Amphibian

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Amphibians (class Amphibia; from Greek αμφις "both" and βιος "life") are a taxon of animals that include all living tetrapods (four-legged vertebrates) that do not have amniotic eggs, are ectothermic (term for the animals whose body heat is regulated by the external environment; previously known as cold-blooded), and generally spend part of their time on land. Most amphibians do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are around 6,200 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology. Amphibians are able to breathe through their skin.

[edit] Classification

See also: Prehistoric amphibian

Traditionally the 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 cladistic revolution, this classification has been modified, 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.

[edit] Systems

[edit] Reproductive

For the purpose of reproduction most amphibians are bound to 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.

Interestingly, recent scienctific evidence put forward by East Carolina University suggests that some species of frogs, like the Dendrobates captivus, often engage in homosexual activity, and as a result their reproduction rate per year has been slowly decreasing.^{[citation needed]}

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.

[edit] Conservation

Main article: Decline in amphibian populations

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.

[edit] Evolutionary history

The first major groups of amphibians developed in the Devonian Period from fishes similar to the modern coelacanth where the fins had evolved into legs. These amphibians were around five meters long in length, which is rare now except for some species of Japanese Salamander. The land was safe as the giant fishes 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.)

The modern mudskipper provides a rough glimpse into the kind of lifestyle and adaptations that proto-amphibians may have taken.^{[citation needed]} (Mudskippers are not closely related to coelocanths.)

[edit] See also

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

[edit] Further reading

[edit] External links

• Amphibian Ark
• Amphibian Species of the World The online database by Darrel Frost and The American Museum of Natural History
• Declining Amphibian Population Task Force
• AmphibiaWeb
• Global Amphibian Assessment
• Amphibians of central Europe
• USGS--Online Guide for the Identification of Amphibians in North America north of Mexico
• General amphibian biology information - Living UnderWorld

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