Anura

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Anurans represent, by far, the most speciose, diverse, and widespread of the three extant amphibian orders. They are found throughout most of the world, except in polar regions, and some oceanic islands and extremely xeric deserts. Anuran diversity is greatest in the tropics. Twenty-five families are currently recognized, representing more than 4,000 species, with more being discovered regularly. This clade is also referred to as Batrachia and is placed in superorder Salientia.

There is no scientific distinction between "frogs" and "toads," although most anurans are usually referred to as one or the other. Anurans have several synapomorphies that distinguish them from other amphibians. The name, Anura, meaning "without tail," identifies one of these: with one exception (Ascaphus), adult frogs do not have tails. Anurans also have nine or fewer presacral vertebrae (usually eight), and the three or four posterior to the sacrum are fused into a rod called the urostyle. Both the radius and ulna (forearm bones), and the tibia and fibula (shank bones), are fused to each other. Furthermore, the astragalus and calcaneum, ankle bones, are greatly elongate, providing an additional level in the legs of frogs, which they utilize in jumping. The saltatory locomotion by which many recognize frogs is aided, and perhaps even permitted, by these many morphological adaptations. Several skull bones are lacking in frogs, although their heads remain highly ossified. The dentary usually lacks teeth, but the maxilla and premaxilla are usually tooth-bearing. The tongue is often large, and free posteriorly. Males of most species have vocal sacs. The tadpole stage of many anuran life histories is also unique to frogs, with several specializations such as internal gills and the absence of true teeth.

Most anurans have external fertilization, and adopt a mating posture called amplexus to insure contact between eggs and sperm. There are exceptions even to this rule of anuran reproduction, which points to the extreme diversity of reproductive modes found in this animals. The primitive mode involves large numbers of aquatic eggs and feeding larvae, but alternate modes include but are not limited to terrestrial eggs with aquatic larvae, direct development (in which there is no tadpole stage), viviparity, and non-feeding aquatic larvae. Approximately 10% of anurans exhibit some form of parental care. Additionally, many species are highly territorial, defending nests, oviposition sites, or other resources. Many temperate species breed "explosively," congregating in large numbers around water for only a night or two each year, during which time all mating takes place. Some tropical species breed year-round. Anurans are found from tropical rainforests to dry mountaintops, from deserts to swamps. Adults may be arboreal, terrestrial, aquatic, or fossorial. Unlike caecilians and salamanders, no anurans are fully paedomorphic. Most anurans are nocturnal. In the winter, many temperate anurans enter a state of torpor to avoid freezing; in arid regions, frogs may bury themselves underground, or minimize the cutaneous respiration (and thus the water loss) that usually identifies frogs.

Despite extensive research on the evolutionary history of amphibians, phylogenetic relationships among the three orders of extant amphibians remain problematic. Of three possible histories, the only one that has not been seriously considered is an Anura - Gymnophiona (caecilians) sister relationship, with Caudata (salamanders) sister to that group. A salamander-caecilian clade (with Anura sister to that) is supported by soft anatomical characters and ribosomal DNA sequences. Osteological characters support a salamander-frog clade, as does a combination of morphological and molecular evidence. There is little doubt that Anura, and Salientia, are monophyletic. Each group is supported by several synapomorphies. Within Anura, however, and especially among the neobatrachians ("higher frogs"), very few historical relationships are resolved. Anuran phylogenies remain highly contentious.

Fossil anurans are known from the Jurassic in Europe, North America, and South America, extending through the Pleistocene. Given the extensive radiation of many anuran clades, the fossil record is somewhat poor, with several families lacking any fossil representatives. Several fossil genera have not been assigned to recognized families. The superorder Salientia includes the fossil "proto-frog" Triadobatrachus, from Madagascar, and Anura.

Cannatella, D., L. Ford, and L. Bockstanz. 1996. Salientia: Tree of Life. (Website.) http://tolweb.org/tree?group=Salientia&contgroup=Living_Amphibians

Cogger, H. G., and R. G. Zweifel, editors. 1998. Encyclopedia of Reptiles and Amphibians, 2nd edition. Academic Press, San Diego.

Duellman, W. E., and L. Trueb. 1986. Biology of Amphibians. Johns Hopkins University Press, Baltimore, MD.

Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. 1998. Herpetology. Prentice-Hall, Inc., Upper Saddle River, NJ.

Stebbins, R. C., and N. W. Cohen. 1995. A natural history of amphibians. Princeton University Press, Princeton.

Zug, G. R. 1993. Herpetology: an introductory biology of amphibians and reptiles. Academic Press, San Diego.

Contributors

Heather Heying (author).

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.