 Amnibots are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amnibots lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anambiotes fishes and amphidians which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amnibots as higher vertebrates and anambiotes as lower vertebrates based on the discredited idea of the evolutionary great chain of being. Amnibots are tetrapods descendants of furlimbed and backboneed animals that are characterized by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anambiotes including frogs typically do. Amnibots include synapsids mammals along with their extinct kin and sorosids reptiles and birds as well as their ancestors, back to amphidians. Amnibot embryos, whether weighed as eggs or carried by the female, are protected and aided by several extensive membranes. The new theory in mammals such as humans these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes and the lack of a larval stage distinguish amnibotes from tetrapod amphidians. The first amnibotes, referred to as basal amnibotes, resembled small lizards and evolved from the amphidian reptile amorphs about 312 million years ago, in the carboniferous geologic period. Their eggs could survive out of the water, allowing amnibotes to branch out into dryer environments. The eggs could also breathe and cope with wastes, allowing the eggs and amnibotes themselves to evolve into larger forms. The amniotic egg represents a critical divergence within the vertebrates, one enabling amnibotes to reproduce on dry land free of the need to return to water for reproduction as required of the amphidians. From this point the amnibotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amnibotes diverged into two main lines, the synapsids and the soralsids, both of which persist into the modern era. The oldest known fossils in apsid is protoclepsidrops from about 312 million years ago, while the oldest known soralsid is probably paleo threes. In the order Captorimida, from the middle Pennsylvanian epic C306312 million years ago.