Editing Tetrapod (section)

===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.