Amphibians are considered the oldest taxon of land vertebrates. They first occurred about 400 million years ago in the Devon era. Amphibians can be grouped into three separate amphibian orders: the anurans, i.e. frogs and toads, with about 6,000 known species, the caudates, i.e. salamanders and newts, with about 600 known species, and the legless caecilians with about 200 species. All amphibians are vertebrates with water-permeable, glandular skin with no scales, feathers or fur. They lay eggs without a protective calcareous shell, the spawn. Larvae hatch from these eggs, which breathe through gills and often look completely different to their parents. Only after metamorphosis do they take on the appearance of the adults and become lung-breathing animals. Therefore, they are easily distinguishable from reptiles, even though newts and lizards look very similar at first glance. But reptiles have dry, scaly skin and no larval state.