Evolution of tetrapods

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish such as *Eusthenopteron* exhibited a sequence of adaptations: *Panderichthys*, suited to muddy shallows; *Tiktaalik* with limb-like fins that could take it onto land; early tetrapods in weed-filled swamps, such as *Acanthostega*, which had feet with eight digits, and *Ichthyostega*, which had limbs. Descendants also included pelagic lobe-finned fish such as coelacanth species.

The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes.^[1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water.^[2] The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.

Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period^[3] whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds,^[4] and several modern amphibians.^[5]

The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known.^[6] It is also one of the best understood, largely thanks to a number of significant transitional fossil finds in the late 20th century combined with improved phylogenetic analysis.^[1]

^ ^a ^b 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.
^ Clack, Jennifer A. (1997). "Devonian tetrapod trackways and trackmakers; a review of the fossils and footprints" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 130 (1–4): 227–250. Bibcode:1997PPP...130..227C. doi:10.1016/S0031-0182(96)00142-3.
^ Laurin, M. (2010). How Vertebrates Left the Water. Berkeley, California, USA.: University of California Press. ISBN 978-0-520-26647-6.
^ Canoville, Aurore; Laurin, Michel (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.
^ Laurin, Michel; Canoville, Aurore; Quilhac, Alexandra (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.
^ Long JA, Gordon MS (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 2014-03-09. as PDF Archived 2013-10-29 at the Wayback Machine

[Shubin,_N._2008-1] 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.

[2] Clack, Jennifer A. (1997). "Devonian tetrapod trackways and trackmakers; a review of the fossils and footprints" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 130 (1–4): 227–250. Bibcode:1997PPP...130..227C. doi:10.1016/S0031-0182(96)00142-3.

[3] Laurin, M. (2010). How Vertebrates Left the Water. Berkeley, California, USA.: University of California Press. ISBN 978-0-520-26647-6.

[4] Canoville, Aurore; Laurin, Michel (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.

[5] Laurin, Michel; Canoville, Aurore; Quilhac, Alexandra (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.

[Gordon&Long-6] Long JA, Gordon MS (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 2014-03-09. as PDF Archived 2013-10-29 at the Wayback Machine

[1]

[2]

[3]

[4]

[5]

[6]