Sarcopterygii

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Sarcopterygii
Fossil range: Latest Silurian - Recent

Coelacanth, Latimeria chalumnae

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Superclass:	Osteichthyes
Class:	Sarcopterygii

Subclasses

Coelacanthimorpha - Coelacanths
Dipnoi - Lungfishes
Tetrapodomorpha - Tetrapods

Sarcopterygii (from Greek sarx, flesh, and pteryx, fin) is traditionally the class of lobe-finned fishes, consisting of lungfish and coelacanths. These are bony fish with paired rounded fins. These fins, being similar to limbs, suggest that these fish may be ancestors of land vertebrates.

Most taxonomists who subscribe to the cladistic approach include within this group the superclass Tetrapoda, which in turns consists of all species of four-limbed vertebrates. 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.

[edit] Taxonomy and Phylogeny

Class SARCOPTERYGII
- Actinistia
  - Eoactinistia
  - Subclass Coelacanthimorpha
    - Order Coelacanthiformes
      - Family Latimeriidae
        
        Macropoma
        
        Latimeria
- Rhipidistia
  - Subclass Dipnoi
    - Order Ceratodontiformes
    - Order Lepidosireniformes
  - Subclass Tetrapodomorpha
    - Order Rhizodontida
    - Superorder Osteolepidida
      - Family Tristichopteridae
        
        Eusthenopteron
        
        Hyneria
      - Order Osteolepiformes
    - Order Panderichthyida
    - Tiktaalik
    - Tetrapoda

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.

[edit] Evolution of Sarcopterygii

Sarcopterygians belong to Osteichthyes group or bony fishes, characterized by their bony skeleton instead of cartilage. The oldest Sarcopterygians were found in the Uppermost Silurian. The first Sarcopterygian closely resembled Acanthodians. The Sarcopterygians closest relatives were the Actinopterygians - ray-finned fishes. Sarcopterygians probably evolved in the oceans, but they later came into freshwater habitats to avoid the predatory placoderms - which were dominant in the Early - Middle Devonian seas. As Sarcopterygians evolve in the Early Devonian, the line splits into two main lineages - the Coelacanths, and the Rhipidistians. The Coelacanths appeared in the Early Devonian, and stayed in the oceans; the coelacanths' heyday was the Late Devonian and Carboniferous, as they were more common during those periods than in any other period in the Phanerozoic. Coelacanths still live today in the oceans. Rhipidistians appeared about the same time as the Coelacanths, but unlike them, Rhipidistians left the ocean world and migrated into the freshwater habitats, their ancestors probably lived in the oceans near the river mouths (estuaries). The Rhipidistians in turn split into two major groups - the lungfishes, and the tetrapodomorphs. The lungfishes' greatest diversity was in the Triassic Period, but today, there are fewer than a dozen genera left. The lungfishes evolved the first proto-lungs and proto-limbs. The lungfishes, ancient and modern, used their stubby fins (proto-limbs) to walk on land and find new water if their waterhole was depleted, and used their lungs to breathe air and get sufficient oxygen. The tetrapodomorphs have the same identical anatomy as the lungfishes, who were their closest kin, but the tetrapodomorphs appear to have stayed in water a little longer until the Late Devonian. Tetrapods - four legged vertebrates were the terapodomorphs' descendants. Tetrapods appeared in the Late Devonian epoch.