Editing Amphibian (section)

== Evolutionary history ==
{{Main|Evolution of tetrapods}}
{{See also|List of prehistoric amphibian genera}}
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| image1            = Eusthenopteron BW.jpg
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| caption1          = Restoration of ''[[Eusthenopteron]]'', a fully aquatic lobe-finned fish
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| caption2          = Restoration of ''[[Tiktaalik]]'', an advanced tetrapodomorph fish
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The first major groups of amphibians developed in the [[Devonian]] period, around 370 million years ago, from [[Sarcopterygii|lobe-finned fish]] which were similar to the modern [[coelacanth]] and [[lungfish]].<ref name="evoamphib">{{cite web |url=http://sci.waikato.ac.nz/evolution/AnimalEvolution.shtml#evolutionofamphibian |title=Evolution of amphibians |publisher=University of Waikato: Plant and animal evolution |access-date=September 30, 2012 |archive-date=June 27, 2016 |archive-url=https://web.archive.org/web/20160627175302/http://sci.waikato.ac.nz/evolution/AnimalEvolution.shtml#evolutionofamphibian |url-status=live }}</ref> These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs that help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would [[evolution|evolve]] into limbs and they would become the ancestors to all [[tetrapod]]s, including modern amphibians, reptiles, birds, and [[mammal]]s. Despite being able to crawl on land, many of these prehistoric [[tetrapodomorph]] fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills.<ref name="Carroll">{{cite book |title=Patterns of Evolution, as Illustrated by the Fossil Record |last=Carroll |first=Robert L. |editor=Hallam, Anthony |editor-link=Anthony Hallam |year=1977 |publisher=Elsevier |isbn=978-0-444-41142-6 |pages=405–420 |url=https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |access-date=October 15, 2020 |archive-date=April 14, 2021 |archive-url=https://web.archive.org/web/20210414082736/https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |url-status=live }}</ref>

Many examples of species showing [[transitional fossil|transitional features]] have been discovered. ''[[Ichthyostega]]'' was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, ''[[Eusthenopteron]]''.<ref name="evoamphib" /> Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land.<!--'cope with the increased gravitational effect of life' – The gravitational force is effectively the same on the surface of the land as in the sea; however, in water, a body experiences a buoyancy force.--> They developed "hands" and "feet" with five or more digits;<ref name="Ichthyostega" /> the skin became more capable of retaining body fluids and resisting desiccation.<ref name="Carroll" /> The fish's [[hyomandibula]] bone in the [[hyoid bone|hyoid]] region behind the gills diminished in size and became the [[stapes]] of the amphibian ear, an adaptation necessary for hearing on dry land.<ref>{{cite journal |author1=Lombard, R. E. |author2=Bolt, J. R. |year=1979 |title=Evolution of the tetrapod ear: an analysis and reinterpretation |journal=Biological Journal of the Linnean Society |volume=11 |issue=1 |pages=19–76 |doi=10.1111/j.1095-8312.1979.tb00027.x |url=https://www.researchgate.net/publication/230169547 |access-date=November 10, 2016 |archive-date=October 4, 2018 |archive-url=https://web.archive.org/web/20181004050337/https://www.researchgate.net/publication/230169547 |url-status=live }}</ref> An affinity between the amphibians and the [[teleost]] fish is the multi-folded structure of the teeth and the paired [[Occipital bone|supra-occipital bones]] at the back of the head, neither of these features being found elsewhere in the animal kingdom.<ref name="Spoczynska" />

[[File:Diplocaulus vale21DB.jpg|thumb|left|upright|alt=''Diplocaulus''|The Permian lepospondyl ''[[Diplocaulus]]'' was largely aquatic]]
At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates,<ref name="Spoczynska"/> though some, such as ''Ichthyostega'', may have sometimes hauled themselves out of the water. It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the [[elephant seal]].<ref name="Ichthyostega">{{cite web |url=http://tolweb.org/Ichthyostega |title=''Ichthyostega'' |author=Clack, Jennifer A. |year=2006 |work=Tree of Life Web Project |access-date=September 29, 2012 |archive-date=June 14, 2020 |archive-url=https://web.archive.org/web/20200614075729/http://tolweb.org/Ichthyostega |url-status=live }}</ref> In the early [[Carboniferous]] (360 to 323 million years ago), the climate was relatively wet and warm. Extensive swamps developed with [[moss]]es, [[fern]]s, [[horsetail]]s and [[calamites]]. Air-breathing [[arthropod]]s evolved and invaded the land where they provided food for the [[Carnivore|carnivorous]] amphibians that began to adapt to the terrestrial environment. There were no other tetrapods on the land and the amphibians were at the top of the food chain, with some occupying ecological positions currently held by crocodiles. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail.<ref name="Spoczynska">{{cite book |title=Fossils: A Study in Evolution |last=Spoczynska |first=J. O. I. |year=1971 |publisher=Frederick Muller Ltd |isbn=978-0-584-10093-8 |pages=120–125 }}</ref> Others were the top land predators, sometimes reaching several metres in length, preying on the large insects of the period and the many types of fish in the water. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the [[amniote|amniotic]] egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their [[Dominance (ecology)|dominance]] in the period that followed.<ref name="evoamphib"/>

After the [[Carboniferous rainforest collapse]] amphibian dominance gave way to reptiles,<ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal | author=Sahney, S. | author2=Benton, M.J. | author3=Ferry, P.A. | name-list-style=amp | year=2010 | title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal=Biology Letters | doi=10.1098/rsbl.2009.1024 | volume=6 | pages=544–547 | issue=4 | pmid=20106856 | pmc=2936204 }}</ref> and amphibians were further devastated by the [[Permian–Triassic extinction event]].<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |author1=Sahney, S.  |author2=Benton, M.J. | year=2008 | title=Recovery from the most profound mass extinction of all time | journal=Proceedings of the Royal Society B: Biological Sciences | doi=10.1098/rspb.2007.1370 | volume = 275 | pages = 759–65| pmid=18198148 | issue=1636 | pmc=2596898}}</ref> During the [[Triassic|Triassic Period]] (252 to 201 million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the [[biosphere]]. According to the fossil record, [[Lissamphibia]], which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups [[Temnospondyli]] and [[Lepospondyli]] at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating,<ref name="Carroll" /> but the most recent molecular study, based on [[multilocus sequence typing]], suggests a Late Carboniferous/[[Cisuralian|Early Permian]] origin for extant amphibians.<ref>{{cite journal|last=San Mauro|first=D.|year=2010|title=A multilocus timescale for the origin of extant amphibians|journal=Molecular Phylogenetics and Evolution|volume=56|pages=554–561|doi=10.1016/j.ympev.2010.04.019|pmid=20399871|issue=2|bibcode=2010MolPE..56..554S |url=https://www.researchgate.net/publication/43182452|access-date=November 10, 2016|archive-date=December 12, 2019|archive-url=https://web.archive.org/web/20191212091746/https://www.researchgate.net/publication/43182452_A_multilocus_timescale_for_the_origin_of_extant_amphibians|url-status=live}}</ref>

[[File:Eryops - National Museum of Natural History - IMG 1974.JPG|thumb|alt=''Eryops''|The temnospondyl ''[[Eryops]]'' had sturdy limbs to support its body on land]]
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on [[Ribosomal DNA|rDNA]] analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that the divergence of the three groups took place in the [[Paleozoic]] or early [[Mesozoic]] (around 250 million years ago), before the breakup of the supercontinent [[Pangaea]] and soon after their divergence from the lobe-finned fish. The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils.<ref>{{cite journal |author1=San Mauro, Diego |author2=Vences, Miguel |author3=Alcobendas, Marina |author4=Zardoya, Rafael |author5=Meyer, Axel |year=2005 |title=Initial diversification of living amphibians predated the breakup of Pangaea |journal=The American Naturalist |volume=165 |issue=5 |pages=590–599 |doi=10.1086/429523 |pmid=15795855 |bibcode=2005ANat..165..590S |s2cid=17021360 |url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-33053 |access-date=November 10, 2016 |archive-date=February 7, 2017 |archive-url=https://web.archive.org/web/20170207031527/http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-33053 |url-status=live }}</ref> There are large gaps in the [[fossil record]], the discovery of the [[Dissorophoidea|dissorophoid]] temnospondyl [[Gerobatrachus hottoni|''Gerobatrachus'']] from the Early Permian in Texas in 2008 provided a missing link with many of the characteristics of modern frogs.<ref name="Anderson">{{cite journal |author1=Anderson, J. |author2=Reisz, R. |author3=Scott, D. |author4=Fröbisch, N. |author5=Sumida, S. |year=2008 |title=A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders |journal=Nature |volume=453 |issue=7194 |pages=515–518 |doi=10.1038/nature06865 |pmid=18497824 |bibcode=2008Natur.453..515A |s2cid=205212809 |url=https://www.academia.edu/13288317 |access-date=November 10, 2016 |archive-date=July 14, 2021 |archive-url=https://web.archive.org/web/20210714102942/https://www.academia.edu/13288317 |url-status=live }}</ref> [[Molecular phylogenetics|Molecular analysis]] suggests that the frog–salamander divergence took place considerably earlier than the [[Paleontology|palaeontological]] evidence indicates.<ref name="Anderson" /> One study suggested that the last common ancestor of all modern amphibians lived about 315 million years ago, and that [[Stereospondyli|stereospondyl]] temnospondyls are the closest relatives to the caecilians.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2017/06/170619151534.htm|title=Tiny fossils reveal backstory of the most mysterious amphibian alive|website=ScienceDaily|access-date=June 28, 2017|archive-date=November 5, 2019|archive-url=https://web.archive.org/web/20191105163759/https://www.sciencedaily.com/releases/2017/06/170619151534.htm|url-status=live}}</ref> However, most studies support a single [[monophyletic]] origin of all modern amphibians within the dissorophoid temnospondyls.<ref name="Atkins-2019" />

As they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their [[lateral line]] systems and adapted their sensory systems to receive stimuli via the medium of the air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful [[ultraviolet]] rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.{{sfn | Dorit | Walker | Barnes | 1991 | pp=843–859 }}