Amniote

Not to be confused with Ammonite.

Amniotes
Temporal range: Late Carboniferous–Present[1] (Possible Mississippian record)
From top to bottom and left to right, examples of amniotes: Edaphosaurus, red fox (two synapsids), king cobra and a white-headed buffalo weaver (two sauropsids).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Haeckel, 1866
Clades

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. Those of Amniota are defined as the smallest crown clade containing humans, the Greek tortoise, and the Nile crocodile.[4][5]

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), 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.[6][7][8][9][10]: 600  [10]: 552 [11] [10]: 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 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). 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 during the Carboniferous period.[12] After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates.[12]

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). Among the earliest known crown group amniotes, the oldest known sauropsid is Hylonomus and the oldest known synapsid is Asaphestera, both of which are from Nova Scotia during the Bashkirian age of the Late Carboniferous around 318 million years ago.[1][13]

This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma,[14] or 312–330 Ma,[15] and by a fossilized birth–death process study at 322–340 Ma.[16]

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  2. ^ Paton, R. L.; Smithson, T. R.; Clack, J. A. (8 April 1999). "An amniote-like skeleton from the Early Carboniferous of Scotland". Nature. 398 (6727): 508–513. Bibcode:1999Natur.398..508P. doi:10.1038/19071. ISSN 0028-0836. S2CID 204992355.
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  8. ^ Janis, C. M., Napoli, J. G., & Warren, D. E. (2020). Palaeophysiology of pH regulation in tetrapods. Philosophical Transactions of the Royal Society B: Biological Sciences, 375 (1793), 20190131. https://doi.org/10.1098/rstb.2019.0131
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  11. ^ Clack, Jennifer A. (27 August 2023). Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 370. ISBN 978-0-253-35675-8.
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