Frog

A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (literally without tail in Ancient Greek). The oldest fossil "proto-frog" appeared in the early Triassic of Madagascar, but molecular clock dating suggests their origins may extend further back to the Permian, 265 million years ago. Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforests. There are over 7,000 recorded species, accounting for over 85% of extant amphibian species. They are also one of the five most diverse vertebrate orders. Warty frog species tend to be called toads, but the distinction between frogs and toads is informal, not from taxonomy or evolutionary history.

An adult frog has a stout body, protruding eyes, anteriorly-attached tongue, limbs folded underneath, and no tail (except in tailed frogs). Frogs have glandular skin, with secretions ranging from distasteful to toxic. Their skin varies in colour from well-camouflaged dappled brown, grey and green to vivid patterns of bright red or yellow and black to show toxicity and ward off predators. Adult frogs live in fresh water and on dry land; some species are adapted for living underground or in trees.

Frogs typically lay their eggs in water. The eggs hatch into aquatic larvae called tadpoles that have tails and internal gills. They have highly specialized rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets. The life cycle is completed when they metamorphose into adults. A few species deposit eggs on land or bypass the tadpole stage. Adult frogs generally have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frog skin has a rich microbiome which is important to their health. Frogs are extremely efficient at converting what they eat into body mass. They are an important food source for predators and part of the food web dynamics of many of the world's ecosystems. The skin is semi-permeable, making them susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. Frogs produce a wide range of vocalizations, particularly in their breeding season, and exhibit many different kinds of complex behaviours to attract mates, to fend off predators and to generally survive.

Frogs are valued as food by humans and also have many cultural roles in literature, symbolism and religion. Frog populations have declined significantly since the 1950s. More than one third of species are considered to be threatened with extinction and over 120 are believed to have become extinct since the 1980s. The number of malformations among frogs is on the rise and an emerging fungal disease, chytridiomycosis, has spread around the world. Conservation biologists are working to understand the causes of these problems and to resolve them.

Etymology and taxonomy
The use of the common names "frog" and "toad" has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term "toad" generally refers to species that are terrestrial with dry, warty skins. There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.

Etymology
The origin of the order name Anura — and its original spelling Anoures — is the Ancient Greek "alpha privative" prefix ἀν- (an-) "without", and οὐρά (ourá), meaning "animal tail". It refers to the tailless character of these amphibians.

The origins of the word frog are uncertain and debated. The word is first attested in Old English as frogga, but the usual Old English word for the frog was frosc (with variants such as frox and forsc), and it is agreed that the word frog is somehow related to this. Old English frosc remained in dialectal use in English as frosh and frosk into the nineteenth century, and is paralleled widely in other Germanic languages, with examples in the modern languages including German Frosch, Icelandic froskur, and Dutch (kik)vors. These words allow us to reconstruct a Common Germanic ancestor *froskaz. The third edition of the Oxford English Dictionary finds that the etymology of *froskaz is uncertain, but agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of *preu = "jump".

How Old English frosc gave rise to frogga is, however, uncertain, as the development does not involve a regular sound-change. Instead, it seems that there was a trend in Old English to coin nicknames for animals ending in -g, with examples—themselves all of uncertain etymology—including dog, hog, pig, stag, and (ear)wig. Frog appears to have been adapted from frosc as part of this trend.

Meanwhile, the word toad, first attested as Old English tādige, is unique to English and is likewise of uncertain etymology. It is the basis for the word tadpole, first attested as Middle English taddepol, apparently meaning 'toad-head'.

Taxonomy
About 88% of amphibian species are classified in the order Anura. These include over 7,000 species in 56 families, of which the Craugastoridae (831 spp.), Hylidae (720 spp.), Microhylidae (670 spp.), and Bufonidae (610 spp.) are the richest in species.

The Anura include all modern frogs and any fossil species that fit within the anuran definition. The characteristics of anuran adults include: 9 or fewer presacral vertebrae, the presence of a urostyle formed of fused vertebrae, no tail, a long and forward-sloping ilium, shorter fore limbs than hind limbs, radius and ulna fused, tibia and fibula fused, elongated ankle bones, absence of a prefrontal bone, presence of a hyoid plate, a lower jaw without teeth (with the exception of Gastrotheca guentheri) consisting of three pairs of bones (angulosplenial, dentary, and mentomeckelian, with the last pair being absent in Pipoidea), an unsupported tongue, lymph spaces underneath the skin, and a muscle, the protractor lentis, attached to the lens of the eye. The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.

Frogs and toads are broadly classified into three suborders: Archaeobatrachia, which includes four families of primitive frogs; Mesobatrachia, which includes five families of more evolutionary intermediate frogs; and Neobatrachia, by far the largest group, which contains the remaining families of modern frogs, including most common species throughout the world. The Neobatrachia suborder is further divided into the two superfamilies Hyloidea and Ranoidea. This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated.

Some species of anurans hybridize readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (P. lessonae) and the marsh frog (P. ridibundus). The fire-bellied toads Bombina bombina and B. variegata are similar in forming hybrids. These are less fertile than their parents, giving rise to a hybrid zone where the hybrids are prevalent.
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