Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen.They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required.Amphibians evolved adaptations that allowed them to stay out of the water for longer periods.Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land.They had started to develop lungs, but still breathed predominantly with gills.Many examples of species showing transitional features have been discovered.Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom.
It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes.
The earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land.
They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates.
The number of known amphibian species is approximately 7,000, of which nearly 90% are frogs.
The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in).