Cladistic classification of Sarcopterygii

Cladistic classification of Sarcopterygii is the classication of Sarcopterygii as a clade containing not only the lobe-finned fishes (coelacanths and lungfish) but also the tetrapods, which are closely related to lungfish. The taxon Sarcopterygii was traditionally classified as a paraphyletic group considered either a class or a subclass of Osteichthyes (bony fish). Identification of the group is based on several characteristics, such as the presence of fleshy, lobed, paired fins, which are joined to the body by a single bone.^[1]

Taxonomic and fossil history

The properties defining the sarcopterygians are in contrast to the other group of bony fish, the Actinopterygii, which have ray-fins made of bony rods, called lepidotrichia. These two bony fish groups were classified together as Osteichthyes at one time, the whole combined group was seen as parallel to the tetrapods (mammals, birds, reptiles, and amphibians).

The extensive fossil record and numerous morphological and molecular studies have shown, however, 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.^[2]^[3] This abides to cladistics in that in order for a group to be valid, it must have 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 and tooth enamel.^[4] Additionally, lungfish and tetrapods share a divided atrium.^[5]

Classification

Multiple Linnean classifications have been proposed with the explicit intent to incorporate Sarcopterygii as a monophyletic taxon instead of maintaining its traditional paraphyletic definition.^[6]^[7]^[8]^[9]

Ahlberg (1991)

Class Osteichthyes

Subclass Actinopterygii
Subclass Sarcopterygii
- Plesion Onychodontia
- Infraclass Actinistia
- Infraclass Rhipidistia
  - Superdivision Tetrapodomorpha
    - Plesion Rhizodontida
    - Plesion Osteolepiformes
    - Plesion Panderichthyidae
    - Division Tetrapoda
  - Superdivision Dipnomorpha
    - Plesion Porolepiformes
    - Plesion Powichthys
    - Plesion Youngolepis
    - Plesion Diabolepis
    - Division Dipnoi

Nelson et al. (2016)

Class Osteichthyes

Subclass Actinopterygii
Subclass Sarcopterygii
- Infraclass Actinistia
- †Infraclass Onychodontida
- Infraclass Dipnomorpha
- †Infraclass Rhizodontida
- †Infraclass Osteolepidida
- †Infraclass Elpistostegalia
- Infraclass Tetrapoda

Betancur-Rodrigues et al. (2017)

Superclass Sarcopterygii

Class Coelacanthimorpha
Class Dipnotetrapodomorpha
- Subclass Dipnomorpha
- Subclass Tetrapodomorpha

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 Craniata

Class Cyclostomi
Class Chondrichthyes
Class Cladistia
Class Actinopteri
Class unspecified
Class Ceratodontimorpha
Class Amphibia
Class Mammalia
Class Sphenodontea
Class Squamatea
Class Testudinea
Class Crocodylea
Class Aves

References

^ Clack JA (2012). Gaining Ground: The origin and evolution of tetrapods. Indiana University Press. ISBN 978-0-253-35675-8.
^ Tudge C (2000). The variety of life. Oxford: Oxford University Press. ISBN 978-0-19-860426-6.
^ Heiser JB, Janis CM, Pough FH (2005). Vertebrate life. Pearson/Prentice Hall. ISBN 978-0-321-77336-4.
^ Benton, Michael J. (2014). Vertebrate Palaeontology (4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 74. ISBN 978-1-118-40764-6. OCLC 867852756.
^ Pough, F. Harvey (2018). Vertebrate Life. Christine M. Janis, Sergi López-Torres (10th ed.). New York: Oxford University Press. p. 123. ISBN 978-1-60535-607-5. OCLC 1022979490.
^ Ahlberg PE (1991). "A re-examination of sarcopterygian interrelationships, with special reference to the Porolepiformes". Zoological Journal of the Linnean Society. 103 (3): 241–287. doi:10.1111/j.1096-3642.1991.tb00905.x.
^ Nelson JS, Grande TC, Wilson MV (April 2016). Fishes of the World. John Wiley & Sons. ISBN 978-1-118-34233-6.
^ Betancur-R R, Wiley EO, Arratia G, Acero A, Bailly N, Miya M, et al. (July 2017). "Phylogenetic classification of bony fishes". BMC Evolutionary Biology. 17 (1): 162. Bibcode:2017BMCEE..17..162B. doi:10.1186/s12862-017-0958-3. PMC 5501477. PMID 28683774.
^ Tedersoo L (2017). "Proposal for practical multi-kingdom classification of eukaryotes based on monophyly and comparable divergence time criteria". bioRxiv. doi:10.1101/240929. S2CID 90691603.

[1] Clack JA (2012). Gaining Ground: The origin and evolution of tetrapods. Indiana University Press. ISBN 978-0-253-35675-8.

[2] Tudge C (2000). The variety of life. Oxford: Oxford University Press. ISBN 978-0-19-860426-6.

[3] Heiser JB, Janis CM, Pough FH (2005). Vertebrate life. Pearson/Prentice Hall. ISBN 978-0-321-77336-4.

[4] Benton, Michael J. (2014). Vertebrate Palaeontology (4th ed.). Chichester, West Sussex: Wiley-Blackwell. p. 74. ISBN 978-1-118-40764-6. OCLC 867852756.

[5] Pough, F. Harvey (2018). Vertebrate Life. Christine M. Janis, Sergi López-Torres (10th ed.). New York: Oxford University Press. p. 123. ISBN 978-1-60535-607-5. OCLC 1022979490.

[6] Ahlberg PE (1991). "A re-examination of sarcopterygian interrelationships, with special reference to the Porolepiformes". Zoological Journal of the Linnean Society. 103 (3): 241–287. doi:10.1111/j.1096-3642.1991.tb00905.x.

[7] Nelson JS, Grande TC, Wilson MV (April 2016). Fishes of the World. John Wiley & Sons. ISBN 978-1-118-34233-6.

[8] Betancur-R R, Wiley EO, Arratia G, Acero A, Bailly N, Miya M, et al. (July 2017). "Phylogenetic classification of bony fishes". BMC Evolutionary Biology. 17 (1): 162. Bibcode:2017BMCEE..17..162B. doi:10.1186/s12862-017-0958-3. PMC 5501477. PMID 28683774.

[9] Tedersoo L (2017). "Proposal for practical multi-kingdom classification of eukaryotes based on monophyly and comparable divergence time criteria". bioRxiv. doi:10.1101/240929. S2CID 90691603.

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