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Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a
taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
(traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly
terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
superclass including amphibians, sauropsids ( reptiles, including
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and therefore
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s) and
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
s (with mammals being the only extant group), evolved from certain sarcopterygians; under a
cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
view, tetrapods are themselves considered a subgroup within Sarcopterygii. The known extant non-tetrapod sarcopterygians include two species of
coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s and six species of
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
es.


Characteristics

Early lobe-finned fishes are bony fish with fleshy, lobed, paired
fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...
, 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 A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
surrounded by layers of vascular bone,
dentine Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by e ...
-like
cosmine Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the class Sarcopterygii. Fish scales that include layers of cosmine are known as cosmoid scales. Description As traditionally d ...
, and external
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
. The morphology of tetrapodomorphs, fish that are similar-looking to tetrapods, give indications of the transition from water to terrestrial life. 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
dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...
s 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 was ''Rhizodus hibberti'' from the Carboniferous period of
Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
which may have exceeded 7 meters in length. Among the two groups of extant (living) species, the
coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s and the
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
es, the largest species is the
West Indian Ocean coelacanth The West Indian Ocean coelacanth (''Latimeria chalumnae'') (sometimes known as gombessa, African coelacanth, or simply coelacanth) is a crossopterygian, one of two extant species of coelacanth, a rare order of vertebrates more closely related t ...
, reaching in length and weighing up . The largest lungfish is the
African lungfish ''Protopterus'' is the genus of four species of lungfish found in Africa. ''Protopterus'' was formerly thought to be the sole genus in the family Protopteridae, but more recent studies have classified it with ''Lepidosiren'' in the family Lepi ...
which can reach 2 m (6.6 ft) in length and weigh up to .


Classification

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 Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, and the
Tetrapodomorpha The Tetrapodomorpha (also known as Choanata) are 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. Advance ...
which include the Tetrapoda) and the
Actinistia The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
(coelacanths).


Taxonomy

The classification below follows Benton (2004), and uses a synthesis of rank-based
Linnaean taxonomy Linnaean taxonomy can mean either of two related concepts: # The particular form of biological classification (taxonomy) set up by Carl Linnaeus, as set forth in his ''Systema Naturae'' (1735) and subsequent works. In the taxonomy of Linnaeus t ...
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. * Subclass Sarcopterygii ** †Order Onychodontida ** Order
Actinistia The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
** Infraclass Dipnomorpha *** †Order Porolepiformes *** Subclass Dipnoi **** Order Ceratodontiformes **** Order Lepidosireniformes ** Infraclass
Tetrapodomorpha The Tetrapodomorpha (also known as Choanata) are 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. Advance ...
*** †Order Rhizodontida *** Superorder Osteolepidida **** †Order Osteolepiformes ***** †Family Tristichopteridae **** †Order Panderichthyida **** Superclass
Tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
a


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). * Sarcopterygii ''
incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' **†''
Guiyu oneiros ''Guiyu oneiros'' is the earliest articulated bony fish discovered. The generic name ''Guiyu'' is a transliteration of the Mandarin 鬼魚 ''guǐyú'' "ghost fish" and the specific name ''oneiros'' is from Greek ὄνειρος "dream". Fossils ...
'' Zhu ''et al.'', 2009 **†'' Diabolepis speratus'' (Chang & Yu, 1984) **†'' Langdenia campylognatha'' Janvier & Phuong, 1999 **†'' Ligulalepis'' Schultze, 1968 **†'' Meemannia eos'' Zhu, Yu, Wang, Zhao & Jia, 2006 **†'' Psarolepis romeri'' Yu 1998 sensu Zhu, Yu, Wang, Zhao & Jia, 2006 **†'' Megamastax ambylodus'' Choo, Zhu, Zhao, Jia, & Zhu, 2014 **†'' Sparalepis tingi'' Choo, Zhu, Qu, Yu, Jia & Zhaoh, 2017 * paraphyletic Osteolepida ''
incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'', **†'' Bogdanovia orientalis'' Obrucheva 1955 as been treated as Coelacanthinimorph sarcopterygian**†'' Canningius groenlandicus'' Säve-Söderbergh, 1937 **†'' Chrysolepis'' **†'' Geiserolepis'' **†'' Latvius'' ***†''L. grewingki'' (Gross, 1933) ***†''L. porosus'' Jarvik, 1948 ***†''L. obrutus'' Vorobyeva, 1977 **†'' Lohsania utahensis'' Vaughn, 1962 **†'' Megadonichthys kurikae'' Vorobyeva, 1962 **†'' Platyethmoidia antarctica'' Young, Long & Ritchie, 1992 **†'' Shirolepis ananjevi'' Vorobeva, 1977 **†'' Sterropterygion brandei'' Thomson, 1972 **†'' Thaumatolepis edelsteini'' Obruchev, 1941 **†'' Thysanolepis micans'' Vorobyeva, 1977 **†'' Vorobjevaia dolonodon'' Young, Long & Ritchie, 1992 * paraphyletic
Elpistostegalia Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Middle Devonian to Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph stock, the fishes ...
/ Panderichthyida ''
incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' **†'' Parapanderichthys stolbovi'' (Vorobyeva, 1960) Vorobyeva, 1992 **†'' Howittichthys warrenae'' Long & Holland, 2008 **†'' Livoniana multidentata'' Ahlberg, Luksevic & Mark-Kurik, 2000 * Stegocephalia ''incertae sedis'' **†'' Antlerpeton clarkii'' Thomson, Shubin & Poole, 1998 **†'' Austrobrachyops jenseni'' Colbert & Cosgriff, 1974 **†'' Broilisaurus raniceps'' (Goldenberg, 1873) Kuhn, 1938 **†'' Densignathus rowei'' Daeschler, 2000 **†'' Doragnathus woodi'' Smithson, 1980 **†'' Jakubsonia livnensis'' Lebedev, 2004 **†'' Limnerpeton dubium'' Fritsch, 1901 (''nomen dubium'') **†'' Limnosceloides'' Romer, 1952 ***†'' L. dunkardensis'' Romer, 1952 (Type) ***†'' L. brahycoles'' Langston, 1966 **†''
Occidens portlocki ''Occidens'' is an extinct genus of stem tetrapod that lived during the earliest part of the Carboniferous in what is now Northern Ireland. It is known from a single type species, ''Occidens portlocki'', named in 2004 on the basis of a left lower ...
'' Clack & Ahlberg, 2004 **†'' Ossinodus puerorum'' emend Warren & Turner, 2004 **†'' Romeriscus periallus'' Baird & Carroll, 1968 **†''
Sigournea multidentata ''Sigournea'' is a genus of stem tetrapod from the Early Carboniferous. The genus contains only one species, the type species ''Sigournea multidentata'', which was named in 2006 by paleontologists John R. Bolt and R. Eric Lombard on the basis of ...
'' Bolt & Lombard, 2006 **†'' Sinostega pani'' Zhu ''et al.'', 2002 **†'' Ymeria denticulata'' Clack ''et al.'', 2012


Evolution

Lobe-finned fishes (sarcopterygians) and their relatives the ray-finned fishes (
actinopterygians Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or hor ...
) 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 were found in the uppermost Silurian, about 418 Ma ( million years ago). 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 split into two main lineages: the
coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s and the rhipidistians. 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. Coelacanths of the genus ''
Latimeria ''Latimeria'' is a rare genus of fish which contains the only living species of coelacanth. It includes two extant species: the West Indian Ocean coelacanth (''Latimeria chalumnae'') and the Indonesian coelacanth (''Latimeria menadoensis''). T ...
'' still live today in the open (pelagic) oceans. The Rhipidistians, whose ancestors probably lived in the oceans near the
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...
mouths ( estuaries), left the ocean world and migrated into freshwater habitats. In turn, they split into two major groups:
lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
and the
tetrapodomorphs The Tetrapodomorpha (also known as Choanata) are 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. Advance ...
. Lungfish radiated into their greatest diversity during the
Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
period; today fewer than a dozen genera remain. They evolved the first proto-lungs and proto-limbs, adapting to living outside a submerged water environment by the middle Devonian (397–385 Ma).


Hypotheses for means of pre-adaption

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 Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...
, 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 adaption: Niedźwiedzki, Szrek, Narkiewicz, ''et al''. (2010) proposed a second, the "inter-tidal hypothesis": That sarcopterygians may have first emerged unto land from intertidal zones rather than inland bodies of water, based on the discovery of the 395 million-year-old Zachełmie tracks in Zachełmie, Świętokrzyskie Voivodeship,
Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populou ...
, the oldest discovered fossil evidence of tetrapods. ;Woodland swamp adaption: Retallack (2011) 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
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...
s. ;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 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
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s (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

* List of sarcopterygian genera * Cladistic Classification of Class Sarcopterygii


Footnotes


References

{{Taxonbar, from=Q160830 Fish classes Silurian bony fish Extant Silurian first appearances Pridoli first appearances Taxa described in 1955 Taxa named by Alfred Romer