Sarcopterygii

Sarcopterygii Fossil range: Late Silurian–Recent, 418–0 Ma PreЄ Є O S D C P T J K Pg N
Modern coelacanth, Latimeria chalumnae
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Superclass:	Osteichthyes
Class:	Sarcopterygii Romer, 1955
Subclasses
Coelacanthimorpha — Coelacanths Dipnoi — Lungfishes Tetrapodomorpha — Tetrapods and their extinct relatives.

Sarcopterygii ("fleshy-finned fishes", from Greek σαρξ, sarx, flesh, and πτερυξ, pteryx, fin) -- sometimes considered synonymous with Crossopterygii ("fringe-finned fishes", from Greek κροσσός, krossos, fringe) is a clade (traditionally a class or subclass) of fleshy-finned or lobe-finned fishes, the living descendants of which are the coelacanths, lungfishes and all tetrapods.

Characteristics

Sarcopterygians - crossopterygians are bony fish with fleshy, lobed-paired fins, which are joined to the body by a single bone.^[1] The fins of crossopterygians differ from those of all other fishes in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fishes). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopts have a symmetrical tail.

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.^[2] The fin-limbs of sarcopterygiians show such a strong similarity to the expected ancestral form of tetrapod limbs that they have been universally considered the direct ancestors of tetrapods in the scientific literature. The crossopterygians apparently followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia (including the Dipnoi or lungfishes, and the Tetrapodomorpha which evolved into the Tetrapoda) and the Actinistia (coelacanths).

Evolution

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.

Sarcopterygians and their relatives the actinopterygians ("ray-finned fishes") comprise the superclass Osteichthyes, the "bony fishes", 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. The first sarcopterygians, found in the uppermost Silurian (ca 418 Ma), closely resembled the Acanthodians (the "spiny fishes", a taxon that went 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 coelacanths and the rhipidistians. The former 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 still live today in the oceans (genus Latimeria).

The Rhipidistians, whose ancestors probably lived in the oceans near the river mouths (estuaries), left the ocean world and migrated into freshwater habitats. They in turn split into two major groups : the lungfishes and the tetrapodomorphs. The lungfishes evolved the first proto-lungs and proto-limbs; they learned in the middle Devonian (397 - 385 Ma) how to live outside a water environment, using their stubby fins (proto-limbs) to walk on land and find new water if their waterhole was depleted, and their lungs to breathe air and get sufficient oxygen. The lungfishes' greatest diversity was in the Triassic period; today there are fewer than a dozen genera left.

The tetrapodomorphs, which included the gigantic rhizodonts, had the same general anatomy as the lungfishes, who were their closest kin, but they appear not to have left their water habitat until the late Devonian epoch (385 - 359 Ma), when they progressively transformed into tetrapods (four-legged vertebrates).

Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian-Triassic extinction event (251 Ma).

Classification

A modern coelacanth, Latimeria chalumnae

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 fishes, despite the former being assigned a higher taxonomic rank.^[3]

• Subclass Sarcopterygii
  • Order Dipnoi
  • Infraclass Crossopterygii
    • †Order Porolepiformes
    • †Order Onychodontida
    • Order Actinistia
  • 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 Philippe Janvier and others for the Tree of Life Web Project.^[4]

See also

• List of Sarcopterygii
• Actinopterygii — ray-finned fishes

References

1. ↑ Clack, J. A. (2002) Gaining Ground. Indiana University
2. ↑ Nelson, Joseph S. (2006). Fishes of the World. John Wiley & Sons, Inc. ISBN 0-471-25031-7. 
3. ↑ Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd
4. ↑ Janvier, Philippe. 1997. Vertebrata. Animals with backbones. Version 01 January 1997 (under construction). http://tolweb.org/Vertebrata/14829/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/