Amniote#Adaptation for terrestrial living

The term amniote comes from the amnion, which derives from Greek ἀμνίον (amnion), which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός (amnos), meaning "lamb".Oxford English Dictionary

File:Chicken egg diagram.svg

Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis, as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis.{{Cite journal|author1= Alexander M. Schreiber |author2= Donald D. Brown |title= Tadpole skin dies autonomously in response to thyroid hormone at metamorphosis |journal= Proceedings of the National Academy of Sciences |volume= 100 |issue= 4 |pages= 1769–1774 |doi= 10.1073/pnas.252774999 |pmid= 12560472 |pmc= 149908 |year= 2003 |bibcode= 2003PNAS..100.1769S |doi-access= free }} The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys (metanephros) and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta.

The ancestors of true amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic).{{cite web|url= http://www.southampton.ac.uk/oes/research/projects/the_mid_palaeozoic_biotic_crisis.page#overview|title= the_mid_palaeozoic_biotic_crisis – Ocean and Earth Science, National Oceanography Centre Southampton – University of Southampton}} The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell.Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating.{{cite journal|last= Cunningham|first= B.|author2= Huene, E.|title= Further Studies on Water Absorption by Reptile Eggs|journal= The American Naturalist|date= Jul–Aug 1938|volume= 72|issue= 741|pages= 380–385|jstor= 2457547|doi= 10.1086/280791|s2cid= 84258651}} Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion.

The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later.[https://www.americanscientist.org/article/shell-game Shell Game » American Scientist] It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4-inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.{{Cite journal|url= https://www.academia.edu/1177175|title= The evolution of body size, Cope's rule and the origin of amniotes|journal= Systematic Biology|volume= 53|issue= 4|pages= 594–622|author= Michel Laurin|pmid= 15371249|year= 2004|doi= 10.1080/10635150490445706|doi-access= }}

A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water.{{cite journal| url=https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.2012.01715.x | doi=10.1111/j.1558-5646.2012.01715.x | title=Phylogenetic Analyses Reveal Unexpected Patterns in the Evolution of Reproductive Modes in Frogs | date=2012 | last1=Gomez-Mestre | first1=Ivan | last2=Pyron | first2=Robert Alexander | last3=Wiens | first3=John J. | journal=Evolution | volume=66 | issue=12 | pages=3687–3700 | pmid=23206128 | hdl=10261/63940 | hdl-access=free }}

An alternative hypothesis is that amniotes evolved as a result of extended embryo retention (EER), where the extraembryonic membranes originated in the oviducts of the fertilized female to control the interaction between the embryos and the female. The eggs in groups like turtles, crocodilians and birds, which are laid at a much earlier developmental stage, would be a secondary evolved trait.[https://pmc.ncbi.nlm.nih.gov/articles/PMC10333127/ Extended embryo retention and viviparity in the first amniotes]

Fish and amphibian eggs have only one inner membrane, the embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.{{citation needed|date=November 2014}}

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage.Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation. Biological Journal of the Linnean Society No 11: pp 19–76 [http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1979.tb00027.x/abstract Abstract] There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of holes behind each eye. In anapsids, the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians, squamates, and tuataras), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this. Molecular testing firmly places them in the diapsid line of descent.

Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle.

Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion, hence the name. A problem with this definition is that the trait (apomorphy) in question does not fossilize, and the status of fossil forms has to be inferred from other traits.

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| caption1 = Archaeothyris, one of the most basal synapsids, first appears in the fossil records about 306 million years ago.{{cite journal | last1 = Falcon-Lang | first1 = H J | last2 = Benton | first2 = M J | last3 = Stimson | first3 = M | year = 2007 | title = Ecology of early reptiles inferred from Lower Pennsylvanian trackways | journal = Journal of the Geological Society | volume = 164 | issue = 6| pages = 1113–1118 | doi = 10.1016/j.palaeo.2010.06.020 }}

| image2 = Europasaurus holgeri Scene 2.jpg

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| caption2 = By the Mesozoic, 150 million years ago, sauropsids included the largest animals anywhere. Shown are some late Jurassic dinosaurs, including the early bird Archaeopteryx perched on a tree stump.

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Older classifications of the amniotes traditionally recognised three classes based on major traits and physiology:Romer A S and Parsons T S (1985) The Vertebrate Body. (6th ed.) Saunders, Philadelphia.Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.{{cite book|page=429|isbn=978-0-471-29505-1 |author=Hildebrand, M. |author2=G. E. Goslow Jr |year=2001|publisher=Wiley|location=New York|title=Analysis of vertebrate structure}}Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York — {{ISBN|978-0-471-38461-8}}.

• Class Reptilia (reptiles)
• Subclass Anapsida ("proto-reptiles", possibly including turtles)
• Subclass Diapsida (majority of reptiles,{{Cite journal | doi=10.1371/journal.pone.0118199| pmid=25803280| pmc=4372529|title = Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa| journal=PLOS ONE| volume=10| issue=3| pages=e0118199|year = 2015|last1 = Reeder|first1 = Tod W.| last2=Townsend| first2=Ted M.| last3=Mulcahy| first3=Daniel G.| last4=Noonan| first4=Brice P.| last5=Wood| first5=Perry L.| last6=Sites| first6=Jack W.| last7=Wiens| first7=John J.| bibcode=2015PLoSO..1018199R| doi-access=free}} progenitors of birds)
• Subclass Euryapsida (plesiosaurs, placodonts, and ichthyosaurs)
• Subclass Synapsida (stem or proto-mammals, progenitors of mammals)
• Class Aves (birds)
• Subclass Archaeornithes (reptile-like birds, progenitors of all other birds)
• Subclass Enantiornithes (early birds with an alternative shoulder joint)*Hope, S. (2002) The Mesozoic record of Neornithes (modern birds). In: Chiappe, L.M. and Witmer, L.M. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs: 339–388. University of California Press, Berkeley. {{ISBN|0-520-20094-2}}
• Subclass Hesperornithes (toothed aquatic flightless birds)
• Subclass Ichthyornithes (toothed, but otherwise modern birds)
• Subclass Neornithes (all living birds)
• Class Mammalia (mammals)
• Subclass Prototheria (Monotremata, egg-laying mammals)
• Subclass Theria (metatheria (such as marsupials) and eutheria (such as placental mammals))

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic—it has given rise to two other classes not included in Reptilia.

Most species described as microsaurs, formerly grouped in the extinct and prehistoric amphibian group lepospondyls, has been placed in the newer clade Recumbirostra, and shares many anatomical features with amniotes which indicates they were amniotes themselves.[https://www.livescience.com/snakelike-microsaur-310-million-years-old.html Tiny ancient reptile named after Thor's world-ending nemesis]

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below.{{cite book|chapter=Appendix: Classification of the Vertebrates |author=Benton, M.J.|year=2015|title=Vertebrate Paleontology|edition=4th|publisher=Wiley Blackwell |pages=433–447|isbn=978-1-118-40684-7|no-pp=true}}

• Series Amniota
• (Class) Clade Synapsida
• A series of unassigned families, corresponding to Pelycosauria †
• (Order) Clade Therapsida
• Class Mammalia – mammals
• (Class) Clade Sauropsida
• Subclass Parareptilia †
• Family Mesosauridae †
• Family Millerettidae †
• Family Bolosauridae †
• Family Procolophonidae †
• Order Pareiasauromorpha
• Family Nycteroleteridae †
• Family Pareiasauridae †
• (Subclass) Clade Eureptilia
• Family Captorhinidae †
• (Infraclass) Clade Diapsida
• Family Araeoscelididae †
• Family Weigeltisauridae †
• Order Younginiformes †
• (Infraclass) Clade Neodiapsida
• Order Testudinata
• Suborder Testudines – turtles
• Infraclass Lepidosauromorpha
• Unnamed infrasubclass
• Infraclass Ichthyosauria †
• Order Thalattosauria †
• Superorder Lepidosauriformes
• Order Sphenodontida – tuatara
• Order Squamata – lizards and snakes
• Infrasubclass Sauropterygia †
• Order Placodontia †
• Order Eosauropterygia †
• Suborder Pachypleurosauria †
• Suborder Nothosauria †
• Order Plesiosauria †
• (Infraclass) Clade Archosauromorpha
• Family Trilophosauridae †
• Order Rhynchosauria †
• Order Protorosauria †
• Division Archosauriformes
• Subdivision Archosauria
• Infradivision Crurotarsi
• Order Phytosauria†
• Family Ornithosuchidae †
• Family Stagonolepididae †
• Family Rauisuchidae †
• Superfamily Poposauroidea †
• Superorder Crocodylomorpha
• Order Crocodylia – crocodilians
• Infradivision Avemetatarsalia
• Infrasubdivision Ornithodira
• Order Pterosauria †
• Family Lagerpetidae †
• Family Silesauridae †
• (Superorder) Clade Dinosauria – dinosaurs
• Order Ornithischia †
• (Order) Clade Saurischia
• (Suborder) Clade Theropoda – theropods
• Class Aves – birds

{{Further |Phylogenetic nomenclature}}

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants".Gauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103–155 in Benton, M.J. (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press. As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy.Lee, M.S.Y. & Spencer, P.S. (1997): Crown clades, key characters and taxonomic stability: when is an amniote not an amniote? In: Sumida S.S. & Martin K.L.M. (eds.) Amniote Origins: completing the transition to land. Academic Press, pp 61–84. [https://books.google.com/books?id=9f7rafLJbWUC&pg=PA61 Google books] Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy.{{Cite journal |author=David S. Berman |year=2013 |title=Diadectomorphs, amniotes or not? |url=https://books.google.com/books?id=5f4oCgAAQBAJ&pg=PA22 |journal=New Mexico Museum of Natural History and Science Bulletin |volume=60 |pages=22–35}}{{Cite journal |author1=Jozef Klembara |author2=Miroslav Hain |author3=Marcello Ruta |author4=David S. Berman |author5=Stephanie E. Pierce |author6=Amy C. Henrici |year=2019 |title=Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high-resolution x-ray microcomputed tomography, and the origin of the amniote crown group |journal=Palaeontology |volume=63 |pages=131–154 |doi=10.1111/pala.12448 |doi-access=free|url=http://eprints.lincoln.ac.uk/id/eprint/36078/3/postprint.pdf }}{{Cite journal |last1=Klembara |first1=Jozef |last2=Ruta |first2=Marcello |last3=Hain |first3=Miroslav |last4=Berman |first4=David S. |date=2021 |title=Braincase and Inner Ear Anatomy of the Late Carboniferous Tetrapod Limnoscelis dynatis (Diadectomorpha) Revealed by High-Resolution X-ray Microcomputed Tomography |journal=Frontiers in Ecology and Evolution |volume=9 |doi=10.3389/fevo.2021.709766 |issn=2296-701X|doi-access=free |url=https://eprints.lincoln.ac.uk/id/eprint/48118/1/fevo-09-709766.pdf }}

The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995),{{cite journal | last1 = Laurin | first1 = M. | last2 = Reisz | first2 = R.R. | year = 1995 | title = A reevaluation of early amniote phylogeny | url = http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf | journal = Zoological Journal of the Linnean Society | volume = 113 | issue = 2| pages = 165–223 | doi = 10.1111/j.1096-3642.1995.tb00932.x | access-date = 2 November 2017 | archive-url = https://web.archive.org/web/20190608150549/http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf | archive-date = 2019-06-08 }} with the exception of turtles, which more recent morphological and molecular phylogenetic studies placed firmly within diapsids.{{cite journal | last1 = Rieppel | first1 = O. | year = 1996 | last2 = DeBraga | first2 = M. | title = Turtles as diapsid reptiles | journal = Nature | volume = 384 | issue = 6608 | pages = 453–5 | doi = 10.1038/384453a0 | bibcode = 1996Natur.384..453R| s2cid = 4264378 | url = http://doc.rero.ch/record/16242/files/PAL_E3477.pdf }}{{cite book | last = Müller | first = Johannes | year = 2004 | chapter = The relationships among diapsid reptiles and the influence of taxon selection | editor1-last = Arratia | editor1-first = G | editor2-last = Wilson | editor2-first = M.V.H. | editor3-last = Cloutier | editor3-first = R.

| title = Recent Advances in the Origin and Early Radiation of Vertebrates | publisher = Verlag Dr. Friedrich Pfeil | pages = 379–408 | isbn = 978-3-89937-052-2 }}{{Cite journal |author1=Tyler R. Lyson |author2=Erik A. Sperling |author3=Alysha M. Heimberg |author4=Jacques A. Gauthier |author5=Benjamin L. King |author6=Kevin J. Peterson | title = MicroRNAs support a turtle + lizard clade | journal = Biology Letters | volume = 8 | issue = 1 | pages = 104–107 | doi = 10.1098/rsbl.2011.0477 | pmid=21775315 | pmc=3259949|date=23 February 2012 }}{{Cite journal | last = Iwabe | first = N. |author2=Hara, Y.|author3=Kumazawa, Y.|author4=Shibamoto, K.|author5=Saito, Y.|author6=Miyata, T.|author7= Katoh, K. | title = Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins | journal = Molecular Biology and Evolution | volume = 22 | issue = 4 | pages = 810–813 | date = 29 December 2004 | doi = 10.1093/molbev/msi075 | pmid = 15625185 | doi-access = free }}{{Cite journal | last = Roos | first = Jonas |author2= Aggarwal, Ramesh K.|author3= Janke, Axel | title = Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary | journal = Molecular Phylogenetics and Evolution | volume = 45 | issue = 2 | pages = 663–673 | date = Nov 2007 | doi = 10.1016/j.ympev.2007.06.018 | pmid = 17719245| bibcode = 2007MolPE..45..663R }}{{Cite journal | last = Katsu | first = Y. |author2= Braun, E. L.|author3= Guillette, L. J. Jr.|author4= Iguchi, T.|title = From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes|journal = Cytogenetic and Genome Research|volume = 127|issue = 2–4|pages = 79–93|date = 17 March 2010|doi = 10.1159/000297715 | pmid = 20234127| s2cid = 12116018 }} The cladogram covers the group as defined under Gauthier's definition.

{{clade| style=font-size:100%;line-height:80%

|label1=Reptiliomorpha

|1={{clade

|1={{extinct}}Diadectomorpha 80 px

|label2=Amniota

|2={{clade

|1=Synapsida (mammals and their extinct relatives)80 px

|label2=Sauropsida

|2={{clade

|1={{extinct}}Mesosauridae 80 px

|label2=Reptilia

|2={{clade

|label1=Parareptilia

|1={{clade

|1={{extinct}}Millerettidae80 px

|label2=unnamed

|2={{clade

|1={{extinct}}Pareiasauria80px

|label2=unnamed

|2={{clade

|1={{extinct}}Procolophonoidea 80px

}}

}}

}}

|label2=Eureptilia

|2={{clade

|1={{extinct}}Captorhinidae 80px

|label2=Romeriida

|2={{clade

|1={{extinct}}Protorothyrididae 70px

|2=Diapsida (lizards, snakes, turtles, crocodiles, dinosaurs, birds, etc.)80px

}}

}}

}}

}}

}}

}}

}}

Following studies in 2022 and 2023,{{Cite journal |last1=Simões |first1=T. R. |last2=Kammerer |first2=C. F. |last3=Caldwell |first3=M. W. |last4=Pierce |first4=S. E. |year=2022 |title=Successive climate crises in the deep past drove the early evolution and radiation of reptiles |journal=Science Advances |volume=8 |issue=33 |pages=eabq1898 |doi=10.1126/sciadv.abq1898 |pmc=9390993 |pmid=35984885 |doi-access=free|bibcode=2022SciA....8.1898S }}{{Cite journal |last1=Wolniewicz |first1=Andrzej S |last2=Shen |first2=Yuefeng |last3=Li |first3=Qiang |last4=Sun |first4=Yuanyuan |last5=Qiao |first5=Yu |last6=Chen |first6=Yajie |last7=Hu |first7=Yi-Wei |last8=Liu |first8=Jun |date=2023-08-08 |editor-last=Ibrahim |editor-first=Nizar |title=An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications |journal=eLife |volume=12 |pages=e83163 |doi=10.7554/eLife.83163 |issn=2050-084X |doi-access=free|pmid=37551884 |pmc=10499374 }} with Drepanosauromorpha placed sister to Weigeltisauridae (Coelurosauravus) in Avicephala based on Senter (2004):{{Cite journal |last=Senter |first=Phil |date=January 2004 |title=Phylogeny of Drepanosauridae (Reptilia: Diapsida) |url=http://dx.doi.org/10.1017/s1477201904001427 |journal=Journal of Systematic Palaeontology |language=en |volume=2 |issue=3 |pages=257–268 |doi=10.1017/S1477201904001427 |bibcode=2004JSPal...2..257S |s2cid=83840423 |issn=1477-2019}}

{{clade| style=font-size:80%;line-height:80%

|1={{clade

|1={{extinct}}Seymouriamorpha 50 px

|label2=Amniota?

|2={{clade

|1={{extinct}}Diadectomorpha 50 px

|2={{clade

|1={{clade

|1={{extinct}}Araeoscelida 50 px

|2={{clade

|1={{extinct}}Captorhinidae 50 px

|2={{extinct}}Protorothyris 50 px

}}

}}

|label2=Amniota

|sublabel2=(crown group)

|2={{clade

|label1=Synapsida

|1={{clade

|label1=Caseasauria

|1={Caseasauria}

|label2=Eupelycosauria

|2={Eupelycosauria}

}}

|label2=Sauropsida

|2={{clade

|1={{extinct}}Acleistorhinidae 50 px

|2={{clade

|1={{clade

|1={{extinct}}Microleter 50 px

|2={{clade

|1={{extinct}}Australothyris

|2={{extinct}}Millerettidae 50 px

}}

}}

|2={{clade

|1={{extinct}}Mesosauria 50 px

|label2=Neoreptilia

|2={{clade

|1={{extinct}}Procolophonia 50 px

|label2=Neodiapsida

|2={{clade

|1={{extinct}}Orovenator 50 px

|2={{clade

|1={{extinct}}Lanthanolania 50 px

|2={{clade

|1={{extinct}}Younginiformes 50 px

|2={{clade

|1={{extinct}}Eunotosaurus 50 px

|label2=Sauria

|2={{clade

|1={Lepidosauromorpha}

|2={{clade

|1={Avicephala}

|2={{clade

|1={{extinct}}Choristodera 50 px

|label2=Archelosauria

|2={{clade

|1={Pantestudines}

|2={{clade

|1={Archosauromorpha}

|2={{clade

|1={{extinct}}Thalattosauria 50 px

|2={{clade

|label1=Ichthyosauromorpha

|1={Ichthyosauromorpha}

|label2=Sauropterygia

|2={Sauropterygiformes}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

|targetA = {Caseasauria}

|subcladeA={{clade hidden |id=1 |mode=left

|1={{extinct}}Vaughnictis 50 px

|2={{clade

|1={{extinct}}Eothyris 50 px

|2={{clade

|1={{extinct}}Caseidae 50 px

|2={{extinct}}Oedaleops 50 px

}}

}}

}}

|targetB = {Eupelycosauria}

|subcladeB={{clade hidden |id=2 |mode=left

|1={{extinct}}Varanopsidae 50 px

|label2=Metopophora

|2={{clade

|1={{extinct}}Ophiacodontidae 50 px

|label2=Sphenacomorpha

|2={{clade

|1={{extinct}}Edaphosauridae 50 px

|label2=Sphenacodontia

|2={{clade

|1={{extinct}}Haptodus 50 px

|label2=Sphenacodontoidea

|2={{clade

|1={{extinct}}Sphenacodontidae 50 px

|2=Therapsida 50 px 50 px 50 px

}}

}}

}}

}}

}}

|targetC = {Lepidosauromorpha}

|subcladeC={{clade

|label1=Lepidosauromorpha

|1={{clade hidden |id=3 |mode=left

|1={{clade

|1={{extinct}}Kuehneosauridae 50 px

|2={{extinct}}Pamelina 50 px

}}

|2={{clade

|1={{extinct}}Sophineta 50 px

|2=Lepidosauria 50 px 50 px

}}

}}

}}

|targetD = {Avicephala}

|subcladeD={{clade

|label1=Avicephala?

|1={{clade hidden |id=4 |mode=left

|1={{extinct}}Drepanosauromorpha 50 px 50 px

|2={{extinct}}Weigeltisauridae 50 px

}}

}}

|targetE = {Pantestudines}

|subcladeE={{clade

|label1=Pantestudines

|1={{clade hidden |id=5 |mode=left

|1={{extinct}}Pappochelys 50 px

|2={{clade

|1={{extinct}}Odontochelys 50 px

|2=Testudinata50 px50 px50 px

}}

}}

}}

|targetF = {Archosauromorpha}

|subcladeF={{clade

|label1=Archosauromorpha

|sublabel1=sensu stricto

|1={{clade hidden |id=6 |mode=left

|1={{extinct}}Aenigmastropheus 50 px

|2={{clade

|1={{extinct}}Protorosaurus 50 px

|2={{clade

|1={{extinct}}Tanystropheidae 50 px

|2={{clade

|1={{extinct}}Prolacertidae 50 px

|2={{clade

|1={{extinct}}Allokotosauria 50 px

|2={{clade

|1={{extinct}}Rhynchosauria 50 px

|2=Archosauriformes

|image2=50 px50 px80 px50 px50 px

}}

}}

}}

}}

}}

}}

}}

|targetG = {Ichthyosauromorpha}

|subcladeG={{clade hidden |id=7 |mode=left

|1={{extinct}}Hupehsuchia 50 px

|2={{extinct}}Ichthyosauriformes

|image2=50 px50 px

}}

|targetH = {Sauropterygiformes}

|subcladeH={{clade hidden |id=8 |mode=left

|1={{extinct}}Helveticosaurus 50 px

|2={{clade

|1={{extinct}}Sauropterygia

|image1=50 px50 px

|2={{clade

|1={{extinct}}Eusaurosphargis 50 px

|2={{extinct}}Palatodonta 50 px

}}

}}

}}

}}

{{Reflist|30em}}

{{Chordata}}

{{Early tetrapods|state=autocollapse|A.}}

{{Taxonbar|from=Q181537}}

{{Authority control}}

Category:Extant Pennsylvanian first appearances

Category:Taxa named by Ernst Haeckel

Category:Zoological nomenclature