Clade of tetrapods including reptiles, birds and mammals
Not to be confused with Ammonite.
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Amniote" – news · newspapers · books · scholar · JSTOR(August 2022) (Learn how and when to remove this message)
Amniotes
Temporal range: Pennsylvanian–Present 312–0 Ma
PreꞒ
Ꞓ
O
S
D
C
P
T
J
K
Pg
N
(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
Domain:
Eukaryota
Kingdom:
Animalia
Phylum:
Chordata
Superclass:
Tetrapoda
Clade:
Reptiliomorpha
Clade:
Amniota Haeckel, 1866
Clades
Synapsida
Sauropsida
incertae sedis
†Casineria?[1]
†Kraterokheirodon[2]
†Diadectomorpha?
†Varanopidae
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 amphibian ancestors during the Carboniferous period and further diverged into two groups, namely the sauropsids (including all reptiles and birds) and synapsids (including mammals and extinct ancestors like "pelycosaurs" and therapsids). They 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, and costal respiration (breathing by expanding/constricting the rib cage).[3][4][5][6]
All three main amniote features listed above, namely the presence of an amniotic buffer, water-impermeable cutes and a robust air-breathing respiratory system, are very important for living on land as true terrestrial animals — the ability to survive and procreate in locations away from water bodies, better homeostasis in drier environments, and more efficient non-aquatic gas exchange 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 semiaquatic reptiliomorphs about 312 million years ago[9] during the Carboniferous period. After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates,[9] and soon diverged into 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.
^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.
^Irmis, R. B.; Parker, W. G. (2005). "Unusual tetrapod teeth from the Upper Triassic Chinle Formation, Arizona, USA". Canadian Journal of Earth Sciences. 42 (7): 1339–1345. Bibcode:2005CaJES..42.1339I. doi:10.1139/e05-031. S2CID 46418796.
^Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109. ISBN 978-0-412-73810-4.
^Cieri, R.L., Hatch, S.T., Capano, J.G. et al. (2020). Locomotor rib kinematics in two species of lizards and a new hypothesis for the evolution of aspiration breathing in amniotes. Sci Rep10. 7739. https://doi.org/10.1038/s41598-020-64140-y
^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
^Hickman, Cleveland P. Jr (17 October 2016). Integrated principles of zoology (Seventeenth ed.). McGraw-Hill. pp. 563–567. ISBN 978-1-259-56231-0.
^ abcKardong, Kenneth V. (16 February 2011). Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill. ISBN 978-0-07-352423-8.
^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.
^ abcBenton, 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.
Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic...
Pan-Amniota) is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians)...
(/tɛˈtræpədə/). Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids...
The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians, the equivalent structure is the blastopore. During early...
Sauropsida (Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to include...
examples include Westlothiana (sometimes considered a stem-amniote rather than a true amniote) and Paleothyris, both of similar build and presumably similar...
fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves...
advanced reptiliomorphs (transitional between "amphibians" sensu lato and amniotes) positioned close to, but outside of the clade Amniota, though some recent...
Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane. Amniotes include reptiles...
sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues)...
Serpentes (/sɜːrˈpɛntiːz/). Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have...
the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes" (PDF). Frontiers in Earth Science. 5: 88. Bibcode:2017FrEaS...5...88L...
(lissamphibians, as well as the extinct temnospondyls and lepospondyls) Amniotes or true land vertebrates Sauropsids (reptiles and birds, as well as the...
gametes was achieved through the transition to internal fertilization. Among amniotes, the development of an erectile penis occurred independently for mammals...
lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either by...
Others prioritize the clade Sauropsida, which typically refers to all amniotes more closely related to modern reptiles than to mammals. The earliest known...
Russia. The Permian witnessed the diversification of the two groups of amniotes, the synapsids and the sauropsids (reptiles). The world at the time was...
animal groups that have adapted to life in dry environments; the other is amniotes, whose living members are reptiles, birds and mammals. Both the smallest...
displayed; recent research cannot establish with confidence if the first amniotes were fully terrestrial, or only amphibious. Most authors consider mesosaurs...
primitive node (or primitive knot) is the organizer for gastrulation in most amniote embryos. In birds it is known as Hensen's node, and in amphibians it is...
Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used...
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass...
a cloaca, which is thought to be a feature inherited from the earliest amniotes. Marsupials have a single orifice for excreting both solids and liquids...
were amniotes, which developed in eggs with internal membranes which allowed the developing embryo to breathe but kept water in. The first amniotes arose...
water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water. The first amniotes apparently arose in...
such are among the most highly cited papers in amniote morphology and paleobiology. The 1988 amniote paper is also frequently cited to demonstrate the...
it is a paraphyletic group encompassing all tetrapods, excluding the amniotes (tetrapods with an amniotic membrane, such as modern reptiles, birds, and...
bird to have ever lived, with their eggs being the largest known for any amniote. Elephant birds are palaeognaths (whose flightless representatives are...
had become very rare. Most of the Reptiliomorpha, stem-amniotes that gave rise to the amniotes, disappeared in the Triassic, but two water-dwelling groups...
Global Information
