Amphibian

Amphibians (class Amphibia) are a taxon of animals that include all tetrapods, four-legged vertebrates, that do not have amniotic eggs.(from Greek αμφις "both" and βιος "life") are ectotherms, and generally spend part of their time on land, but most do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are around 6,000 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology.

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 order Temnospondyli) appeared during the early Carboniferous period. During the late Carboniferous, Permian and Triassic periods, amphibians were extremely diverse, including many large and small forms, some newt and salamander-like, others resembling snakes or eels, and some large-snouted forms that were very like small (about 1.5 meter long) crocodiles (e.g. Archegosaurus). The drying out of the coal swamps during the latest Carboniferous and again at the end of the Early Permian diminished many of the environments of these Paleozoic amphibians, with the result that many types died out, and they were supplanted by reptiles. However both large and small amphibians still continued to flourish in rivers and lakes of the Late Permian, alongside the diverse therapsids or mammal-like reptiles that were the dominant land animals of that time. During the following, Triassic period, there were many genera with large, sometimes very flattened heads and very weak limbs; some of these, like Paracyclotosaurus, Cyclotosaurus and Mastodonsaurus were several meters in length. Apart from a few stragglers, all these large amphibians died out at the Triassic-Jurassic extinction event, and the majority of Jurassic amphibians belonged to modern groups, and would look familiar today.

Throughout their history, amphibians have ranged in size from large forms, such as the above mentioned Triassic genera, the 5 foot (150cm) long Eryops of the Permian period, and the Giant Japanese and Chinese Salamanders of today, down to the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus iberia (leptodactylid frog) 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.

Classification

Fire Salamander (Salamandra salamandra)

Traditionally the amphibians have included all tetrapods that are not amniotes. They are divided into three subclasses:
* Subclass Labyrinthodontia (diverse Paleozoic and early Mesozoic group)
* Subclass Lepospondyli (small Paleozoic group)
* Subclass Lissamphibia (frogs, salamanders, etc)

Of these only the last 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 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" of earlier books, that are no longer placed in the formal Amphibia.

All recent amphibians are included in the Lissamphibia, which is usually considered a clade (which means that it is thought that all Lissamphibians 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, 1988).

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. 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.

The Lissamphibia are traditionally divided into three orders, but an extinct salamander-like group, the Albanerpetontidae, is now considered in addition to the other three groups.
* Family Albanerpetontidae - Jurassic to Miocene (extinct)
* Superorder Salientia
** Triadobatrachus (Triassic)
** Order Anura (frogs and toads) Jurassic to recent - 5,296 recent species
* Order Caudata or Urodela (salamanders): Jurassic to recent - 555 recent species
* Order Gymnophiona or Apoda (caecilians): Jurassic to recent - 171 recent species

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

Caecilian from the San Antonio zoo

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, 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

Amphibian conservation

The Golden toad 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, 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.

References

*Carroll, R. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman & Co., New York
*Duellman/Trueb, Biology of Amphibians
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Amphibian: Encyclopedia BETA

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