Amphibian

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For other uses, see Amphibian (disambiguation).

{{taxobox | color=pink | name=Amphibians | image = Caerulea3 crop.jpg | image_width = 230px | image_caption = White's Tree Frog (Litoria caerulea) | regnum = Animalia | phylum = Chordata | subphylum = Vertebrata | classis = Amphibia | classis_authority = Linnaeus 1758 | subdivision_ranks = Orders | subdivision = Subclass Temnospondyli - extinct
Subclass Lepospondyli - extinct
Subclass Lissamphibia
  Anura
  Caudata
  Gymnophiona}} Amphibians (class Amphibia) are a taxon of animals that include all tetrapods (four-legged vertebrates) that do not have amniotic eggs. Amphibians (from Greek αμφις "both" and βιος "life") generally spend part of their time on land, but they do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are about 5,950 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology.

1 History of amphibians
2 Classification
3 Reproduction
4 Amphibian conservation
5 See also
6 References
7 External links

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History of amphibians

Amphibians developed with the characteristics of pharyngeal slits/gills, a dorsal nerve cord, a notochord, and a post-anal tail at different stages of their life. Though early tetrapods (which appeared 390 million years ago in the Devonian period) are often referred to as "amphibians", the first true amphibians (of the subclass Temnospondyli) appeared during the early Carboniferous period. Throughout their history, amphibians have ranged in size from the 5 foot (150cm) long Permian Eryops, and down to the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus iberia from Cuba, with a total length of 9.6-9.8 millimeters (0.4 inches). Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic.

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Classification

Image:Salamandra salamandra CZ.JPG Traditionally the amphibians have included all tetrapods that are not amniotes, but more modern classifications define amphibians as the group that includes frogs and salamanders, and which may also include temnospondyls (traditional placed in the disbanded subclass "labyrinthodontia"). Recent amphibians may all belong to a single subgroup, called the Lissamphibia, though some studies show that salamanders arose seperately from a temnospondyl-like ancestor (Carroll, 1988).

There are two ancient, extinct, subclasses:

Subclass Temnospondyli
Subclass Lepospondyli

Of the remaining modern subclass Lissamphibia there are three orders:

Order Anura (frogs and toads) (in Superorder Salientia): 5,228 species
Order Caudata or Urodela (salamanders): 552 species
Order Gymnophiona or Apoda (caecilians): 171 species

Authorities disagree on whether Salientia is a Superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. In effect Salientia includes all the Anura plus a single Triassic proto-frog species, Triadobatrachus massinoti. Practical considerations seem to favour using the former arrangement now.

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Reproduction

For the purpose of reproduction most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true sea water 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 Image:Caecilian.jpgand 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, bypassing the tadpole stage entirely. 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 get eyelids and adapt to vision outside the water
An eardrum is developed to lock the middle ear
In frogs and toads, the tail disappears

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Amphibian conservation

Template:Main Image:Bufo periglenes1.jpg 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, and disease. However, many of the causes of amphibian declines are still poorly understood, and amphibian declines are currently a topic of much ongoing research.

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References

Duellman/Trueb, Biology of Amphibians
{{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

}}

{{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
| id = Template:Doi
| 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
| title = Initial diversification of living amphibians predated the breakup of Pangaea
| url = http://www.journals.uchicago.edu/AN/journal/issues/v165n5/40546/40546.html
| journal = American Naturalist
| volume = 165
| pages = 590-599
| year = 2005
| month = May

| coauthors = Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer }}

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
| id = Template:Doi

}}

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