Amphibian

taxobox
name=Amphibians or amphibia
fossil_range= Late Devonian - Recent

---

image_width = 300px
image_caption = Western Spadefoot Toad, "Spea hammondii"
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
superclassis = Tetrapoda
classis = Amphibia
classis_authority = Linnaeus, 1758
subdivision_ranks = Subclasses and Orders
subdivision = Order Temnospondyli - "extinct" Subclass Lepospondyli - "extinct" Subclass Lissamphibia Order Anura Order Caudata Order Gymnophiona

Amphibians (class Amphibia), such as frogs, toads, salamanders, newts, gymnophiona, Sirens and amphiumas, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Typically, amphibians have four limbs. Unlike other land animals (amniotes), amphibians lay eggs in water, as their fish ancestors did. Amphibians are superficially similar to reptiles.

In recent decades, there has been a dramatic decline in amphibian populations around the globe and many species are now threatened or extinct. Scientists do not agree on the cause.

Amphibians evolved in the Devonian period. They were a top predator in the Carboniferous Period, but proto-crocodiles evolved and took over that niche.

Taxonomy

Traditionally, amphibians have included all tetrapods that are not amniotes . They are divided into three subclasses, of which two are only known as extinct subclasses:

* Subclass Labyrinthodontia (diverse Paleozoic and early Mesozoic group)
* Subclass Lepospondyli (small Paleozoic group)
* Subclass Lissamphibia (frogs, toads, salamanders, newts, etc.)

Of these only the last subclass includes recent species.

With the phylogenetic revolution, this classification has been modified, or changed, and the Labyrinthodontia discarded as being a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author, and 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, etc) and all their descendants. This may also include extinct groups like the temnospondyls (traditionally placed in the disbanded subclass "labyrinthodontia"), and the Lepospondyls. This means that there are a now large number of basal Devonian and Carboniferous tetrapod groups, described as "amphibians" in earlier books, that are no longer placed in the formal Amphibia.

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 also that salamanders arose separately from a temnospondyl-like ancestor (Carroll, 2007).

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 favour 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
*** Family "Albanerpetontidae" - Jurassic to Miocene (extinct)
** Superorder "Salientia"
*** Genus "Triadobatrachus" - Triassic (extinct)
*** Order "Anura" (frogs and toads): Jurassic to recent - 5,453 recent species in 45 families
*** Order "Caudata" or "Urodela" (salamanders, newts): Jurassic to recent - 560 recent species in 9 families
*** Order "Gymnophiona" or "Apoda" (caecilians): Jurassic to recent - 171 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 (see external links below). The numbers of species cited above follow Frost.

Reproductive system

For the purpose of reproduction most amphibians are bound to have fresh water. A few tolerate brackish water, but there are no true seawater 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 whatsoever. 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.

Conservation

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, 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 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), per 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. [ [http://www.reuters.com/article/latestCrisis/idUSL2038808 Reuters, Giant newt, tiny frog identified as most at risk] ] [ [http://www.guardian.co.uk/environment/2008/jan/21/conservation guardian.co.uk, Drive to save weird and endangered amphibians] ] [ [http://www.guardian.co.uk/environment/gallery/2008/jan/21/wildlife.conservation?picture=332110244 guardian.co.uk/environment, images of the species] ] [ [http://www.guardian.co.uk/environment/gallery/2008/jan/21/wildlife.conservation guardian.co.uk/environment, Gallery: the world's strangest amphibians] ]

Evolutionary history

The first major groups of amphibians developed in the Devonian Period from fish similar to the modern coelacanth where the fins had evolved into legs. These amphibians were around five meters long. The land was safe as the giant fish and sharks in the ocean could not come onto land. However, there were two problems with living out their entire lives on land. Primarily, the food that these amphibians consumed was in the water, but also at this point the skin on most of these amphibians was not water-tight.

In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position where we now find crocodiles. These amphibians were notable for eating the mega-insects on land and many types of fishes in the water. Towards the end of the Permian Period and the Triassic Period, the amphibians started having competition with proto-crocodiles which led to their drop in size in the temperate zones or leaving for the poles. (Amphibians were able to hibernate during the winter whereas crocodiles could not, allowing the amphibians in higher latitudes protection from the reptiles.)

Paleontologists once believed that the kind of lifestyle and adaptations that proto-amphibians was similar to the modern mudskipper.Fact|date=July 2007 (Mudskippers are not closely related to coelocanths.)

See also

*Chytridiomycosis
*Fishapods
*Frog zoology
*List of amphibians
*Prehistoric amphibian
*Sleep in nonhumans
*Tetrapod

References

Further reading

*cite book
last = Carroll
first = Robert L.
title = Vertebrate Paleontology and Evolution
year = 1988
publisher = W.H. Freeman & Co.
location = New York
*cite book
last = Duellman
first = William E.
coauthors = Linda Trueb
title = Biology of Amphibians
year = 1994
publisher = Johns Hopkins University Press
isbn = 978-0801847806
*cite journal
last = Frost
first = Darrel R.
title = The Amphibian Tree of Life
url = http://digitallibrary.amnh.org/dspace/handle/2246/5781
journal = Bulletin of the American Museum of Natural History
volume = 297
pages = 1–291
year = 2006
month = March
coauthors = Taran Grant, Julián Faivovich, Raoul H. Bain, Alexander Haas, Célio F.B. Haddad, Rafael O. De Sá, 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
doi = 10.1206/0003-0090(2006)297 [0001:TATOL] 2.0.CO;2
*cite journal
last = Pounds
first = J. Alan
title = Widespread amphibian extinctions from epidemic disease driven by global warming
url = http://www.nature.com/nature/journal/v439/n7073/full/nature04246.html
journal = Nature
volume = 439
pages = 161–167
year = 2006
month = January
doi = 10.1038/nature04246
coauthors = Martín R. Bustamante, Luis A. Coloma, Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, Enrique La Marca, Karen L. Masters, Andrés Merino-Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo Sánchez-Azofeifa, Christopher J. Still and Bruce E. Young
*cite journal
last = San Mauro
first = Diego
coauthors = Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer
title = Initial diversification of living amphibians predated the breakup of Pangaea
url =
journal = American Naturalist
volume = 165
pages = 590–599
year = 2005
month = May
doi = 10.1086/429523
*Solomon Berg Martin, "Biology"
*cite journal
last = Stuart
first = Simon N.
coauthors = Janice S. Chanson, Neil A. Cox, Bruce E. Young, Ana S. L. Rodrigues, Debra L. Fischman, Robert W. Waller
title = Status and trends of amphibian declines and extinctions worldwide
url = http://www.sciencemag.org/cgi/content/full/306/5702/1783
journal = Science
volume = 306
issue = 5702
pages = 1783–1786
year = 2004
month = December
doi = 10.1126/science.1103538
pmid = 15486254
*cite book
last = S.N.Stuart, M.Hoffmann, J.S.Chanson, N.A.Cox, R.J.Berridge, P.Ramani, B.E. Young (editors)
first = Collective work.
title = Threatened Amphibians of the World
publisher = Published by Lynx Edicions, in association with IUCN-The World Conservation Union, Conservation International and NatureServe.
date = September 2008
url = http://www.hbw.com/lynx/en/lynx-edicions/portada-lynx/MON0017-threatened-amphibians-world.html
id = ISBN 978-84-96553-41-5
pages = 776 pages

External links

* [http://www.amphibians.org/ Amphibian Specialist Group]
* [http://www.amphibianark.org/ Amphibian Ark]
* [http://research.amnh.org/herpetology/amphibia/index.html/ Amphibian Species of the World] The online database by Darrel Frost and The American Museum of Natural History
* [http://www.amphibiaweb.org/ AmphibiaWeb]
* [http://www.globalamphibians.org/ Global Amphibian Assessment]
* [http://www.whose-tadpole.net/ Amphibians of central Europe]
* [http://www.npwrc.usgs.gov/narcam/idguide/index.htm USGS--Online Guide for the Identification of Amphibians in North America north of Mexico]
* [http://www.livingunderworld.org/ General amphibian biology information - Living UnderWorld]
* [http://www.atlantabotanicalgarden.org/site/conservation/amphibian_research/ Atlanta Botanical Garden Amphibian Conservation Program]