Introduction

Evolution

The first tetrapods emerged onto land 360 million years ago at the end of the Devonian period, evolving from the sarcopterygian (lobe-finned) fishes through selective forces imposed upon them.
Strong shoulder and pelvic structures, along with well developed limbs, feet and vertebrae, gave them the support and mechanical means for movement on land.
Amphibians resembling modern species appeared early in the Triassic period, 225 million years ago. The earliest fossil with modern frog characteristics appeared in the early Jurassic period, 190 million years.
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Classification

Extant amphibians are split into three lineages, Urodela (Newts and Salamanders), Apoda (Legless amphibians) and Salientia. Frogs are classed under Salientia as Anura, but are the only living class.
Anurans are the most successful and diverse of the amphibians, with 29 presently recognised families and over 3750 species.
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Characteristics

Anurans are relatively small vertebrates, with the largest species being around 30cm and have a number of morphological characteristics, most characteristically the suppresion of the tail in adults. See Fig.1. (Hickman 1997).
Their hind limbs are elongated and they have a powerful pelvic structure, with elongated ilia, which allows various modes of locomotion. They are particularly specialised for jumping, but can also crawl, hop, or run, with some being proficient climbers. The structure of the muscles in the hind limbs allows maximum contraction forces to be achieved and the muscles also shorten faster, generating more power than most animals. (Lutz 1994).
With simultaneous movement of the hind legs and a rigid body they also make powerful swimmers.

Fig.1. Click on a word to highlight that area

They often exhibit sexual dimorphism, with the female being larger than the male in 90% of species, although in 10% the males are larger and may possess tusks or spines.

The sensory systems of anurans are particularly well developed. The eyes are large and well developed, positioned well forward on the head to allow binocular vision and capable of seeing a wide range of colours. Vision is the most important in prey detection, while the ability to see polarised light and recognise landmarks also aids in orientation.
Olfaction is especially acute and also plays a part in prey detection, but its main purpose is in orientation around a home range or towards a breeding site. Most anurans feed on invertebrates by a rapid protrusion of a strong, sticky, tongue. (Pough 1996. Beebee 1996).

Anurans absorb water and gases through their skin, which is kept moist and permeable by secretary mucus glands. The skin also secretes toxins, which in some species can be particularly potent and is used as a defence mechanism, while a number of species are capable of changing colour to help in camouflage and thermoregulation.
Generally they are limited to damp and humid conditions as a means to help keep the skin moist and maintain their water balance. However, this has not restricted them from holding an impressive distribution over all the continents except Antarctica.
They are found in a diverse range of arboreal, aquatic and terrestrial habitats, thanks to a number of behavioural and physiological specialisations.

Reproduction can be divided into two general categories, explosive and prolonged breeders. Migration to and from a breeding site is often seen, particularly from temperate, explosive breeders. Reproduction usually requires some water in which the eggs and larvae (tadpoles) can develop. Most temperate anurans have external fertilisation, with eggs being deposited and larvae developing in lentic or lotic waters. Often bodies of water are small or sparse, so a diversity of strategies have evolved to overcome this problem, the largest variety occurring in the tropical species.
Anurans are much the noisiest of the amphibians, possessing effective vocal and auditory systems, used during breeding, as well as for prey and predator detection. The majority of males have some form of vocal sac in order to amplify their calls.

Linked topics

Arboreal Distribution Diversity Evolution Feeding Migration Skin Terrestrial Vocalisation