Amphibian

Amphibians, such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (cold-blooded) animals that either metamorphose from a juvenile water-breathing form (tadpoles), to an adult air-breathing form, or paedomorph and retain some juvenile characteristics. Mudpuppies and waterdogs are good examples of paedomorphic species. Adult amphibians typically have four limbs, and a moist, permeable skin; although caecilians are limbless. Amphibians first evolved around 360 mya, in the Late Devonian period.

Evolutionary History
Amphibians likely originated from lobe-finned fish, such as Eusthenopteron and Panderichthys. These fish were similar to the modern lung fish in that they had large, muscular fins that they used to move about in muddy shallows. As with the modern lung fish, the early lobe-finned fish were probably able to use their swim bladder as a pair of simple lungs that allowed them to breathe air for short periods of time. Eventually, the lobe-finned fish began to produce more amphibian-like fish such as Tiktaalik, which may have been able to push itself up onto the land in pursuit of prey. Tiktaalik and other similar lobe-finned fish eventually became the first amphibians, sch as Icthyostega, which could only stay on land for a few minutes. Amphibians began to diversify at the beginning of the Carboniferous period, producing two main groups, the labyrinthodonts and the Temnospondyls, the latter group in turn gave rise to the first reptiles, the reptiliomorphs.

Amphibians began to decline in the Permian due to the drier climate and the diversifying reptiles that outcompeted the amphibians, almost to the group's extinction. The temnospondyls, however, survived and gave rise to the modern amphibians (the Lissamphibians), during the Triassic period. The temnospondyls died out, along with the labyrinthodonts, in the K-T mass extinction event that also claimed the dinosaurs, pterosaurs, and marine reptiles. The lissamphibians quickly became the dominant form of amphibians, eventually outcompeting the other surviving forms of amphibian, although they did not take control of the planet as they were still in direct competition with reptiles and mammals. The modern amphibians consist of frogs, toads, salamanders, newts, waterdogs, and caecilians.

Amphibian varieties
Although only 21% of all amphibians that ever existed are currently extant, they are still extremely varied.

Frogs and toads


Most frogs and toads are characterized by long hind legs, a short body, webbed digits (fingers or toes), protruding eyes and the absence of a tail. Frogs are widely known as exceptional jumpers, and many of the anatomical characteristics of frogs, particularly their long, powerful legs, are adaptations to improve jumping performance. Due to their permeable skin, frogs are often semi-aquatic or inhabit humid areas, but move easily on land. They typically lay their eggs in puddles, ponds or lakes, and their larvae, called tadpoles, have gills and develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.

Salamanders and newts
Salamanders and newts consist of 500 spcies of amphibian, which look much like the Lizards in that they have long, tapering tails, four limbs, and narrow bodies. Salamanders and newts can be distinguished from lizards in having a slimy skin lacking scales; they also spend significantly more time in water; however some must remain on land in order to survive, such as the marbled salamander. Although they belong to the same group, newts generally spend more time in water than salamanders; some have evolved to become completely aquatic. These creatures, too, begin their lives as tadpoles.

Caecilians


The caecilians are a highly unusual order of amphibians that resemble earthworms or snakes. They mostly live hidden in the ground, which makes them one of the least known order of amphibians.

Lepospondyls
Lepospondyls were a group of Carboniferous-Permian amphibians, classified as Labyrinthodonts and were quite varied, containing at least six different clades. Most lepospondyls were slender and newt-like in design. The lepospondyls included some forms that are fairly well known, such as Diplocaulus. The lepospondyls were, as a general rule, quite small, with the biggest forms reaching a metre or so in length.

Temnospondyls
Temnospondyls were some of the most diverse ever amphibians, vaying widely in size and body design. They were extremely long-lasting, lasting from the Carboniferous to the Cretaceous period, and contained such well-known forms as Eryops. The temnospondyls had a worldwide range up to the Triassic period, at which point they began to dissapear, possibly due to competition from aquatic reptiles.

Reproduction
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.

Most amphibians are dedicated parents. Salamanders and frogs, in particular, guard their eggs from predators and rivals, and a few also care for their hatched tadpoles. Like many reptiles, the tadoples stick to the backs of their parents for transportation, again mostly in the case of frogs and salamanders.

Tadpoles
Tadpoles are young amphibians that live in the water. During the tadpole stage of the amphibian life cycle, most respire by means of external or internal gills. Tadpoles can be distinguished from fish by their bulbous shape, and the presence of flaps that are the beginnings of limbs. Like fish, however, tadpoles hatch from jelly-like eggs. Because tadpoles are born lacking lungs, most amphibians go to great lengths to ensure that the eggs hatch in water.

Tadpole Anatomy
As a tadpole matures, it most commonly metamorphosizes by gradually growing limbs (usually the legs first, followed by the arms) and then (most commonly in the case of frogs) outwardly absorbing its tail by apoptosis. Lungs develop around the time of leg development, and tadpoles late in development will often be found near the surface of the water, where they breathe air. During the final stages of external metamorphosis, the tadpole's mouth changes from a small, enclosed mouth at the front of the head to a large mouth the same width as the head.

Conservation Status
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.