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Concept
Amniote
Summary
Amniotes belong to the clade Amniota, a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds) and synapsids (including mammals and mammal ancestors like "pelycosaurs" and therapsids). They are distinguished from the other living tetrapod clade—the lissamphibians—by the development of three extraembryonic membranes (amnion for embryoic protection, chorion for gas exchange, and allantois for metabolic waste disposal or storage), thicker and more keratinized skin, and costal respiration (breathing by expanding/constricting the rib cage).
All three main features listed above, namely the presence of an amniotic buffer, water-impermeable cutes and a robust respiratory system, are very important for amniotes to live on land as true terrestrial animals—the ability to reproduce in locations away from water bodies, better homeostasis in drier environments, and more efficient air respiration to power terrestrial locomotions, although they might still require regular access to drinking water for rehydration like the semiaquatic amphibians do. Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs (reptiles, birds and monotremes) or nurturing fertilized eggs within the mother (marsupial and placental mammals), unlike anamniotes (fish and amphibians) that have to spawn in or closely adjacent to aquatic environments. Additional unique features are the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys, which are more complex, the presence of an astragalus for better extremity range of motion, and the complete loss of metamorphosis (which includes an egg and aquatic larval stage), gill and skin breathing, and any lateral line system.
The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibious reptiliomorphs about 312 million years ago, in the Carboniferous geologic period.
Official source
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