Amniote
Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the Carboniferous period. Amniota is defined as the smallest crown clade containing humans, the Greek tortoise, and the Nile crocodile.Amniotes represent a crucial evolutionary step in vertebrate history, marking the transition from aquatic to fully terrestrial life.
Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians — by: the development of three extraembryonic membranes ; internal fertilization; thicker and keratinized skin; costal respiration ; the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys; more complex kidneys; the presence of an astragalus for better extremity range of motion; the diminished role of skin breathing; and the complete loss of metamorphosis, gills, and lateral lines.
The presence of an amniotic buffer, of a water-impermeable skin, and of a robust, air-breathing, respiratory system, allow amniotes to live on land as true terrestrial animals. Amniotes have the ability to procreate without water bodies. Because the amnion and the fluid it secretes shield the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs, retaining shelled eggs in the mother's body until they hatch, or nurturing fertilized eggs within the mother. This distinguishes amniotes from anamniotes that have to spawn in aquatic environments. Most amniotes still require regular access to drinking water for rehydration, like the semiaquatic amphibians do.
They have better homeostasis in drier environments, and more efficient non-aquatic gas exchange to power terrestrial locomotion, which is facilitated by their astragalus.
Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs, with fossil evidence suggesting they appeared no later than the earliest Carboniferous or late Devonian period. After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates.
Until 2025, it was assumed that amniotes originated during the mid-late Carboniferous, as the earliest body fossils of the group dated to this time. However, the discovery of clawed footprints made by a crown group-amniote from the earliest Carboniferous-aged Snowy Plains Formation of Australia suggests that they likely originated even earlier, probably during the Devonian. After their origins, they almost immediately diverged into two groups, namely the sauropsids and synapsids. Excluding the early fossil footprints, the earliest known crown group amniotes known from body fossils are the sauropsid Hylonomus and the synapsid Asaphestera, both of which are from Nova Scotia during the Bashkirian age of the Late Carboniferous around.
This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma, or 312–330 Ma, and by a fossilized birth–death process study at 322–340 Ma. However, the Snowy Plains footprints suggest a minimum divergence of 358.9–354 Ma.
Etymology
The term amniote comes from the amnion, which derives from Greek ἀμνίον, which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός, meaning "lamb".Description
Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a terrestrial form with limbs and a thick stratified epithelium. In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.Adaptation for terrestrial living
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta. The evolution of reproductive strategies such as oviparity, ovoviviparity, and viviparity occurred independently across multiple amniote lineages, likely influenced by environmental pressures such as temperature, humidity, and predation risk.The ancestors of true amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction, all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial. This was confirmed by the discovery of fossil footprints dated to the Gap in 2025.The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating. Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion.
The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes and a calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.
A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water.
An alternative hypothesis is that amniotes evolved as a result of extended embryo retention, where the extraembryonic membranes originated in the oviducts of the fertilized female to control the interaction between the embryos and the female. The eggs in groups like turtles, crocodilians and birds, which are laid at a much earlier developmental stage, would be a secondary evolved trait.
The egg membranes
Fish and amphibian eggs have only one inner membrane, the embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.Amniote traits
While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage. There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of holes behind each eye. In anapsids, the ancestral condition, there are none; in synapsids there is one; and in diapsids, there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this. Molecular testing firmly places them in the diapsid line of descent.Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a sternum in the pectoral girdle and an astragalus bone in the ankle.