Examples of tetrapods who no longer have four limbs but whose ancestors did have four limbs include snakes, amphisbaenians, caecilians, and cetaceans. So, despite the fact that these creatures no longer have four limbs, they are still tetrapods.
Tetrapods evolved from a group of vertebrates known as the tetrapodomorph fishes. These ancient fishes were a lineage of lobe-finned fishes whose paired, fleshy fins evolved into limbs with digits. This transformation took place during the Devonian Period, and is estimated to have occurred about 370 million years ago. Examples of tetrapodomorph fishes are Tiktaalik and Panderichthys.
Early tetrapods were the first vertebrates to leave the water and embark on a life on land. Some early tetrapods that have been described in the fossil record include Acanthostega, Ichthyostega, and Nectridea.
Tetrapods, as a group, are often subdivided based on whether or not their eggs are adapted for life on land or are more suited for aquatic lifestyles. Tetrapods that have eggs adapted for terrestrial habitats are known as amniotes. Their eggs are referred to as amniotic eggs. Amniotes include all present day reptiles, birds, and mammals. Amniotes evolved about 340 million years ago during the Carboniferous Period. They arose from a group of lizard-like amphibians called the amphibian reptiliomorphs. Although amphibians are both vertebrates and tetrapods, they are not amniotes. As a result, they do not lay amniotic eggs and are, to this day, bound to aquatic habitats for reproduction.