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Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...
or subclass) of the
bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...
known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass including
amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s,
sauropsid Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syna ...
s (
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s, including dinosaurs and therefore birds) and synapsids (with
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s being the only
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
group), evolved from certain sarcopterygians; under a cladistic view, tetrapods are themselves considered a subgroup within Sarcopterygii. The known extant non-tetrapod sarcopterygians include two species of coelacanths and six species of lungfishes.


Characteristics

Early lobe-finned fishes are
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...
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 surrounded by layers of vascular bone, dentine-like cosmine, and external keratin. 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 Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
organisms, possessed legs
derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...
from these fins. Sarcopterygians also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of
actinopterygian 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 ho ...
s (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 The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
period of Scotland which may have exceeded 7 meters in length. Among the two groups of
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
(living) species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching in length and weighing up . The largest lungfish is the African lungfish 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 Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of lobe-finned fishes which includes the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a d ...
(including the Dipnoi, the lungfish, and the Tetrapodomorpha which include the Tetrapoda) and the Actinistia (coelacanths).


Taxonomy

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. * Subclass Sarcopterygii ** †Order
Onychodontida Onychodontiformes (also known as Onychodontida and Struniiformes) is an order of prehistoric sarcopterygian fish that lived during the Devonian period. The onychodontiforms are generally regarded as early-diverging members of the coelacanth ...
** Order Actinistia ** Infraclass
Dipnomorpha 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 ...
*** †Order
Porolepiformes Porolepiformes is an order of prehistoric lobe-finned fish which lived during the Devonian period (about 416 to 359 million years ago). The group contains two families: Holoptychiidae and Porolepididae. Porolepiformes was established by the ...
*** Subclass
Dipnoi 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 ...
**** Order
Ceratodontiformes Ceratodontoidei is a suborder of lungfish that is defined as "the clade including all taxa more closely related to ''Lepidosiren'', '' Neoceratodus'' and '' Gnathorhiza'' than to '' Uronemus'', '' Conchopoma'' and ''Sagenodus''". Members of this ...
**** Order
Lepidosireniformes Lev Berg Lev Semyonovich Berg, also known as Leo S. Berg (russian: Лев Семёнович Берг; 14 March 1876 – 24 December 1950) was a leading Russian geographer, biologist and ichthyologist who served as President of the Soviet Geogra ...
** Infraclass Tetrapodomorpha *** †Order
Rhizodontida Rhizodontida is an extinct group of predatory tetrapodomorphs known from many areas of the world from the Givetian through to the Pennsylvanian - the earliest known species is about 377 million years ago (Mya), the latest around 310 Mya. Rhizodo ...
*** Superorder
Osteolepidida Osteolepiformes, also known as Osteolepidida, is a group of prehistoric lobe-finned fishes which first appeared during the Devonian period. The order contains the families Canowindridae, Megalichthyidae, Osteolepididae and Tristichopteridae ...
**** †Order
Osteolepiformes Osteolepiformes, also known as Osteolepidida, is a group of prehistoric lobe-finned fishes which first appeared during the Devonian period. The order contains the families Canowindridae, Megalichthyidae, Osteolepididae and Tristichopterida ...
***** †Family
Tristichopteridae Tristichopterids (Tristichopteridae) were a diverse and successful group of tetrapodomorph fishes living throughout the Middle and Late Devonian. They first appeared in the Eifelian stage of the Middle Devonian. Within the group sizes ranged from ...
**** †Order Panderichthyida **** Superclass Tetrapoda


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'' **†'' Guiyu oneiros'' Zhu ''et al.'', 2009 **†'' Diabolepis speratus'' (Chang & Yu, 1984) **†'' Langdenia campylognatha'' Janvier & Phuong, 1999 **†''
Ligulalepis ''Ligulalepis'' is an extinct genus of prehistoric bony fish. ''Ligulalepis'' was first described from isolated scales found in the Taemas-Wee jasper limestones of New South Wales (Early Devonian age) by Dr Hans-Peter Schultze (1968) and further ...
'' 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 In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
Osteolepida '' incertae sedis'', **†''
Bogdanovia orientalis ''Bogdanovia'' is an extinct genus of lobe-finned fish which lived during the Devonian period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Per ...
'' Obrucheva 1955
as been treated as Coelacanthinimorph sarcopterygian As, AS, A. S., A/S or similar may refer to: Art, entertainment, and media * A. S. Byatt (born 1936), English critic, novelist, poet and short story writer * "As" (song), by Stevie Wonder * , a Spanish sports newspaper * , an academic male voice ...
**†'' Canningius groenlandicus'' Säve-Söderbergh, 1937 **†''
Chrysolepis ''Chrysolepis'' is a small genus of plants in the family Fagaceae, endemic to the western United States. Its two species have the common name chinquapin. The genus occurs from western Washington south to the Transverse Ranges in Southern Califor ...
'' **†'' Geiserolepis'' **†''
Latvius ''Latvius'' is an extinct genus of prehistoric sarcopterygian or lobe-finned fish. See also * Sarcopterygii * List of sarcopterygians * List of prehistoric bony fish A ''list'' is any set of items in a row. List or lists may also refer to: ...
'' ***†''L. grewingki'' (Gross, 1933) ***†''L. porosus'' Jarvik, 1948 ***†''L. obrutus'' Vorobyeva, 1977 **†'' Lohsania utahensis'' Vaughn, 1962 **†''
Megadonichthys kurikae ''Megadonichthys kurikae'' is an extinct species of fish belonging to the family Osteolepididae Osteolepididae is an family of primitive, fish-like tetrapodomorphs (the clade that contains modern tetrapods and their extinct relatives) that l ...
'' 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 In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
Elpistostegalia/ Panderichthyida '' incertae sedis'' **†'' Parapanderichthys stolbovi'' (Vorobyeva, 1960) Vorobyeva, 1992 **†'' Howittichthys warrenae'' Long & Holland, 2008 **†'' Livoniana multidentata'' Ahlberg, Luksevic & Mark-Kurik, 2000 *
Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...
''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'' Clack & Ahlberg, 2004 **†'' Ossinodus puerorum'' emend Warren & Turner, 2004 **†'' Romeriscus periallus'' Baird & Carroll, 1968 **†'' Sigournea multidentata'' 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) comprise the superclass of bony fishes (
Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...
) characterized by their bony skeleton rather than
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
. There are otherwise vast differences in fin, respiratory, and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of
cosmoid 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 ...
layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians were found in the uppermost
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, about 418 Ma ( million years ago). They closely resembled the
acanthodians Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
(the "spiny fish", a taxon that became extinct at the end of the Paleozoic). In the early–middle
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
(416–385 Ma), while the predatory
placoderms Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
dominated the seas, some sarcopterygians came into
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
habitats. In the Early Devonian (416–397 Ma), the sarcopterygians split into two main lineages: the coelacanths and the
rhipidistian Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of lobe-finned fishes which includes the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a de ...
s. Coelacanths never left the oceans and their heyday was the late Devonian and
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, from 385 to 299 Ma, as they were more common during those periods than in any other period in the
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
. Coelacanths of the genus '' Latimeria'' still live today in the open (pelagic) oceans. The Rhipidistians, whose ancestors probably lived in the oceans near the river mouths ( estuaries), left the ocean world and migrated into freshwater habitats. In turn, they split into two major groups: lungfish and the tetrapodomorphs. Lungfish radiated into their greatest diversity during the Triassic 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, 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 zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...
s rather than inland bodies of water, based on the discovery of the 395 million-year-old
Zachełmie tracks The Zachelmie trackways are a series of Middle Devonian-age Trace fossil, trace fossils in Poland, purportedly the oldest evidence of terrestrial Vertebrate, vertebrates (Tetrapod, tetrapods) in the fossil record. These trackways were discovered in ...
in
Zachełmie, Świętokrzyskie Voivodeship Zachełmie is a village in the administrative district of Gmina Zagnańsk, within Kielce County, Świętokrzyskie Voivodeship, in south-central Poland. It lies approximately east of Zagnańsk and north-east of the regional capital Kielce. The ...
, Poland, 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 floodplains. ;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 Rhizodontida is an extinct group of predatory tetrapodomorphs known from many areas of the world from the Givetian through to the Pennsylvanian - the earliest known species is about 377 million years ago (Mya), the latest around 310 Mya. Rhizodo ...
, 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

*
List of sarcopterygian genera This list of lobe-finned fish is a comprehensive listing of all Genus, genera that have ever been included in the class Sarcopterygii, excluding purely vernacular terms and Tetrapods. The list includes all commonly accepted genera, but also genera ...
*
Cladistic Classification of Class Sarcopterygii Sarcopterygii or the lobe-finned fishes ( coelacanths and lungfishes) were usually classified as either a class or a subclass of Osteichthyes based on the traditional Linnaean classification. Identification of the group is based on several charac ...


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