Amphibians (class Amphibia), such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land animals (amniotes), amphibians lay eggs in water. Amphibians are superficially similar to reptiles.

Amphibians are ecological indicators, and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct.

Amphibians evolved in the Devonian Period and were top predators in the Carboniferous and Permian Periods, but many lineages were wiped out during the Permian-Triassic extinction. One group, the metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict temnospondyls like Koolasuchus and the modern orders of Lissamphibia.

See also:

• Science: Biology: Flora and Fauna: Animalia: Chordata: Amphibia (277)

• Xenopus Laevis
  dkfz-heidelberg.de
  Database focusing on gene expression in the frog, xenopus laevis. Includes pictorial representations and expression domains.
• Frog Genetics: Xenopus Tropicalis Jumps into the Future
  faculty.virginia.edu
  Article which explains why amphibian embryos are one of the favoured systems for elucidating the mechanisms of early development.
• Salamander Genome Project
  salamander.uky.edu
  Development of expressed sequence tags (ESTS) and a comprehensive genome map. Includes project overview, linkage maps, and conserved syntenies .