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Amphibians
Fossil range: Late Devonian–present
File:Spea hammondii
Western Spadefoot Toad, Spea hammondii
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Tetrapoda
Class: Amphibia
Linnaeus, 1758
Subclasses and Orders

   Order Temnospondyliextinct
Subclass Lepospondyliextinct
Subclass Lissamphibia
   Order Anura
   Order Caudata
   Order Gymnophiona

Amphibians (class Amphibia, from Amphi- meaning "on both sides" and -bios meaning "life"), such as frogs, salamanders, 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. Mudpuppies, for example, retain juvenile gills in adulthood. The three modern orders of amphibians are Anura (frogs and toads), Caudata (salamanders and newts), and Gymnophiona (caecilians, limbless amphibians that resemble snakes). Like the fish they evolved from, most amphibians lay eggs in water. Amphibians are superficially similar to reptiles, but reptiles are amniotes, along with mammals and birds.

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 Permian–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.

Contents

Etymology

Amphibian is derived from the Ancient Greek term ἀμφίβιος amphíbios which means both kinds of life, amphi meaning “both” 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.[1]

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 Labyrinthodontia† (diverse Paleozoic and early Mesozoic group)
  • Subclass Lepospondyli† (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 “Labyrinthodontia”), and the Lepospondyls. This means that cladistic nomenklature list a large number of basal Devonian and Carboniferous tetrapod groups, undoubtedly were “amphibians” 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.[2]

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.

Class Amphibia

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.[3] The numbers of species cited above follow Frost.

Respiration

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

from the San Antonio zoo]]

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.

Conservation

of Monteverde, Costa Rica was among the first casualties of amphibian declines. Formerly abundant, it was last seen in 1989.]]

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. Amphibian Ark is an organization that was formed to implement the ex-situ conservation recommendations of this plan, and they have been working with zoos and aquaria around the world encouraging them to create assurance colonies of threatened amphibians. One such project is the Panama Amphibian Rescue and Conservation Project that built on existing conservation efforts in Panama to create a country-wide response to the threat of chytridiomycosis rapidly spreading into eastern Panama [4]

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.[5][6][7][8]

References

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. 

See also

External links


frr:Amphibie


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

Contents

English

Pronunciation

  • enPR: ăm'fĭb"ĭ-ən, IPA: /ˈæmˌfɪbɪən/, SAMPA: /"{m%fIbI@n/
  •  Audio (US)help, file

Noun

Singular
amphibian

Plural
amphibians

amphibian (plural amphibians)

  1. Any four-legged vertebrate that does not have amniotic eggs, living both on land and in water.

Translations

Related terms

See also


Simple English

Amphibians
Fossil range: Carboniferous – Recent
File:Eryops megacephalus skeleton
Eryops, a Carboniferous amphibian
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Tetrapoda
Class: Amphibia
Linnaeus, 1758
Subclasses and Orders
†: extinct
File:Oophaga.pumilio.
Strawberry Poison-dart Frog, Oophaga pumilio. Like many living amphibia, poison and warning colouration protects it.

Amphibians are a group of four-legged animals. They form the class Amphibia (Amphi- = both; bios = life). They live in shallow water and on land, and must lay their eggs in water. They have a larval stage in water, and the adult stage usually lives mostly on land.

Early amphibia, from the Carboniferous, were quite large predators with flat skulls. The amphibia which survive today are much smaller and more specialised forms. Frogs and toads (Anura), newts and salamanders (Caudata) and caecilians are all amphibians.

As amphibia grow from a larva to an adult, they change shape. This is metamorphosis. They lose their gills and tails, and grow front and hind legs. Larval frogs, toads or salamanders are called tadpoles.

Adult females lay as many as 4000 eggs, usually in the water or in wet places. They eat other animals, especially insects. Amphibians are cold blooded, which means that their body temperature does not adapt to the weather.

In terms of species numbers, amphibians are still quite successful. They are just limited in the habitats they can live in. There are about 5,700 living species of amphibia. For comparison, there are about 4,000 mammalian species.

Examples

Taxonomy

Look up Amphibia in Wikispecies, a directory of species
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frr:Amfiibie


Citable sentences

Up to date as of December 17, 2010

Here are sentences from other pages on Amphibian, which are similar to those in the above article.








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