Amphibians

                                    Amphibians

                                         

• Amphibians are ectothermic, tetrapod vertebrates of the classAmphibia. They inhabit a wide variety of habitats with most species living within terrestrial, fossorial, arboreal or freshwateraquatic ecosystems. Amphibians typically start out as larva living in water, but some species have developed behavioural adaptations to bypass this. The young generally undergometamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely upon skin. They are superficially similar toreptiles but, along with mammals and birds, reptiles are amniotesand do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
• The earliest amphibians evolved in the Devonian Period fromsarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. The three modern orders of amphibians are Anura (the frogs and toads), Caudata/Urodela (the salamanders), and Gymnophiona/Apoda (the caecilians). The total number of known amphibian species is approximately 7,000, of which nearly 90% are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) Chinese Giant Salamander (Andrias davidianus) but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.

Characteristics

• The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs.^[22] Reptiles, birds and mammals are amniotes, the eggs of which are either laid or carried by the female and are surrounded by several membranes, some of which are impervious.^[23] Lacking these membranes, amphibians require water bodies for reproduction, although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage.^[10] They are not found in the sea with the exception of one or two frogs that live in brackish water in mangrove swamps.^[24] On land, amphibians are restricted to moist habitats because of the need to keep their skin damp.^[10]
  The smallest amphibian (and vertebrate) in the world is a microhylid frog from New Guinea (Paedophryne amauensis) first discovered in 2012. It has an average length of 7.7 mm (0.30 in) and is part of a genus which contains four of the world's ten smallest frog species. The largest living amphibian is the 1.8 m (5 ft 11 in)Chinese Giant Salamander (Andrias davidianus)^[26] but this is a great deal smaller than the largest amphibian that ever existed—the extinct 9 m (30 ft) Prionosuchus, a crocodile-like temnospondyl dating to 270 million years ago from the middle Permian of Brazil.^[The largest frog is the African Goliath frog (Conraua goliath) which can reach 32 cm (13 in) and weigh 3 kg (6.6 lb).^[26]
  Amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through internal physiological processes. Their metabolic rate is low and as a result, their food and energy requirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species have ears that can detect airborne or ground vibrations. They have muscular tongues, which in many species can be protruded. Modern amphibians have fully ossified vertebrae with articular processes. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians. The skin contains many mucous glands and in some species, poison glands. The hearts of amphibians have three chambers, twoatria and one ventricle. They have a urinary bladder and nitrogenous waste products are excreted primarily asurea. Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a pump action in which air is first drawn into thebuccopharyngeal region through the nostrils. These are then closed and the air is forced into the lungs by contraction of the throat. They supplement this with gas exchange through the skin.^]

  [edit]Anura

  

  

  Red-eyed tree frog (Agalychnis callidryas) with limbs and feet specialised for climbing
  The order Anura (from the Latin an- meaning "without" and the Ancient Greek oura meaning "tail") comprises the frogs and toads. They usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin. Members of this order with smooth skins are commonly referred to as frogs, while those with warty skins are known as toads. The difference is not a formal one taxonomically and there are numerous exceptions to this rule. Members of the family Bufonidaeare known as the "true toads".^[29] Frogs range in size from the 30 centimetres (12 in) goliath frog (Conraua goliath) of West Africa^] to the 7.7 millimetres (0.30 in) Paedophryne amauensis, first described in Papua New Guinea in 2012, which is also the smallest known vertebrate.^[31] Although most species are associated with water and damp habitats, some are specialised to live in trees or in deserts. They are found worldwide except for polar areas.
  Anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear. Future molecular studies should provide further insights into their evolutionary relationships.^[33] The suborder Archaeobatrachia contains four families of primitive frogs. These areAscaphidae, Bombinatoridae, Discoglossidae and Leiopelmatidae which have few derived features and are probably paraphyletic with regard to other frog lineages.^[34] The six families in the more evolutionarily advanced suborder Mesobatrachia consist of the fossorial Megophryidae, Pelobatidae, Pelodytidae, Scaphiopodidae andRhinophrynidae and the obligatorily aquatic Pipidae. These have certain characteristics that are intermediate between the two other suborders.^[34] Neobatrachia is by far the largest suborder and includes the remaining families of modern frogs, including most common species. 96% of the over 5,000 extant species of frog are neobatrachians.