Amphibians such as frogs, toads,, salamanders and limbless ones, are cold-blooded animals that metamorphose from water-breathing tadpole to an air-breathing adult. There are 6184 species of amphibians today. The West African goliath frog, which is 30 cm long and weighs 3.3 kilograms, is the largest anuran and the smallest is the Brazilian brachycephalid tree frog, which reaches a length of only 9.8 mm. Frogs and toads show the greatest diversity in tropics while salamanders inhabit the northern hemisphere in cool, moist forests. Caecilians are found throughout the tropics, except in Madagascar.
For the purpose of reproduction most amphibians are dependent on fresh water, a few tolerate brackish water but there are no true saltwater amphibians. 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, namely, the gills are replaced by lungs; the skin develops glands to avoid dehydration on land; the eyes develop eyelids and adapt to terrestrial vision; an eardrum is developed to transmit vibrations from air; tail disappears.
Amphibians are divided into four subclasses, of which three are extinct.
Subclass Labyrinthodontia – Paleozoic and early Mesozoic group. Earliest ancestors with labyrinthine folds in teeth. Extinct.
Subclass Lepospondyli – Paleozoic group with scale-like ossification in vertebrae. Ancestors of caecilians. Extinct.
Subclass Phyllospondyli – Paleozoic ancestors of urodeles and anurans with leaf-like ossification around the notochord. They had short body and tail. Extinct.
Subclass Lissamphibia – frogs, toads, salamanders, newts, etc. Living groups which have smooth scaleless skin. Modern amphibians.
Labyrinthodonts are classified into three orders which are all extinct.
Order Ichthyostegalia includes Tiktaalik, Acanthostega and Ichthyostega.
Order Temnospondyli includes giant amphibians like Eryops, Cacops, Buettneria.
Order Anthracosauria includes ancestors of reptiles such as Seymoria.
Order ANURA or SALIENTIA – Frogs and Toads. Total 5453 species.
Frogs and toads are tail-less amphibians that have large, well-developed eyes, short, thick bodies and four legs. Frogs have long, powerful hind legs that are well adapted for leaping and swimming. Tree frogs have suction pads on their fingers and toes for climbing. Toads have shorter legs than frogs adapted for hopping or walking instead of leaping. The skin of toads has a dry, warty appearance.
Family Alytidae – Painted Frogs or Disc-Tongued frogs, 11 species.
Family Bombinatoridae – Fire Belly toads, 10 species.
Family Leiopelmatidae – New Zealand primitive frogs, 6 species.
Family Megophryidae – Litter frogs or Short Legged toads, 137 species
Family Pelobatidae – European Spade-foot toads, 4 species
Family Pelodytidae – Parsley frogs, 3 species
Family Pipidae – Tongueless frogs or clawed frogs, 31 species.
Family Rhinophrynidae – Mexican Burrowing Toad, 1 species
Family Scaphiopodidae – American spade-foot toads, 7 species
Family Arthroleptidae – Screeching frogs or squeakers, 132 species
Family Brachycephalidae – Saddleback toads, 823 species.
Family Bufonidae – True toads, over 500 species
Family Centrolenidae – Glass frogs, 146 species
Family Dendrobatidae – Poison dart frogs, 264 species
Family Heleophrynidae – Ghost frogs, 6 species
Family Hemisotidae – Shovelnose frogs, 9 species
Family Hylidae – Tree frogs, 894 species
Family Hylodidae – 39 species
Family Hyperoliidae – Sedge frogs or bush frogs, 207 species
Family Leptodactylidae – Southern frogs or tropical frogs, over 330 species.
Family: Limnodynastidae, 44 species
Family Mantellidae, 166 species
Family Micrixalidae – 11 species, sometimes placed in the family Ranidae.
Family Microhylidae – Narrow-mouthed frogs, 452 species
Family Myobatrachidae – Australian ground frogs, 82 species.
Family Nyctibatrachidae – 13 species
Family: Petropedetidae, 16 species
Family Ranidae – True frogs, over 750 species
Family Rhacophoridae – Moss frogs, 286 species
Order CAUDATA or URODELA – Tailed amphibians. Total 560 species.
Salamanders have tail, short bodies, four almost equal legs and moist skin. They are often brightly colored and have great powers of regeneration. Most salamanders are about 4 to 6 inches long but the Chinese giant salamander (Megalobatrachus) grows to 5 feet in length. A tiny species in Mexico measures only about 1 inch.
Family Cryptobranchidae – Giant salamanders, 3 species
Family Hynobiidae – Asiatic salamanders, 51 species
Family Ambystomatidae – Mole salamanders, 37 species.
Family Amphiumidae – Amphiumas or Congo eels, 3 species
Family Plethodontidae – Lungless salamanders, 378 species
Family Proteidae – Mudpuppies and Waterdogs, 6 species
Family Rhyacotritonidae – Torrent salamanders, 4 species
Family Salamandridae – True salamanders and newts, 74 species
Family Sirenidae – Sirens, 4 species
Order GYMNOPHIONA or APODA or CAECILIA – Caecilians or limbless amphibians. Total 170 species
Caecilians look like worms or snakes with a short pointed tail. They are well adapted for burrowing in moist sand. The long, slender body has numerous grooves. The eyes are small and covered by skin. There is a tentacle that is a tactile organ. Caecilian species range in length from about 10 cm to nearly 1.5 meters.
Family Caeciliidae – Common caecilians, 121 species
Family Ichthyophiidae – Fish caecilians, 41 species
Family Rhinatrematidae – Beaked caecilians, 9 species