Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates are characterised by prominent muscular (lobes) within their fish fin, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered lepidotrichia supporting the fins.
The , a mostly terrestrial clade of vertebrates, are now recognized as having evolved from sarcopterygian ancestors and are most closely related to . Their paired pectoral fins and evolved into limbs, and their lung bud eventually evolved into air-breathing . Cladistics, 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 only aquatic animal members that are not tetrapods, i.e. a paraphyletic group.
Non-tetrapod sarcopterygians were once the dominant predators of freshwater ecosystems during the Carboniferous and Permian periods, but suffered significant decline after the Permian–Triassic extinction event. The only known extant non-tetrapod sarcopterygians are the two species of and six species of .
Characteristics
Early lobe-finned fishes are
Osteichthyes 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 that resembles a limb bud. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surrounded by layers of vascular bone, cosmine (similar to dentin), and external keratin.][ The physical structure of tetrapodomorphs, fish bearing resemblance to tetrapods, provides valuable insights into the evolutionary shift from aquatic to terrestrial existence.][ 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 two 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 genuine tooth enamel.
Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish was Rhizodus from the Carboniferous period of Scotland which may have exceeded 7 meters in length. Among the two groups of living species, the and the , the largest species is the West Indian Ocean coelacanth, reaching in length and weighing up . The largest lungfish is the marbled lungfish which can reach 2 m (6.6 ft) in length and weigh up to .][
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Classification
Taxonomists who adhere to the cladistic approach include Tetrapoda within Sarcopterygii,[ sometimes under a Linnean rank such as infraclass or division.]
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.
Multiple Linnean classifications have been proposed with the explicit intent to incorporate Sarcopterygii as a monophyletic taxon instead of maintaining its traditional paraphyletic definition.
Ahlberg (1991)
Class Osteichthyes
Nelson et al. (2016)
Class Osteichthyes
Betancur-Rodrigues et al. (2017)
Superclass Sarcopterygii
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Class
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Class Dipnotetrapodomorpha
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.
Tedersoo (2017)
Phylum Vertebrate
Taxonomy
The classification below follows Benton (2004),[ and uses a synthesis of rank-based Linnaean 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.][
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Evolution
Lobe-finned fishes and their sister group, the Actinopterygii, make up the clade Osteichthyes, characterized by the presence of (which share ancestry with lungs) as well as the evolution of ossified endoskeleton instead of like the skeletons of , and most . There are otherwise vast differences in fin, respiratory and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmine layers in the scales of sarcopterygians. The earliest sarcopterygian fossils were found in the uppermost Silurian, 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–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, or lobe-finned fishes, split into two main lineages: the and the . Coelacanths 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.
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.
The rhipidistians, whose ancestors probably lived in the oceans near river mouths and estuaries, left the marine world and migrated into freshwater habitats. They then split into two major groups: the lungfish and the , and both of them evolved their swim bladders into air-breathing lungs. Lungfish radiated into their greatest diversity during the Triassic 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-limbed and later the fully terrestrial during the Late Devonian, when the Late Devonian Extinction bottlenecked and selected against the more aquatically adapted groups among .
Hypotheses for means of pre-adaptation
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 paleontologist Alfred 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.
- Inter-tidal adaptation
- Niedźwiedzki, Szrek, Narkiewicz, et al. (2010)
- Woodland swamp adaptation
- Retallack (2011)
[ proposed a third hypothesis is dubbed the "woodland hypothesis"]
- 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, and temperature control—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. This suggests they adapted to walking on the ground-bed under water before they advanced onto dry land.
History through to the end-Permian extinction
The first tetrapodomorphs, which included the gigantic rhizodontida, 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 and megalichthyids are the only tetrapodomorphs which survived after the Devonian, with the latter group disappearing during the Permian.
Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian–Triassic extinction event (251 Ma).
Phylogeny
The cladogram presented below is based on studies compiled by Janvier et al. (1997) for the Tree of Life Web Project,[ Mikko's Phylogeny Archive][ and Swartz (2012).][
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Sarcopterygii incertae sedis
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† Guiyu oneiros Zhu et al., 2009
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† Diabolepis (Chang & Yu, 1984)
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† Langdenia Janvier & Phuong, 1999
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† Ligulalepis Schultze, 1968
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† Meemannia Zhu, Yu, Wang, Zhao & Jia, 2006
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† Psarolepis Yu 1998 sensu Zhu, Yu, Wang, Zhao & Jia, 2006
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† Megamastax Choo, Zhu, Zhao, Jia, & Zhu, 2014
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† Sparalepis Choo, Zhu, Qu, Yu, Jia & Zhaoh, 2017
[
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Paraphyly Osteolepida incertae sedis|
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† Bogdanovia orientalis Obrucheva 1955 has
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† Canningius Säve-Söderbergh, 1937
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† Chrysolepis
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† Geiserolepis
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† Latvius
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† L. grewingki (Gross, 1933)
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† L. porosus Jarvik, 1948
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† L. obrutus Vorobyeva, 1977
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† Lohsania Vaughn, 1962
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† Megadonichthys Vorobyeva, 1962
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† Platyethmoidia Young, Long & Ritchie, 1992
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† Shirolepis Vorobeva, 1977
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† Sterropterygion Thomson, 1972
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† Thaumatolepis Obruchev, 1941
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† Thysanolepis Vorobyeva, 1977
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† Vorobjevaia Young, Long & Ritchie, 1992
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Paraphyly Elpistostegalia/Elpistostegalia incertae sedis
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† Parapanderichthys stolbovi (Vorobyeva, 1960) Vorobyeva, 1992
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† Howittichthys Long & Holland, 2008
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† Livoniana Ahlberg, Luksevic & Mark-Kurik, 2000
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Stegocephalia incertae sedis
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† Antlerpeton Thomson, Shubin & Poole, 1998
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† Austrobrachyops Colbert & Cosgriff, 1974
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† Broilisaurus (Goldenberg, 1873) Kuhn, 1938
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† Densignathus Daeschler, 2000
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† Doragnathus Smithson, 1980
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† Jakubsonia Lebedev, 2004
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† Limnerpeton Fritsch, 1901 ( nomen dubium)
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† Limnosceloides Romer, 1952
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† L. dunkardensis Romer, 1952 (Type)
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† L. brahycoles Langston, 1966
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† Occidens portlocki Clack & Ahlberg, 2004
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† Ossinodus emend Warren & Turner, 2004
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† Romeriscus Baird & Carroll, 1968
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† Sigournea Bolt & Lombard, 2006
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† Sinostega Zhu et al., 2002
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† Ymeria Clack et al., 2012
See also
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List of sarcopterygian genera
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Cladistic Classification of Class Sarcopterygii
Footnotes
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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.
