Tetrapods

Tetrapods (meaning "four-limbed") are animals that are mostly known for having four limbs, although there are a few exceptions, where some tetrapods have just two or no legs. Tetrapods are represented today by amphibians, reptiles and mammals. Tetrapods evolved from a group of amphibian-like lobe-finned fish that slowly developed land-dwelling locomotion and the abbility of breathing atmospherical oxygen. Tetrapods first evolved in the Late Devonian, as semiaquatic air-breathing salamander-looking vertebrates. The fact that they were the first and only truly terrestrial groups of vertebrates, is one of the most profound changes in the evolutionary history of life. The first tetrapods needed to stay close to water to survive, and indeed spent most of the time there. Amphibians are mostly semi-aquatic, mostly laying their eggs on water, where the larvae usually breath underwater, in a larval stage, often as what's usually called tadpoles, before developing into mostly air-breathing adults. However, the most evolved types of tetrapods are the amniotes, which evolved to raise their offspring farther from liquid bodies of water, and colonize land with more efficiency. Amniotes differ from many amphibians for laying their eggs on land or retaining a fertilized egg inside the mother. Due to the amniotes success, many primitive amphibians were led to extinction. One group of amniotes evolved to reptiles and the other evolved to mammals. Amniotes include the only vertebrates to successfully conquer the air, through flight. Some tetrapods lost one pair of legs or all two pairs, as a result of evolution. Someother tetrapods evolved to become strictly aquatic, such as in many amphibians that evolved through the eras, as well as some amniotes as well. Generally, tetrapods distinguish from other related vertebrates, such as fish, by having a jaw and set of teeth structured for out-of-water feeding, limb girdles and extremeties for better land locomotion, a pair of lungs for aerial respiration, and senses refined to ear, see and smell on land.