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 (typically) terrestrial form with
limbs and a thick stratified
epithelium (rather than first entering a feeding larval
tadpole stage followed by
metamorphosis, as
amphibians do). 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 (metanephros) 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 (360 million years ago), 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 (some forms would later become secondarily aquatic). This was confirmed by the discovery of fossil footprints dated to the Gap in 2025. 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 (amnion, chorion, and allantois) 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 (0.4-inch) 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 (EER), 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 (mammals and their extinct relatives) there is one; and in
diapsids (including birds,
crocodilians,
squamates, and
tuataras), 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 (some amniotes have lost it) and an
astragalus bone in the ankle. ==Definition and classification==