Amphibians are cold-blooded vertebrates, like reptiles, but require a moist environment to survive. Amphibian fossils were first discovered 419 million years ago during the Devonian period. Amphibians are distinguished by their permeable skin, which they use for breathing, absorbing water, and protection through the generation of poison in their skin glands. Amphibians lay eggs, which then go through the process of metamorphosis: the eggs hatch into limbless larvae, which are water hosted creatures that swim, that then transform into limbed adults that live primarily on land and breathe air. An important ecological indicator, due to their especially restrictive habitat requirements, amphibians are the first to die off when their environment is disturbed; this is why over half of all frog species are facing extinction.