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Temporal range: Late Silurian–Recent, 418–0Ma
The Sarcopterygii // or lobe-finned fish (from Greek σαρξ sarx, flesh, and πτερυξ pteryx, fin) – sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek κροσσός krossos, fringe) – constitute a clade (traditionally a class or subclass) of the bony fish, though a strict cladistic view includes the terrestrial vertebrates. The living sarcopterygians are the coelacanths, lungfish, and the tetrapods.
Early lobe-finned fishes are bony fish with fleshy, lobed, paired fins, which are joined to the body by a single bone. The fins of lobe-finned fishes differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surronded by layers of vascular bone, dentine-like cosmine, and external keratin. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish). The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopterygians have a symmetrical tail. All sarcopterygians possess teeth covered with true enamel.
Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish is the prehistoric predatory Hyneria that lived during the Devonian period around 360 million years ago. Hyneria grew up to 5 m (16 ft) in length and may have weighed as much as two tons. Among the two groups of extant (living) species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching 2 m (6.5 ft) in length and weighing up 110 kg (240 lb). The largest lungfish is the African lungfish which can reach 2 m (6.6 ft) in length and weigh up to 50 kg (110 lb).
Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within this group, which in turn consists of all species of four-limbed vertebrates. The fin-limbs of lobe-finned fishes such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fishes apparently followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia (including the Dipnoi, the lungfish, and the Tetrapodomorpha which include the Tetrapoda) and the Actinistia (coelacanths).
The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.
||Actinistia, coelacanths, is a subclass of mostly fossil lobe-finned fishes. This subclass contains the coelacanths, including the two living coelacanths, the West Indian Ocean coelacanth and the king of the sea.|
||Dipnoi, lungfish, also known as salamanderfish, is a subclass of freshwater fish. Lungfish are best known for retaining characteristics primitive within the bony fishes, including the ability to breathe air, and structures primitive within the lobe-finned fishes, including the presence of lobed fins with a well-developed internal skeleton. Today, lungfish live only in Africa, South America and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests advanced lungfish had a widespread freshwater distribution and the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangaea, Gondwana and Laurasia.|
||Tetrapodomorpha, tetrapods and their extinct relatives, is a clade of vertebrates, consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advanced forms transitional between fish and the early labyrinthodonts, like Tiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish half-tetrapods, in appearance and limb morphology. Tetrapodomorpha contains the crown group tetrapods (the last common ancestor of living tetrapods and all of its descendants) and several groups of early stem tetrapods. Tetrapodomorpha contains several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda is the Stem Tetrapoda, a paraphyletic unit encompassing the fish to tetrapod transition. Among the characters defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Tetrapodomorph fossils are known from the early Devonian onwards, and include Osteolepis, Panderichthys, Kenichthys and Tungsenia.|
- Subclass Sarcopterygii
- Infraclass Dipnomorpha
- Infraclass Crossopterygii
- Infraclass Tetrapodomorpha
The cladogram presented below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project.
Lobe-finned fishes (sarcopterygians) and their relatives the ray-finned fishes (actinopterygians) comprise the superclass of bony fishes (Osteichthyes) characterized by their bony skeleton rather than cartilage. There are otherwise vast differences in fin, respiratory, and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians, found in the uppermost Silurian (ca 418 Ma), closely resembled the acanthodians (the "spiny fish", a taxon that went extinct at the end of the Paleozoic). In the early–middle Devonian (416 - 385 Ma), while the predatory placoderms dominated the seas, some sarcopterygians came into freshwater habitats.
In the Early Devonian (416 - 397 Ma), the sarcopterygians split into two main lineages — the coelacanths and the rhipidistians. The former never left the oceans and their heyday was the late Devonian and Carboniferous, from 385 to 299 Ma, as they were more common during those periods than in any other period in the Phanerozoic; coelacanths still live today in the oceans (genus Latimeria).
The Rhipidistians, whose ancestors probably lived in the oceans near the river mouths (estuaries), left the ocean world and migrated into freshwater habitats. They in turn split into two major groups: the lungfish and the tetrapodomorphs. The lungfish's greatest diversity was in the Triassic period; today there are fewer than a dozen genera left. The lungfish evolved the first proto-lungs and proto-limbs; developing the ability to live outside a water environment in the middle Devonian (397 - 385 Ma).
There are three major hypotheses as to how they evolved their stubby fins (proto-limbs). The traditional explanation is the "shrinking waterhole hypothesis" or "desert hypothesis" posited by the American paleontologist Alfred Romer. He believed that limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.
The second hypothesis is the "inter-tidal hypothesis" put forward in 2010 by a team of Polish paleontologists (Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, & Per Ahlberg). They argued that sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water. Their hypothesis is based on the discovery of the 395 million-year-old Zachełmie tracks in Zachełmie, Świętokrzyskie Voivodeship, Poland, the oldest ever discovered fossil evidence of tetrapods.
The third hypothesis is dubbed the "woodland hypothesis" and was proposed by the American paleontologist Gregory J. Retallack in 2011. He argues that limbs may have developed in shallow bodies of water in woodlands as a means of navigating in environments filled with roots and vegetation. He based his conclusions on the evidence that transitional tetrapod fossils are consistently found in habitats that were formerly humid and wooded floodplains.
The first tetrapodomorphs, which included the gigantic rhizodonts, had the same general anatomy as the lungfish, who were their closest kin, but they appear not to have left their water habitat until the late Devonian epoch (385 - 359 Ma), with the appearance of tetrapods (four-legged vertebrates). Tetrapods are the only tetrapodomorphs which survived after the Devonian.
Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian-Triassic extinction event (251 Ma).
See also 
- Zhu M, W Zhao, L Jia, J Lu, T Qiao and Q Qu (2009) "The oldest articulated osteichthyan reveals mosaic gnathostome characters" Nature, 458: 469–474. doi:10.1038/nature07855
- Coates, M.I. (2009) "Palaeontology: Beyond the Age of Fishes" Nature, 458: 413–414. doi:10.1038/458413a
- Post details: Critical transitions in fish evolution lack fossil documentation Science Literature, 27 March 2009.
- PharyngulaScience blogs, 1 April 2009.
- Clack, J. A. (2002) Gaining Ground. Indiana University
- Kardong, Kenneth V. (1998). Vertebrates: Comparative Anatomy, Function, Evolution, second edition, USA: McGraw-Hill, 747 pp.. ISBN 0-07-115356-X/0-697-28654-1.
- NOVA. Transcripts. The Missing Link. PBS (2002-02-26)
- Haines, Tim; Chambers, Paul (2006). The Complete Guide to Prehistoric Life (illustrated ed.). Canada: Firefly Books. pp. 32–33. ISBN 978-1-55407-181-4.
- Froese, Rainer, and Daniel Pauly, eds. (2009). "Lepidosirenidae" in FishBase. January 2009 version.
- Protopterus aethiopicus. Fishing-worldrecords.com
- Nelson 2006.
- Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd
- Ernst Heinrich Philipp August Haeckel, Edwin Ray Lankester, L. Dora Schmitz (1892). The History of Creation, Or, The Development of the Earth and Its Inhabitants by the Action of Natural Causes: A Popular Exposition of the Doctrine of Evolution in General, and of that of Darwin, Goethe, and Lamarck in Particular : from the 8. German Ed. of Ernst Haeckel. D. Appleton. p. 422. page 289
- Jing Lu, Min Zhu, John A. Long, Wenjin Zhao, Tim J. Senden, Liantao Jia and Tuo Qiao (2012). "The earliest known stem-tetrapod from the Lower Devonian of China". Nature Communications 3. doi:10.1038/ncomms2170.
- Janvier, Philippe. 1997. Vertebrata. Animals with backbones. Version 01 January 1997 (under construction). http://tolweb.org/Vertebrata/14829/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/
- Benton 2005.
- "Fish-Tetrapod Transition Got A New Hypothesis In 2011". Science 2.0. December 27, 2011. Retrieved January 2, 2012.
- Grzegorz Niedźwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg (2010). "Tetrapod trackways from the early Middle Devonian period of Poland". Nature (Nature Publishing Group) 463 (7277): 43–48. Bibcode:2010Natur.463...43N. doi:10.1038/nature08623. PMID 20054388. Retrieved January 3, 2012.
- Shanta Barley (January 6, 2010). "Oldest footprints of a four-legged vertebrate discovered". New Scientist. Retrieved January 3, 2010.
- Retallack, Gregory (May 2011). "Woodland Hypothesis for Devonian Tetrapod Evolution". Journal of geology (University of Chicago Press) 119 (3): 235–258. Bibcode:2011JG....119..235R. doi:10.1086/659144.