AMPHEBIANS

Amphibians (class Amphibia , from Amphi- meaning "on both sides" and - bios meaning "life") are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic (or cold-blooded) tetrapods. Most Amphibians undergo metamorphosis from a juvenile water-breathing form to an adult air-breathing form, but some are paedomorphs that retain the juvenile water-breathing form throughout life. 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), and in total they number approximately 6,500 species.[1] Many amphibians lay their eggs in water. Amphibians are superficially similar to reptiles, but reptiles are amniotes, along with mammals and birds. The study of amphibians is called batrachology. Amphibians are ecological indicators,[2] and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct. The earliest amphibians evolved in the Devonian period from lobe-finned fish that used their strong, bony fins to venture onto dry land. [3] They were the top predators in the Carboniferous and Permian periods,[4] but they later faced competition from their descendants, the reptiles, and 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. 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.[5] Evolutionary history Main article: Labyrinthodontia See also: List of prehistoric amphibians The first major groups of amphibians developed in the Devonian period from lobe- finned fish similar to the modern coelacanth and lungfish,[3] which had evolved multi-jointed leg-like fins that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breath air when the stagnant pools of the Devonian swamps were lacking in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances required it. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. Amphibians evolved adaptations which allowed them to stay out for longer periods. However, they never developed the ability to live their entire lives on land, having a fully aquatic tadpole stage and still needing to return to water to lay their shell-less eggs. The first true amphibians appeared in the Carboniferous Period, by which time they were already moving up the food chain and occupying the ecological position currently claimed by such animals as crocodiles. Amphibians were once the top land predators, sometimes reaching several meters in length, preying on the large insects on land and many types of fish in the water. During the Triassic Period, the better-adapted reptiles began to compete with amphibians, leading to the reduction of their size and importance in the biosphere. Lissamphibia, which includes all modern amphibians and is the only surviving lineage of amphibians left, could have branched off from the extinct groups Temnospondyli and Lepospondyli anytime between the mid- Permian to the early Triassic, but the relative scarcity of fossil evidence does not permit an exact date. [4] 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, which may actually be more closely related to amniotes than Lissamphibia) 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. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is by no means satisfactory understood, and lissamphibia may possibly include extinct groups like the temnospondyls (traditionally placed in the subclass “Labyrinthodontia”), and the Lepospondyls, and in some analysis even the amniotes. This means that phylogenetic nomenclature 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, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniots.[6][7] 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 Subclass Lissamphibia Family Albanerpetontidae — Jurassic to Miocene (extinct) Superorder Salientia Genus Triadobatrachus — Triassic (extinct) — A stem Anuran Order Anura ( frogs and toads): Jurassic to recent — 5,602 recent species in 48 families Order Caudata or Urodela (salamanders, newts ): Jurassic to recent — 571 recent species in 9 families Order Gymnophiona or Apoda (caecilians): Jurassic to recent — 174 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. [8] 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 and 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.