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 fossilizedbirth–death process study at 322–340 Ma.[16]
The termamniote comes from theamnion, which derives fromGreek ἀμνίον (amnion), which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood ofsacrificed animals was caught, and derived from ἀμνός (amnos), meaning "lamb".[17]
Zoologists characterize amniotes in part byembryonic development that includes the formation of several extensive membranes, theamnion,chorion, andallantois. Amniotes develop directly into a (typically) terrestrial form withlimbs and a thick stratifiedepithelium (rather than first entering a feeding larvaltadpole stage followed bymetamorphosis, asamphibians do). In amniotes, the transition from a two-layered periderm to acornified epithelium is triggered bythyroid hormone during embryonic development, rather than by metamorphosis.[18] 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.
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hardeggshell and anallantois that facilitatesrespiration while providing a reservoir for disposal of wastes. Theirkidneys (metanephros) andlarge intestines are also well-suited to water retention.Most mammals do not lay eggs, but corresponding structures develop inside theplacenta.
The ancestors of true amniotes, such asCasineria kiddi, which lived about 340 million years ago, evolved fromamphibianreptiliomorphs and resembled small lizards. At the lateDevonian 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 duringRomer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic).[19] The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell.[20] Indeed, many modern-day amniotes require moisture to keep their eggs fromdesiccating.[21] 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. Afterinternal 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, likeextraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later.[22] 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.[23]
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.[24]
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 terrestrialfood-chain, which was restricted tolevel three and below, with onlyinvertebrates occupying level two. Amniotes would eventually experienceadaptive radiations when some species evolved the ability to digest plants and newecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.[citation needed]
While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of anotic notch at the back margin of theskull roof. In their ancestors, this notch held aspiracle, an unnecessary structure in an animal without an aquatic larval stage.[25] There are three main lines of amniotes, which may be distinguished by the structure of theskull and in particular the number ofholes behind each eye. Inanapsids, the ancestral condition, there are none; insynapsids (mammals and their extinct relatives) there is one; and indiapsids (including birds,crocodilians,squamates, andtuataras), 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.
Amniota was first formally described by the embryologistErnst Haeckel in 1866 on the presence of theamnion, hence the name. A problem with this definition is that the trait (apomorphy) in question does notfossilize, and the status of fossil forms has to be inferred from other traits.
Amniotes
Archaeothyris, one of the mostbasal synapsids, first appears in the fossil records about 306 million years ago.[27]
By theMesozoic, 150 million years ago, sauropsids included the largest animals anywhere. Shown are somelate Jurassic dinosaurs, including the early birdArchaeopteryx perched on a tree stump.
This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique fromcladistics, as the class Reptilia isparaphyletic—it has given rise to two other classes not included in Reptilia.
Most species described asmicrosaurs, formerly grouped in the extinct and prehistoric amphibian grouplepospondyls, has been placed in the newer cladeRecumbirostra, and shares many anatomical features with amniotes which indicates they were amniotes themselves.[34]
A different approach is adopted by writers who reject paraphyletic groupings. One such classification, byMichael Benton, is presented in simplified form below.[35]
With the advent of cladistics, other researchers have attempted to establish new classes, based onphylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example,Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor ofextant mammals and reptiles, and all its descendants".[26] As Gauthier makes use of acrown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy.[36] Though traditionally considered reptiliomorphs, some recent research has recovereddiadectomorphs as the sister group toSynapsida within Amniota, based on inner ear anatomy.[37][38][39]
Thecladogram presented here illustrates thephylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995),[40] with the exception of turtles, which more recent morphological andmolecularphylogenetic studies placed firmly withindiapsids.[41][42][43][44][45][46] The cladogram covers the group as defined under Gauthier's definition.
^Benton, Michael J. (1997).Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109.ISBN978-0-412-73810-4.
^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.ISBN978-1-259-56231-0.
^abcKardong, Kenneth V. (16 February 2011).Vertebrates: Comparative Anatomy, Function, Evolution. McGraw-Hill.ISBN978-0-07-352423-8.
^Clack, Jennifer A. (27 August 2023).Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 370.ISBN978-0-253-35675-8.
^Mann, Arjan; Gee, Bryan M.; Pardo, Jason D.; Marjanović, David; Adams, Gabrielle R.; Calthorpe, Ami S.; Maddin, Hillary C.; Anderson, Jason S. (5 May 2020). Sansom, Robert (ed.). "Reassessment of historic 'microsaurs' from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem".Papers in Palaeontology.6 (4). Wiley:605–625.Bibcode:2020PPal....6..605M.doi:10.1002/spp2.1316.ISSN2056-2802.
^Stewart J. R. (1997):Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press.
^Cunningham, B.; Huene, E. (July–August 1938). "Further Studies on Water Absorption by Reptile Eggs".The American Naturalist.72 (741):380–385.doi:10.1086/280791.JSTOR2457547.S2CID84258651.
^Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation.Biological Journal of the Linnean Society No 11: pp 19–76Abstract
^abGauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103–155 in Benton, M.J. (ed.),The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press.
^*Hope, S. (2002) The Mesozoic record of Neornithes (modern birds).In: Chiappe, L.M. and Witmer, L.M. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs: 339–388. University of California Press, Berkeley.ISBN0-520-20094-2
^Benton, M.J. (2015). "Appendix: Classification of the Vertebrates".Vertebrate Paleontology (4th ed.). Wiley Blackwell.433–447.ISBN978-1-118-40684-7.
^Lee, M.S.Y. & Spencer, P.S. (1997): Crown clades, key characters and taxonomic stability: when is an amniote not an amniote? In: Sumida S.S. & Martin K.L.M. (eds.)Amniote Origins: completing the transition to land. Academic Press, pp 61–84.Google books
^Müller, Johannes (2004). "The relationships among diapsid reptiles and the influence of taxon selection". In Arratia, G; Wilson, M.V.H.; Cloutier, R. (eds.).Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag Dr. Friedrich Pfeil. pp. 379–408.ISBN978-3-89937-052-2.
^Katsu, Y.; Braun, E. L.; Guillette, L. J. Jr.; Iguchi, T. (17 March 2010). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes".Cytogenetic and Genome Research.127 (2–4):79–93.doi:10.1159/000297715.PMID20234127.S2CID12116018.
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