Sarcopterygii

Class of fishes

Lobe-finned fishes Temporal range: Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
From top to bottom and left to right, examples of sarcopterygians: tetrapodomorph Panderichthys rhombolepis .
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Euteleostomi
Clade:	Sarcopterygii Romer, 1955
Subgroups
?†"Psarolepids" Guiyu Sparalepis Psarolepis Achoania Actinistia – coelacanths & relatives †Onychodontiformes Rhipidistia Dipnomorpha – lungfish & relatives Tetrapodomorpha – four-limbed vertebrates & relatives

Sarcopterygii (

extant 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

Guiyu oneiros, the earliest-known bony fish, lived during the Late Silurian, 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 are

derived from these fins. Sarcopterygians 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 was

extant (living) species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching 2 m (6 ft 7 in) 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).^[8][9]

Classification

See also:

Cladistic Classification of Class Sarcopterygii

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.

Actinistia

(coelacanths).

Taxonomy

The classification below follows Benton (2004),[11] 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.^[11]

Actinistia	West Indian Ocean coelacanth	Actinistia, coelacanths, are a subclass of lobe-finned fishes, all but two of which are fossil species. The subclass Actinistia contains the coelacanths, including the two living coelacanths: the West Indian Ocean coelacanth and the Indonesian coelacanth.
Dipnoi	Queensland lungfish	Dipnoi, lungfish, also known as salamanderfish,[12] 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 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	Advanced tetrapodomorph Tiktaalik	Tetrapodomorpha, tetrapods and their extinct relatives, 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.[13] 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. The Tetrapodomorpha contain the crown group tetrapods (the last common ancestor of living tetrapods and all of its descendants) and several groups of early stem tetrapods, and several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda are 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.[14]

A modern coelacanth, Latimeria chalumnae

Queensland lungfish

• Subclass Sarcopterygii
  • †Order
    Onychodontida
  • Order
    Actinistia
  • Infraclass Dipnomorpha
    • †Order Porolepiformes
    • Subclass Dipnoi
      • Order Ceratodontiformes
      • Order
        Lepidosireniformes
  • Infraclass Tetrapodomorpha
    • †Order Rhizodontida
    • Superorder
      Osteolepidida
      • †Order Osteolepiformes
        • †Family Tristichopteridae
      • †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,^[15] Mikko's Phylogeny Archive^[16] and Swartz (2012).^[17]

• Sarcopterygii incertae sedis
  • †Guiyu oneiros 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^[18][19]

Life restoration of Sparalepis tingi and other fauna from the Silurian of Yunnan

• paraphyletic Osteolepida incertae sedis|^[a]
  • †
    Bogdanovia orientalis
    Obrucheva 1955 [has 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/Panderichthyida incertae sedis
  • †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
    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

Evolution of lobe-finned fishes

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish—like Eusthenopteron

— exhibited a sequence of adaptations:

• Panderichthys, suited to muddy shallows;
• Tiktaalik with limb-like fins that could take it onto land;
• Early tetrapods in weed-filled swamps, such as:
  • Acanthostega, which had feet with eight digits,
  • Ichthyostega with limbs.

Descendants also included pelagic lobe-finned fish such as coelacanth species.

Tooth from the sarcopterygian Onychodus from the Devonian of Wisconsin

Lobe-finned fishes (sarcopterygians) and their relatives the ray-finned fishes (

In the Early Devonian (416–397 Ma), the sarcopterygians split into two main lineages: the

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.^[21]

Inter-tidal adaption

Niedźwiedzki, Szrek, Narkiewicz, et al. (2010)

Zachełmie tracks in Zachełmie, Świętokrzyskie Voivodeship, Poland, the oldest discovered fossil evidence of tetrapods.^[22][23]

Woodland swamp adaption

Retallack (2011)^[24] 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.^[21][24]

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,^[27] and temperature control^[29]—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.^[32] 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 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
• Cladistic Classification of Class Sarcopterygii

Footnotes

References