Sarcopterygii (/ˌsɑːrkɒptəˈrɪdʒi.aɪ/; from Ancient Greek σάρξ (sárx)'flesh' and πτέρυξ (ptérux)'wing, fin')—sometimes considered synonymous withCrossopterygii (κροσσός,krossós, 'fringe')—is aclade (traditionally aclass or subclass) ofvertebrateanimals which includes a group ofbony fish commonly referred to aslobe-finned fish. These vertebrates are characterised by prominent muscularlimb buds (lobes) within theirfins, which are supported by articulatedappendicular skeletons. This is in contrast to the other clade of bony fish, theActinopterygii, which have onlyskin-coveredbony spines supporting the fins.
Thetetrapods, a mostlyterrestrial clade of vertebrates, are now recognized as having evolved from sarcopterygian ancestors and are most closely related tolungfishes. Their pairedpectoral andpelvic fins evolved intolimbs, and theirforegut diverticulum eventually evolved into air-breathinglungs.Cladistically, this would make the tetrapods a subgroup within Sarcopterygii and thus sarcopterygians themselves. As a result, the phrase "lobe-finned fish" normally refers to not the entire clade but onlyaquatic members that are not tetrapods, i.e. aparaphyletic group.
Guiyu oneiros, the earliest-known bony fish, lived during the LateSilurian, 419 million years ago).[1] It has the combination of both ray-finned and lobe-finned features, although analysis of the totality of its features places it closer to lobe-finned fish.[2][3][4]
Early lobe-finned fishes arebony fish with fleshy, lobed, paired fins, which are joined to the body by a single bone.[5] 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 that resembles alimb bud. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surrounded by layers of vascular bone,cosmine (similar todentin), and externalkeratin.[6] The physical structure of tetrapodomorphs, fish bearing resemblance to tetrapods, provides valuable insights into the evolutionary shift from aquatic to terrestrial existence.[7] Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. The first tetrapod land vertebrates, basal amphibian organisms, possessed legs derived from these fins. Sarcopterygians also possess twodorsal fins with separate bases, as opposed to the single dorsal fin in ray-finned fish. The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Early sarcopterygians commonly exhibit a symmetrical tail, while all sarcopterygians possess teeth that are coated with genuineenamel.
Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish wasRhizodus hibberti from theCarboniferous period ofScotland which may have exceeded 7 meters in length. Among the two groups of living species, thecoelacanths and thelungfishes, the largest species is theWest Indian Ocean coelacanth, reaching 2 m (6 ft 7 in) in length and weighing up 110 kg (240 lb). The largest lungfish is themarbled lungfish which can reach 2 m (6.6 ft) in length and weigh up to 50 kg (110 lb).[8][9]
Taxonomists who adhere to the cladistic approach include Tetrapoda within Sarcopterygii,[10] sometimes under a Linnean rank such as infraclass or division.[11][12][13] The fin-limbs found in lobe-finned fishes like the coelacanths display a strong resemblance to the presumed ancestral form of tetrapod limbs. Lobe-finned fishes seemingly underwent two distinct evolutionary paths, leading to their classification into two clades: theRhipidistia (comprising the Dipnoi, orlungfish, and theTetrapodomorpha, which includes the Tetrapoda) and theActinistia (represented by coelacanths).
The extensive fossil record and numerous morphological and molecular studies have shown that lungfish and some fossil lobe-finned fish ("rhipidistians") are more closely related to tetrapods than they are to coelacanths; as a result tetrapods are nested within Sarcopterygii.[14][15] This abides tocladistics in that in order for a group to be valid, it musthave an ancestral species and all descendants of that common ancestor based on shared characteristics. As such mammals, sauropsids (birds and "reptiles"), and amphibians are highly derived sarcopterygians despite superficially looking nothing like the standard lobe-finned fish anatomically speaking. However, similarities can be noticed in their limb bones andtooth enamel.[16] Additionally, lungfish and tetrapods share a dividedatrium.[17]
Multiple Linnean classifications have been proposed with the explicit intent to incorporate Sarcopterygii as a monophyletic taxon instead of maintaining its traditional paraphyletic definition.[18][19][20][21]
Other classifications do not use Sarcopterygii as a ranked taxon but still nonetheless still reject traditional paraphyletic assemblages. In the scheme below, sarcopterygian groups are marked in bold letters.
The classification below follows Benton (2004),[22] and uses a synthesis of rank-basedLinnaean taxonomy and also reflects evolutionary relationships. Benton included the clade 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.[22]
Actinistia, coelacanths, are a subclass of lobe-finned fishes, all but two of which are species known only through fossils. The subclass Actinistia contains thecoelacanths, including the two living coelacanths: theWest Indian Ocean coelacanth and theIndonesian coelacanth.
Dipnoi, commonly referred to as lungfish, but also known as salamanderfish,[23] are 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 inAfrica,South America, andAustralia. Whilevicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinentGondwana, 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 ofPangaea,Gondwana, andLaurasia.
Tetrapodomorpha, tetrapods and their extinct relatives, are a clade of vertebrates consisting oftetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish.[24] Advanced forms transitional between fish and the earlylabyrinthodonts, likeTiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish, half-tetrapods, in appearance and limb morphology. The Tetrapodomorpha contain thecrown group tetrapods (the last common ancestor of living tetrapods and all of its descendants) and several groups of earlystem tetrapods, and several groups of related lobe-finned fishes, collectively known as theosteolepiforms. The Tetrapodamorpha minus thecrown group Tetrapoda are the stem tetrapoda, aparaphyletic unit encompassing the fish to tetrapod transition. Among the characters defining tetrapodomorphs are modifications to the fins, notably ahumerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Tetrapodomorph fossils are known from the earlyDevonian onwards, and includeOsteolepis,Panderichthys,Kenichthys, andTungsenia.[25]
Lobe-finned fishes and their sister group, theray-finned fishes, make up the cladeOsteichthyes, characterized by the presence ofswim bladders (which share ancestry with lungs) as well as the evolution ofossifiedendoskeleton instead ofcartilages like the skeletons ofacanthodians,chondrichthyians and mostplacoderms. There are otherwise vast differences in fin, respiratory and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence ofcosmoid layers in the scales of sarcopterygians. The earliest sarcopterygian fossils were found in the uppermostSilurian, about 418 Ma. They closely resembled the acanthodians (the "spiny fish", a taxon that became extinct at the end of the Paleozoic). In the early–middleDevonian (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, or lobe-finned fishes, split into two main lineages: thecoelacanths and therhipidistians. Coelacanths never left the oceans and their heyday was the late Devonian andCarboniferous, from 385 to 299 Ma, as they were more common during those periods than in any other period in thePhanerozoic. Actinistians, a group within the lobe-finned fish, have been around for almost 380 million years. Over time, researchers have identified 121 species spread across 47 genera. Some species are well-documented in their evolutionary placement, while others are harder to track. The greatest boom in actinistian diversity happened during the Early Triassic, just after the Great Dying.[27]Coelacanths of the genusLatimeria still live today in the open oceans and retained many primordial features of ancient sarcopterygians, earning them a reputation as living fossils.
The rhipidistians, whose ancestors probably lived in the oceans near river mouths andestuaries, left the marine world and migrated into freshwater habitats. They then split into two major groups: thelungfish and thetetrapodomorphs, and both of them evolved their swim bladders into air-breathing lungs. Lungfish radiated into their greatest diversity during theTriassic period; today, fewer than a dozen genera remain, having evolved the first proto-lungs and proto-limbs, adapting to living outside a submerged water environment by the middle Devonian (397–385 Ma). The tetrapodomorphs, on the other hand, evolved into the fully-limbedstegocephalians and later the fully terrestrialtetrapods during theLate Devonian, when theLate Devonian Extinction bottlenecked and selected against the more aquatically adapted groups amongstem-tetrapods.[28][29] The surviving tetrapods then underwentadaptive radiation on dry land and become the dominant terrestrial animals during the Carboniferous and thePermian periods.
There are three major hypotheses as to how lungfish evolved their stubby fins (proto-limbs).
Shrinking waterhole
The first, traditional explanation is the "shrinking waterhole hypothesis", or "desert hypothesis", posited by the American paleontologistAlfred Romer, who believed that limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.[30]
Inter-tidal adaptation
Niedźwiedzki, Szrek, Narkiewicz,et al. (2010)[31] proposed a second, the "inter-tidal hypothesis": That sarcopterygians may have first emerged unto land fromintertidal zones rather than inland bodies of water, based on the discovery of the 395 million-year-oldZachełmie tracks, the oldest discovered fossil evidence of tetrapods.[31][32]
Woodland swamp adaptation
Retallack (2011)[33] proposed a third hypothesis is dubbed the "woodland hypothesis": Retallack 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.[30][33]
Habitual escape onto land
A fourth, minority hypothesis posits that advancing onto land achieved more safety from predators, less competition for prey, and certain environmental advantages not found in water—such as oxygen concentration,[36] and temperature control[38]—implying that organisms developing limbs were also adapting to spending some of their time out of water. However, studies have found that sarcopterygians developed tetrapod-like limbs suitable for walking well before venturing onto land.[41] This suggests they adapted to walking on the ground-bed under water before they advanced onto dry land.
The first tetrapodomorphs, which included the giganticrhizodonts, 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 andmegalichthyids are the only tetrapodomorphs which survived after the Devonian, with the latter group disappearing during the Permian.[42]
Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during thePermian–Triassic extinction event (251 Ma).
The cladogram presented below is based on studies compiled by Janvieret al. (1997) for theTree of Life Web Project,[43] Mikko's Phylogeny Archive[44] and Swartz (2012).[45]
^Tudge C (2000).The variety of life. Oxford: Oxford University Press.ISBN978-0-19-860426-6.
^Heiser JB, Janis CM, Pough FH (2005).Vertebrate life. Pearson/Prentice Hall.ISBN978-0-321-77336-4.
^Benton, Michael J. (2014).Vertebrate Palaeontology (4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 74.ISBN978-1-118-40764-6.OCLC867852756.
^Pough, F. Harvey (2018).Vertebrate Life. Christine M. Janis, Sergi López-Torres (10th ed.). New York: Oxford University Press. p. 123.ISBN978-1-60535-607-5.OCLC1022979490.
^Tedersoo L (2017). "Proposal for practical multi-kingdom classification of eukaryotes based on monophyly and comparable divergence time criteria".bioRxiv10.1101/240929.
^Benton, M. J. (2005)Vertebrate Palaeontology, Blackwell, 3rd edition, Fig 2.10 on page 35 and Fig 3.25 on page 73
^R., Cloutier; P. L., Forey (1991). "Diversity of extinct and living actinistian fishes (Sarcopterygii)".The Biology of Latimeriachalumnae and Evolution of Coelacanths.1 (1):59–74.doi:10.1007/978-94-011-3194-0_4.
^abHohn-Schulte, B.; Preuschoft, H.; Witzel, U.; Distler-Hoffmann, C. (2013). "Biomechanics and functional preconditions for terrestrial lifestyle in basal tetrapods, with special consideration ofTiktaalik roseae".Historical Biology.25 (2):167–181.Bibcode:2013HBio...25..167H.doi:10.1080/08912963.2012.755677.S2CID85407197.
^Janvier, Philippe (1 January 1997)."Vertebrata: Animals with backbones".tolweb.org (Version 01 January 1997 (under construction) ed.). The Tree of Life Web Project.
Cloutier, R., & Forey, P. L. (1991). Diversity of extinct and living actinistian fishes (Sarcopterygii). In J. A. Musick, M. N. Bruton, & E. K. Balon (Eds.), The biology of Latimeriachalumnae and evolution of coelacanths (pp. 59–74). Springer Netherlands.doi:10.1007/978-94-011-3194-0
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