What Are Amphibians?
The word amphibians comes from two Greek words: amphi meaning “of both kinds” and bios meaning “life”. Amphibians are a class of vertebrate (back-boned) animals that can live both in water and on land. They are descended from fishes that lived more than 300 million years ago.
The first amphibians to crawl out of the water were heavily built, and slow and clumsy on land, but more active in water. They had long bodies and tails, and some developed into the highly efficient class of reptiles.
About 160 million years ago many amphibians became extinct. But a few survived to develop into the present-day frogs, toads, newts, salamanders and the worm-like caecilians. Modern amphibians usually have moist, tough skins. They breathe partly through their skins, although they also have lungs, they usually lay their eggs in water. Here the young live, breathing chiefly through gills, until they change into their adult forms.
They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioral adaptations to bypass this. They eat insects, snails, worms and similar food, and are eaten by fish, snakes and birds.
They are usually small. But the Giant Salamander (Andrias davidianus), of the Far East is 5 feet 11 inches long but is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil, and the Giant Frog of West Africa grows to a mature length of almost one foot. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
With their complex reproductive needs and permeable skins, amphibians are often ecological indicators; 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 from sarcopterygian 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), Urodela (the salamanders), and Apoda (the caecilians). The 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).