Temporal range:
PennsylvanianPresent (Possible Mississippian record)
From top to bottom and left to right, examples of amniotes : Edaphosaurus and a red fox (two synapsids) & king cobra and a white-headed buffalo weaver (two sauropsids).
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Haeckel, 1866

Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distinguished from the other tetrapod clade — the amphibians — 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).[3][4][5][6]

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[7]: 600  near their kidneys, which are more complex,[7]: 552  the presence of an astragalus for better extremity range of motion,[8] and the complete loss of metamorphosis (which includes an egg and aquatic larval stage), gill and skin breathing, and any lateral line system.[7]: 694 

The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibious reptiliomorphs about 312 million years ago,[9] in the Carboniferous geologic period. Amniotes spread around Earth's land and became the dominant land vertebrates.[9] They soon diverged into two clades, the synapsids and sauropsids, whose lineages both still persist today. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago,[9] while the oldest known sauropsid are probably Hylonomus and Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago). Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being.

  1. ^ 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.
  2. ^ Irmis, R. B.; Parker, W. G. (2005). "Unusual tetrapod teeth from the Upper Triassic Chinle Formation, Arizona, USA" (PDF). Canadian Journal of Earth Sciences. 42 (7): 1339–1345. Bibcode:2005CaJES..42.1339I. doi:10.1139/e05-031. S2CID 46418796.
  3. ^ Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109. ISBN 978-0-412-73810-4.
  4. ^ Cieri, R.L., Hatch, S.T., Capano, J.G. et al. Locomotor rib kinematics in two species of lizards and a new hypothesis for the evolution of aspiration breathing in amniotes. Sci Rep 10, 7739 (2020).
  5. ^ 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.
  6. ^ Hickman, Cleveland P., Jr. Integrated principles of zoology (Seventeenth ed.). McGraw-Hill. pp. 563–567. ISBN 9781259562310.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ a b c Kardong, Kenneth V. Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill. ISBN 9780073524238.
  8. ^ Clack, Jennifer A. Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 370. ISBN 9780253356758.
  9. ^ a b c Benton, M.J.; Donoghue, P.C.J. (2006). "Palaeontological evidence to date the tree of life". Molecular Biology and Evolution. 24 (1): 26–53. doi:10.1093/molbev/msl150. PMID 17047029.

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