Caudata

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Salamanders are distributed throughout most of the holarctic region, with the greatest diversity in the United States. Of ten currently recognized extant families (comprising more than 420 extant species), all but one (Hynobiidae) are represented in North America. Five families are restricted to the temperate Western hemisphere. Representatives of the remaining four families are also found in Europe and/or Asia, and one family has radiated into tropical America (Plethodontidae).

Caudates are easily distinguished from other amphibians on the basis of several morphological characters, including the presence of a tail in all larvae, juveniles and adults. Limbs extend at right angles from the elongate, cylindrical body, and hindlimbs and forelimbs tend to be of approximately equal size (with some notable exceptions, such as Sirenidae, which lack hindlimbs). The otic notch and middle ear are absent, as are several skull bones, including but not limited to the postorbital, postparietal, jugal, basioccipital, and ectopterygoid. When present, aquatic larvae have true teeth on both upper and lower jaws, gill slits and external gills. Salamanders have the largest genomes of any tetrapods. Adult sizes range from less than one inch to almost six feet.

Salamanders exhibit a wide range of life history and courtship patterns. Basal, so-called "primitive" salamanders have external fertilization (Cryptobranchoidea, and probably Sirenoidea), but the majority of extant families (Salamandroidea) have internal fertilization. Copulatory organs do not exist in any salamanders. In internally fertilizing salamanders, specialized cloacal glands in males produce spermatophores, pyramidal structures capped with sperm. Females pick the sperm cap up, and eggs are usually fertilized as they pass through the cloaca. Most salamanders deposit aquatic eggs, which hatch into aquatic larvae. Some have direct development of terrestrial eggs, and even ovovivipary and true viviparity are known in caudates. Several salamanders exhibit parental care, which usually takes the form of egg guarding by one or both parents. Salamanders run the gamut of possible developmental patterns, from complete, obligate metamorphosis, to facultative or partial metamorphosis, to obligate paedomorphosis, in which adults retain larval characters throughout their lives (all cryptobranchids, sirenids, amphiumids, and proteids are obligate paedomorphs).

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 (frogs and toads) - Gymnophiona (caecilians) sister relationship, with Caudata sister to that group. A salamander-caecilian clade is supported by soft anatomical characters and ribosomal DNA sequences. Osteological characters support a salamander-frog clade, and combined morphological and molecular evidence marginally support the salamander-frog sister relationship. There is no controversy as what is or is not a caudate, although some family definitions remain troubling (e.g. Proteidae), and there is still considerable disagreement over the identity of suborders within Caudata.

Fossil salamanders are known from most extant families, as well as four extinct families. The fossil record reveals salamanders beginning in the upper Jurassic of North America and Eurasia, extending through the Pleistocene. Some fossil caudates have not yet been assigned to families. While the terms Caudata and Urodela are usually used interchangeably, some authors have suggested using Urodela to describe only extant forms, while retaining Caudata as the more inclusive group including all known extant and fossil species.

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.

Larson, A., D. Heyse, T. Jackman, D. Maddison, F. Moffitt, and T. Titus. 1996. Caudata: Tree of life. (Website.) http://tolweb.org/tree?group=Caudata&contgroup=Living_Amphibians

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.

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.