Editing Tetrapod (section)

==Classification==
{{see also|List of chordate orders|List of tetrapod families}}
[[File:Linnaeus - Regnum Animale (1735).png|thumb|upright=1.4|[[Carl Linnaeus]]'s 1735 classification of animals, with tetrapods occupying the first three classes]]
The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits.<ref name=Romer>{{cite book |author=Romer, A.S. |title=The Vertebrate Body |publisher=W.B. Saunders |location=Philadelphia |year=1949 }} (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)</ref> [[Snake]]s and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply to [[caecilians]] and [[aquatic mammals]]. Newer taxonomy is frequently based on [[cladistics]] instead, giving a variable number of major "branches" ([[clade]]s) of the tetrapod [[phylogenetic tree|family tree]].

As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In [[phylogenetic nomenclature]], in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type ''of'' dinosaurs.

===History of classification===
The tetrapods, including all large- and medium-sized land animals, have been among the best understood animals since earliest times. By [[Aristotle]]'s time, the basic division between mammals, birds and egg-laying tetrapods (the "[[Herpetology|herptiles]]") was well known, and the inclusion of the legless snakes into this group was likewise recognized.<ref>{{cite journal|last=Lloyd|first=G.E.R.|title=The Development of Aristotle's Theory of the Classification of Animals|journal=Phronesis|year=1961|volume=6|issue=1|pages=59–81|jstor=4181685|doi=10.1163/156852861X00080}}</ref> With the birth of modern [[biological classification]] in the 18th century, [[Carl Linnaeus|Linnaeus]] used the same division, with the tetrapods occupying the first three of his six classes of animals.<ref name="Linn1758" >{{cite book |first=Carolus |last=Linnaeus |title=Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis |publisher=Laurentius Salvius |location=Stockholm |year=1758 |url=https://www.biodiversitylibrary.org/bibliography/542 |language=la |edition=[[10th edition of Systema Naturae|10th edition]] |access-date=2018-01-13 |archive-date=2008-10-10 |archive-url=https://web.archive.org/web/20081010032456/http://www.biodiversitylibrary.org/bibliography/542 |url-status=live }}</ref> While reptiles and amphibians can be quite similar externally, the French zoologist [[Pierre André Latreille]] recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.<ref>Latreielle, P.A. (1804): Nouveau Dictionnaire à Histoire Naturelle, xxiv., cited in Latreille's ''Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique'', 1825</ref>

===Modern classification===
With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominantly done by experts working within classes. In the early 1930s, American [[Vertebrate paleontology|vertebrate palaeontologist]] [[Alfred Romer]] (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his ''[[Vertebrate Paleontology (Romer)|Vertebrate Paleontology]]''.<ref>Smith, C. H. (2005). [http://www.wku.edu/~smithch/chronob/ROME1894.htm "Romer, Alfred Sherwood (United States 1894-1973)"]. {{Webarchive|url=https://web.archive.org/web/20081012014018/http://www.wku.edu/~smithch/chronob/ROME1894.htm |date=2008-10-12 }}. [[Western Kentucky University]]</ref> This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. [[Michael Benton|Benton]] (1998) and Knobill and Neill (2006).<ref>Benton, M. J. (1998). "The quality of the fossil record of vertebrates". pp. 269–303 in Donovan, S. K. and Paul, C. R. C. (eds), ''The adequacy of the fossil record'', Fig. 2. New York: Wiley.</ref><ref>Neill, J. D. (ed.) (2006). ''Knobil and Neill's Physiology of Reproduction'' (3rd ed.). Vol 2. [[Academic Press]]. p. 2177.</ref> While mostly seen in general works, it is also still used in some specialist works like Fortuny et al. (2011).<ref>{{cite journal | last1 = Fortuny | first1 = J. | last2 = Bolet | first2 = A. | last3 = Sellés | first3 = A. G. | last4 = Cartanyà | first4 = J. | last5 = Galobart | first5 = À. | year = 2011 | title = New insights on the Permian and Triassic vertebrates from the Iberian Peninsula with emphasis on the Pyrenean and Catalonian basins | url = http://www.ucm.es/info/estratig/JIG/vol_content/vol_37_1/JIG_37_1_Fortuny_65-86.pdf | journal = [[Journal of Iberian Geology]] | volume = 37 | issue = 1 | pages = 65–86 | doi = 10.5209/rev_JIGE.2011.v37.n1.5 | access-date = 2012-12-04 | archive-date = 2011-05-17 | archive-url = https://web.archive.org/web/20110517050605/http://www.ucm.es/info/estratig/JIG/vol_content/vol_37_1/JIG_37_1_Fortuny_65-86.pdf | url-status = live | doi-access = free }}</ref> The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):<ref name="hilde">{{cite book|page=429|isbn=978-0-471-29505-1|last1=Hildebrand |first1=M. |author2=G. E. Goslow Jr |others=ill. Viola Hildebrand |year=2001 |publisher=Wiley |location=New York |title=Analysis of vertebrate structure}}</ref>

*'''Superclass [[Tetrapoda]]''' – four-limbed vertebrates
**'''Class [[Amphibia]]''' – amphibians
***'''Subclass''' '''[[Ichthyostegalia]]''' – early fish-like amphibians (paraphyletic group outside leading to the crown-clade Neotetrapoda)
***'''Subclass''' '''[[Anthracosauria]]''' – reptile-like amphibians (often thought to be the ancestors of the [[amniote]]s)
***'''Subclass''' '''[[Temnospondyli]]''' – large-headed Paleozoic and Mesozoic amphibians
***'''Subclass''' '''[[Lissamphibia]]''' – modern amphibians
**'''Class [[Reptilia]]''' – reptiles
***'''Subclass''' '''[[Diapsida]]''' – diapsids, including crocodiles, dinosaurs, birds, lizards, snakes and turtles
***'''Subclass [[Euryapsida]]''' – euryapsids
***'''Subclass [[Synapsida]]''' – synapsids, including mammal-like reptiles-now a separate group (often thought to be the ancestors of mammals)
***'''Subclass [[Anapsida]]''' – anapsids
**'''Class [[Mammalia]]''' – mammals
***'''Subclass [[Prototheria]]''' – egg-laying mammals, including monotremes
***'''Subclass [[Allotheria]]''' – multituberculates
***'''Subclass [[Theria]]''' – live-bearing mammals, including marsupials and placentals

This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique from [[cladistics]]. The earliest tetrapods are grouped under class Amphibia, although several of the groups are more closely related to [[amniote]]s than to [[Lissamphibia|modern day amphibians]]. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendants of [[Theropoda|theropod dinosaurs]]. [[Basal (phylogenetics)|Basal]] non-mammalian [[synapsid]]s ("mammal-like reptiles") traditionally also sort under class Reptilia as a separate subclass,<ref name=Romer/> but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on [[phylogeny]], disregarding the anatomy and physiology.