Amphibians (class Amphibia, from Amphi- meaning "on both sides" and -bios meaning "life"), such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that metamorphose from a juvenile water-breathing form, either to an adult air-breathing form, or to a paedomorph that retains some juvenile characteristics. Proteidae (mudpuppies and waterdogs) are good examples of paedomorphic species. Though amphibians typically have four limbs, the caecilians are notable for being limbless. Unlike other land vertebrates (amniotes), most amphibians lay eggs in water. Amphibians are superficially similar to reptiles.
Amphibians are ecological indicators, and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct.
Amphibians evolved in the Devonian Period and were top predators in the Carboniferous and Permian Periods, but many lineages were wiped out during the Permianâ€“Triassic extinction. One group, the metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict temnospondyls like Koolasuchus and the modern orders of Lissamphibia.
Amphibian is derived from the Ancient Greek term áĽ€îĽĎ†îŻî˛îąî¿Ď‚ amphĂbios which means both kinds of life, amphi meaning â€śbothâ€ť and bio meaning life. The term was initially used for all kinds of combined natures. Eventually it was used to refer to animals that live both in the water and on land.
 Evolutionary history
The first major groups of amphibians developed in the Devonian Period from fish similar to the modern coelacanth and lungfish which had evolved multi-jointed leg-like fins that enabled them to crawl along the sea bottom. These amphibians were as much as one to five meters in length. However, amphibians never developed the ability to live their entire lives on land, having to return to water to lay their shell-less eggs.
In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position currently occupied by crocodiles. These amphibians were notable for eating the mega insects on land and many types of fishes in the water. During the Triassic Period, the better land-adapted proto-crocodiles began to compete with amphibians, leading to their reduction in size and importance in the biosphere.
 Taxonomic history
Traditionally, amphibians have included all tetrapod vertebrates that are not amniotes. They are divided into three subclasses, of which two are only known as extinct subclasses:
- Subclass Labyrinthodontiaâ€ (diverse Paleozoic and early Mesozoic group)
- Subclass Lepospondyliâ€ (small Paleozoic group, sometimes included in the Labyrinthodontia)
- Subclass Lissamphibia (frogs, toads, salamanders, newts, etc.)
Of these only the last subclass includes recent species.
With the phylogenetic classification Labyrinthodontia has been discarded as it is a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author, whether they use a stem-based or node-based classification. Generally amphibians are defined as the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants. This may also include extinct groups like the temnospondyls (traditionally placed in the subclass â€śLabyrinthodontiaâ€ť), and the Lepospondyls. This means that cladistic nomenklature list a large number of basal Devonian and Carboniferous tetrapod groups, undoubtedly were â€śamphibiansâ€ť in biology, that are formally placed in Amphibia in Linnaean taxonomy, but not in cladistic taxonomy.
All recent amphibians are included in the subclass Lissamphibia, superorder Salientia, which is usually considered a clade (which means that it is thought that they evolved from a common ancestor apart from other extinct groups), although it has also been suggested that salamanders arose separately from a temnospondyl-like ancestor.
Authorities also disagree on whether Salientia is a Superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. Practical considerations seem to favor using the former arrangement now. The Lissamphibia, superorder Salientia, are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of the Lissamphibia, besides the superorder Salientia. Furthermore, Salientia includes all three recent orders plus a single Triassic proto-frog, Triadobatrachus.
- Subclass Lissamphibia
- Superorder Salientia
- Genus Triadobatrachus â€” Triassic (extinct)
- Order Anura (frogs and toads): Jurassic to recent â€” 5,602 recent species in 48 families
- Order Caudata or Urodela (salamanders, newts): Jurassic to recent â€” 571 recent species in 9 families
- Order Gymnophiona or Apoda (caecilians): Jurassic to recent â€” 174 recent species in 3 families
The actual number of species partly also depends on the taxonomic classification followed, the two most common classifications being the classification of the website AmphibiaWeb, University of California (Berkeley) and the classification by herpetologist Darrel Frost and The American Museum of Natural History, available as the online reference database Amphibian Species of the World. The numbers of species cited above follow Frost.
The lungs in amphibians are primitive compared to that of the amniotes, possessing few internal septa, large alveoli and therefore a slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping. However, most amphibians are able to exchange gasses with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularized skin must remain moist in order for the oxygen to diffuse at a sufficient rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog or hellbender salamanders. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.
 Reproductive system
For the purpose of reproduction most amphibians require fresh water. A few (e.g. Fejervarya raja) can inhabit brackish water and even survive (though not thrive) in seawater, but there are no true marine amphibians. Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae (tadpoles or polliwogs) breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults, but there are many interesting exceptions to this general way of reproduction.
The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:
- The gills are replaced by other respiratory organs, i.e., lungs.
- The skin changes and develops glands to avoid dehydration.
- The eyes develop eyelids and adapt to vision outside the water.
- An eardrum is developed to lock the middle ear.
- In frogs and toads, the tail disappears.
Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion. A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan (available at http://www.amphibians.org). Developed by over 80 leading experts in the field, this call to action details what would be required to curtail amphibian declines and extinctions over the next 5 years - and how much this would cost. The Amphibian Specialist Group of the World Conservation Union (IUCN) is spearheading efforts to implement a comprehensive global strategy for amphibian conservation.
On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE), as given by chief Helen Meredith, identified nature's most endangered species: "The EDGE amphibians are amongst the most remarkable and unusual species on the planet and yet an alarming 85% of the top 100 are receiving little or no conservation attention." The top 10 endangered species (in the List of endangered animal species) include: the Chinese giant salamander, a distant relative of the newt, the tiny Gardiner's Seychelles, the limbless Sagalla caecilian, South African ghost frogs, lungless Mexican salamanders, the Malagasy rainbow frog, Chile's Darwin frog (Rhinoderma rufum) and the Betic Midwife Toad.
 Further reading
- Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York: W.H. Freeman & Co..
- Carroll, Robert L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Baltimore: The Johns Hopkins University Press. ISBN 978-0-8018-9140-3.
- Duellman, William E.; Linda Trueb (1994). Biology of Amphibians. Johns Hopkins University Press. ISBN 978-0801847806.
- Frost, Darrel R.; Taran Grant, JuliĂ¡n Faivovich, Raoul H. Bain, Alexander Haas, CĂ©lio F.B. Haddad, Rafael O. De SĂ¡, Alan Channing, Mark Wilkinson, Stephen C. Donnellan, Christopher J. Raxworthy, Jonathan A. Campbell, Boris L. Blotto, Paul Moler, Robert C. Drewes, Ronald A. Nussbaum, John D. Lynch, David M. Green, Ward C. Wheeler (March 2006). "The Amphibian Tree of Life". Bulletin of the American Museum of Natural History 297: 1â€“291. doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2. http://hdl.handle.net/2246/5781.
- Pounds, J. Alan; MartĂn R. Bustamante, Luis A. Coloma, Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, Enrique La Marca, Karen L. Masters, AndrĂ©s Merino-Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo SĂ¡nchez-Azofeifa, Christopher J. Still and Bruce E. Young (January 2006). "Widespread amphibian extinctions from epidemic disease driven by global warming". Nature 439 (7073): 161â€“167. doi:10.1038/nature04246. PMID 16407945. http://www.nature.com/nature/journal/v439/n7073/full/nature04246.html.
- San Mauro, Diego; Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer (May 2005). "Initial diversification of living amphibians predated the breakup of Pangaea". American Naturalist 165 (5): 590â€“599. doi:10.1086/429523. PMID 15795855.
- San Mauro, Diego (2010). "A multilocus timescale for the origin of extant amphibians". Molecular Phylogenetics and Evolution 56 (2): 554â€“561. doi:10.1016/j.ympev.2010.04.019. PMID 20399871.
- Solomon Berg Martin, Biology
- Stuart, Simon N.; Janice S. Chanson, Neil A. Cox, Bruce E. Young, Ana S. L. Rodrigues, Debra L. Fischman, Robert W. Waller (December 2004). "Status and trends of amphibian declines and extinctions worldwide". Science 306 (5702): 1783â€“1786. doi:10.1126/science.1103538. PMID 15486254. http://www.sciencemag.org/cgi/content/full/306/5702/1783.
- S.N.Stuart, M.Hoffmann, J.S.Chanson, N.A.Cox, R.J.Berridge, P.Ramani, B.E. Young (editors), Collective work. (September 2008). Threatened Amphibians of the World. Published by Lynx Edicions, in association with IUCN-The World Conservation Union, Conservation International and NatureServe.. ISBN 978-84-96553-41-5. http://www.hbw.com/lynx/en/lynx-edicions/portada-lynx/MON0017-threatened-amphibians-world.html. 776 pages
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