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.^[6][7]

Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians (frogs/toads, salamanders/newts and caecilians) — by: the development of three extraembryonic membranes (amnion for embryonic protection, chorion for gas exchange, and allantois for metabolic waste disposal or storage); internal fertilization; thicker and keratinized skin; costal respiration (breathing by expanding/constricting the rib cage); 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.^{[8][9][10][11][12]: 600 [12]: 552 [13][12]: 694}

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 (birds, monotremes, and most reptiles), retaining shelled eggs in the mother's body until they hatch (ovoviviparity, in some reptiles), or nurturing fertilized eggs within the mother (viviparity in marsupial and placental mammals). This distinguishes amniotes from anamniotes (fish and amphibians) 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.^[1] After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates.^[14]

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 (potentially a sauropsid) from the earliest Carboniferous-aged Snowy Plains Formation of Australia (358.9 to 354 million years ago) suggests that they likely originated even earlier, probably during the Devonian.^[1] After their origins, they almost immediately diverged into two groups, namely the sauropsids (including all reptiles and birds) and synapsids (including mammals and extinct ancestors like "pelycosaurs" and therapsids). 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 318 million years ago.^[15]

This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma,^[16] or 312–330 Ma,^[17] and by a fossilized birth–death process study at 322–340 Ma.^[18] However, the Snowy Plains footprints suggest a minimum divergence of 358.9–354 Ma.^[1]