1=Queiroz |first1=Kevin de |url=https://www.taylorfrancis.com/books/mono/10.1201/9780429446276/phylonyms-kevin-de-queiroz-philip-cantino-jacques-gauthier |title=Phylonyms: A Companion to the PhyloCode |last2=Cantino |first2=Philip D. |last3=Gauthier |first3=Jacques A. |editor-first1=Kevin |editor-first2=Philip |editor-first3=Jacques |editor-last1=De Queiroz |editor-last2=Cantino |editor-last3=Gauthier |publisher=CRC Press |year=2020 |edition=1st |location=Boca Raton |chapter=Stegocephali E. D. Cope 1868 [M. Laurin], converted clade name |doi=10.1201/9780429446276|isbn=9780429446276 |s2cid=242704712 }}</ref> A crown group is a subset of a category of animal defined by the most recent common ancestor of living representatives. This cladistic approach defines "tetrapods" as the nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a [[Node-based taxon|node-based]] [[clade]], a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).<ref name="RCQ03" />

Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such as ''Ichthyostega'' and ''Acanthostega'', certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along the [[Crown group|stem group]], a subset of animals related to, but not within, the crown group. The stem and crown group together are combined into the [[total group]], given the name [[Tetrapodomorpha]], which refers to all animals closer to living tetrapods than to Dipnoi ([[lungfish]]es), the next closest group of living animals.<ref>{{harvnb|Clack|2012|pp=87–9}}</ref> Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as the presence of limbs and digits.

Laurin's approach to the definition of tetrapods is rooted in the belief that the term has more relevance for [[Neontology|neontologists]] (an informal term used for biologists specializing in living organizms) than paleontologists (who primarily use the apomorphy-based definition).<ref name=":3" /> In 1998, he re-established the defunct historical term '''[[Stegocephali]]''' to replace the apomorphy-based definition of tetrapod used by many authors.<ref name="LaurinGirondot2000">{{cite journal|last1=Laurin|first1=Michel|last2=Girondot|first2=Marc|last3=de Ricqlès|first3=Armand|title=Early tetrapod evolution|journal=Trends in Ecology & Evolution|volume=15|issue=3|year=2000|pages=118–123|url=http://max2.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf|issn=0169-5347|doi=10.1016/S0169-5347(99)01780-2|pmid=10675932|access-date=2015-06-08|archive-date=2012-07-22|archive-url=https://web.archive.org/web/20120722083254/http://max2.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf|url-status=dead}}</ref> Other paleontologists use the term [[stem-tetrapod]] to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes.<ref>{{harvnb|Laurin|2010|p=9}}</ref> The term "fishapod" was popularized after the discovery and 2006 publication of ''[[Tiktaalik]]'', an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits.

The two subclades of crown tetrapods are [[Batrachomorpha]] and [[Reptiliomorpha]]. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians.<ref>{{harvnb|Benton|2009|p=99}}</ref> Gaffney (1979) provided the name '''Neotetrapoda''' to the crown group of tetrapods, though few subsequent authors followed this proposal.<ref name=":3" />

The earliest fossils attributed to crown-group tetrapods are footprints from the earliest Carboniferous ([[Tournaisian]]) of Australia, which appear to belong to early [[Amniote|amniotes]] or potentially even [[Sauropsida|sauropsids]]. Prior to the discovery of these prints, the earliest evidence of crown-group tetrapods were [[Temnospondyli|temnospondyl]] footprints from slightly later in the Tournaisian, with the earliest body fossils being of the temnospondyl ''[[Balanerpeton]]'' from the [[Viséan]].<ref name=":4">{{Cite journal |last=Long |first=John A. |last2=Niedźwiedzki |first2=Grzegorz |last3=Garvey |first3=Jillian |last4=Clement |first4=Alice M. |last5=Camens |first5=Aaron B. |last6=Eury |first6=Craig A. |last7=Eason |first7=John |last8=Ahlberg |first8=Per E. |date=2025-05-14 |title=Earliest amniote tracks recalibrate the timeline of tetrapod evolution |url=https://www.nature.com/articles/s41586-025-08884-5 |journal=Nature |language=en |pages=1–8 |doi=10.1038/s41586-025-08884-5 |issn=1476-4687|doi-access=free }}</ref>