Temporal range:
Four-limbed vertebrates (tetrapods sensu lato) originated in the Eifelian stage of the Middle Devonian[2]
a collage of six images of tetrapod animals. clockwise from top left: Mercurana myristicapaulstris, a shrub frog; Dermophis mexicanus, a legless amphibian looking like a naked snake; Equus quagga, a plains zebra; Sterna maxima, a tern (seabird); Pseudotrapelus sinaitus, a Sinai agama; Tachyglossus aculeatus, a spiny anteater
Clockwise from top left: Mercurana myristicapaulstris, a shrub frog; Dermophis mexicanus, a legless amphibian; Equus quagga, a plains zebra; Sterna maxima, a tern (seabird); Pseudotrapelus sinaitus, a Sinai agama; Tachyglossus aculeatus, a short-beaked echidna
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Eotetrapodiformes
Clade: Elpistostegalia
Clade: Stegocephali
Superclass: Tetrapoda
Hatschek & Cori, 1896

Tetrapods (/ˈtɛtrəˌpɒdz/;[5] from Ancient Greek τετρα- (tetra-) 'four', and πούς (poús) 'foot') are four-limbed vertebrate animals constituting the superclass Tetrapoda (/tɛˈtræpədə/).[6] It includes extant and extinct amphibians, and the amniotes which in turn divide into the sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (extinct pelycosaurs, extinct therapsids and all extant mammals).

Tetrapods evolved from a clade of primitive semiaquatic animals known as the Tetrapodomorpha which, in turn, evolved from ancient lobe-finned fish (sarcopterygians) around 390 million years ago in the Middle Devonian period;[7] their forms were transitional between lobe-finned fishes and true four-limbed tetrapods. Limbed vertebrates (tetrapods in the broad sense of the word) are first known from Middle Devonian trackways, and body fossils became common near the end of the Late Devonian but these were all aquatic. The first crown-tetrapods (last common ancestors of extant tetrapods capable of terrestrial locomotion) appeared by the very early Carboniferous, 350 million years ago.[8]

The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The transition from a body plan for gill-based aquatic respiration and tail-propelled locomotion to one that enables the animal to survive out of water and move around on land is one of the most profound evolutionary changes known.[9][10] Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic fish ancestors. These include distinct head and neck structures for feeding and movements, appendicular skeletons (shoulder and pelvic girdles in particular) for weight bearing and locomotion, more versatile eyes for seeing, middle ears for hearing, and more efficient heart and lungs for oxygen circulation and exchange outside water.

The first tetrapods (stem) or "fishapods" were primarily aquatic. Modern amphibians, which evolved from earlier groups, are generally semiaquatic; the first stages of their lives are as waterborne eggs and fish-like larvae known as tadpoles, and later undergo metamorphosis to grow limbs and become partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous. The key innovation in amniotes over amphibians is the amnion, which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water. (Some amniotes later evolved internal fertilization, although many aquatic species outside the tetrapod tree had evolved such before the tetrapods appeared, e.g. Materpiscis.) Some tetrapods, such as snakes and caecilians, have lost some or all of their limbs through further speciation and evolution; some have only concealed vestigial bones as a remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the Carboniferous period,[11] others as recently as the Cenozoic.[12][13]

One group of amniotes diverged into the reptiles, which includes lepidosaurs, dinosaurs (which includes birds), crocodilians, turtles, and extinct relatives; while another group of amniotes diverged into the mammals and their extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from among the dinosaurs, pterosaurs from the archosaurs, and bats from among the mammals.

  1. ^ Irisarri, I., Baurain, D., Brinkmann, H. et al. Phylotranscriptomic consolidation of the jawed vertebrate timetree. Nat Ecol Evol 1, 1370–1378 (2017). https://doi.org/10.1038/s41559-017-0240-5
  2. ^ Cite error: The named reference NiedźwiedzkiSzrek2010 was invoked but never defined (see the help page).
  3. ^ Hatschek, B.; Cori, C. J. (1896). Elementarcus der Zootomie in fünfzen Vorlesungen [Elementary Zootomy in Fifteen Lectures] (in German). Jena: Gustav Fischer.
  4. ^ de Queiroz, K.; Cantino, P. D.; Gauthier, J. A., eds. (2020). "Tetrapoda B. Hatschek and C. J. Cori 1896 [M. Laurin], converted clade name". Phylonyms: A Companion to the PhyloCode. Boca Raton: CRC Press. pp. 759–764. ISBN 978-1-138-33293-5.
  5. ^ "tetrapod". Dictionary.com Unabridged (Online). n.d.
  6. ^ "tetrapoda". Merriam-Webster Dictionary. Retrieved 2022-12-30.
  7. ^ Cite error: The named reference NarkiewiczNarkiewicz2015 was invoked but never defined (see the help page).
  8. ^ Hedges, S. B., Marin, J., Suleski, M., Paymer, M. & Kumar, S. Tree of Life reveals clock-like speciation and diversification. Mol. Biol. Evol. 32, 835–845 (2015).
  9. ^ Long JA, Gordon MS (Sep–Oct 2004). "The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition". Physiol. Biochem. Zool. 77 (5): 700–19. doi:10.1086/425183. PMID 15547790. S2CID 1260442. Archived from the original on 2016-04-12. Retrieved 2011-04-09. as PDF Archived 2013-10-29 at the Wayback Machine
  10. ^ Shubin, N. (2008). Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body. New York: Pantheon Books. ISBN 978-0-375-42447-2.
  11. ^ Laurin 2010, pp. 163
  12. ^ Canoville, Aurore; Laurin, Michel (June 2010). "Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences". Biological Journal of the Linnean Society. 100 (2): 384–406. doi:10.1111/j.1095-8312.2010.01431.x.
  13. ^ Laurin, Michel; Canoville, Aurore; Quilhac, Alexandra (August 2009). "Use of paleontological and molecular data in supertrees for comparative studies: the example of lissamphibian femoral microanatomy". Journal of Anatomy. 215 (2): 110–123. doi:10.1111/j.1469-7580.2009.01104.x. PMC 2740958. PMID 19508493.

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