Sarcopterygii

Sarcopterygii Fossil range: Late Silurian – Recent
Coelacanth, Latimeria chalumnae
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Superclass: Osteichthyes Class: Sarcopterygii
Subclasses
Coelacanthimorpha—Coelacanths Dipnoi or Dipnotetrapodomorpha—Lungfishes Tetrapodomorpha or Infraclass Tetrapoda—Tetrapods and their extinct relatives.

Sarcopterygii traditionally is a class of vertebrates known as lobe-finned fishes, consisting of living and fossil lungfishes and coelacanths, and related extinct fishes. Members of this group are characterized by lobed paired fins, joined to the body by a single bone (Clark 2002) and two dorsal fins with separate bases.

Among the coelacanths, the first living species, Latimeria chalumnae, was found in 1938, although diverse fossil specimens (now placed in several families) were well known at the time (Nelson 2006). Two living species are currently known. The dipnoans (lungfishes) also are well represented in the fossil record, but there are only six extant species, with the first living lungfish formally described in 1837 (Nelson 2006).

In order to make the taxon monophyletic, the tetrapods are sometimes included as part of class Sarcopterygii; that is, fishes in this group are considered more closely related to mammals and other tetrapods than to other fishes (Nelson 2006). The fact that lungfish would be placed in the same class as such diverse animals as eagles, alligators, and chimpanzees, but not with sharks or salmon, shows the emphasis placed on lineage in current taxonomy.

Overview of classification

Sarcopterygii (from Greek sarx, flesh, and pteryx, fin) is traditionally viewed as a class of fishes, including the lungfishes and the coelacanths. Historically, it has been considered, along with Actinopterygii, one of two extant (living) classes of fishes in the taxon Osteichthyes, or bony fishes, characterized by their bony skeleton instead of cartilage. However, Nelson, in his authoritative Fishes of the World (Nelson 2006) discontinues Osteichthyes for formal taxonomic use and instead places Actinopterygii and Sarcopterygii (along with the extinct Acanthodii) in grade Teleostomi. The term osteichthyians is retained for vernacular use for those actinopterygians and sarcopterygians conventionally termed fishes.

Some taxonomists, who continue the use of the term Osteichthyes, nonetheless consider Sarcopterygii a sister superclass to Osteichthyes, instead of a class below them, based on the vast differences between Sarcopterygii and Osteichthyes in fin structure, respiratory structure, and circulatory structure.

Nelson (2006), in order to make Sacropterygii a monophyletic group, includes as part of Sarcopterygii not only the fishes, but also all the tetrapods (amphibians, reptiles, birds, and mammals). This is the approach of many taxonomists who subscribe to the cladistic approach. Indeed, the fin limbs of sarcopterygiians show such a strong similarity to the expected ancestral form of tetrapod limbs that they typically are considered the direct ancestors of tetrapods in the scientific literature. Thus, while there are eight extant species of fishes in Sarcopterygii, Nelson recognizes almost 27,000 extant species in this class. Traditionally, the term Sarcopterygii was used to include only fishes called dipnoans (lobe-finned fishes) and crossopterygians (Nelson 2006).

There are various taxonomic schemes for the class Sarcopterygii. Nelson (2006) recognizes two subclasses of Sarcopterygii: Coelacanthimorpha and Dipnotetrapodomorpha. Coelacanthimorpha includes one order, Coelacanthiformes (coelacanths), with eight families that include only fossil members and one family that includes two living species of coelacanth as well as fossil members. The subclass Dipnotetrapodomopha is a new term that includes several orders and families, with the order Ceratodontiformes including the three extant families, three extant genera, and six modern species.

Characteristics

Sarcopterygians are bony fish with lobed paired fins, which are joined to the body by a single bone (Clack 2002). These fins are considered to have evolved into the legs of the first tetrapod land vertebrates, amphibians. Sarcopterygians 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.

Coelacanths (order Coelacanthiformes), living and extinct, are characterized by a caudal fin with three lobes, external nostrils, and an anterior dorsal fin in front of the center of the body (Nelson 2004). There are two living species, Latimeria chalumne and L. menadoensis (Nelson 2006). The first living species ever found, Latimeria chalumnae, is a marine species known from South Africa, Comoros Archipelago, and the Mozambique area (Nelson 2006). It was first found off of South Africa in 1938. Adults reach 1.8 meters and it has an unusual method of swimming in keeping its body rigid (Nelson 2006). It is the only living chordate with an intracranial joint, although this feature appears in other coelacanths (Nelson 2006).

The lungfishes (dipnoi) have platelike teeth useful for crushing and grinding (Nelson 2006). The six extant species of lungfishes are placed by Nelson in the order Ceratodontiformes of the superorder Ceratodontimorpha. These are characterized by functional lungs, a caudal fin that confluent with the dorsal and anal fins, and the absence of premaxilla and maxilla (Nelson 2006). The extant species are all freshwater forms. There is one species of Australian lungfishes (family Ceratodontidae), one species of South American lungfishes (family Lepidosirenidae; found in Brazil and Paraguay), and four species of African lungfishes (family Protopteridae; all placed in the genus Protopterus). The air bladder (lung) of the Australian lungfishes are unpaired, but the air bladder of the other families of lungfishes are paired (Nelson 2006). Likewise, the Australian lungfishes have flipper-like pectoral and pelvic fins, large scales, and larvae without external gills, while the other species have filamentous pectoral and pelvic fins, without rays, small scales, and larvae with external gills (Nelson 2006).

Evolution of Sarcopterygii

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.

The oldest sarcopterygians were found in the Uppermost Silurian. The first Sarcopterygian closely resembled Acanthodians. The sarcopterygians closest relatives at the time were the actinopterygians (ray-finned fishes). Sarcopterygians probably evolved in the oceans, but they later came into freshwater habitats, possibly as an adaptation to avoid the predatory placoderms, which were dominant in the Early–Middle Devonian seas.

During the Early Devonian, the sarcopterygian line split into two main lineages: the coelacanths and the Rhipidistia.

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, are adapted to use their stubby fins (proto-limbs) to walk on land and find new water if their waterhole is depleted, and use their lungs to breathe air and get sufficient oxygen.

The tetrapodomorphs have the same basic anatomy as the lungfishes, who were their closest kin, but the tetrapodomorphs appear to have inhabited water a little longer, until the Late Devonian. Tetrapods, four legged vertebrates, were the terapodomorphs' descendants. Tetrapods appeared in the Late Devonian epoch.

Non-tetrapod sarcopterygians continued to survive towards the end of Paleozoic era. They suffered heavy losses during the Permian-Triassic extinction event.

Taxonomy

Other than aquatic tetrapods, such as turtles or whales, coelacanths are the only sarcopterygians that still live in the ocean.

The following taxonomy is drawn from Nelson (2006).

• Class SARCOPTERYGII

• Subclass Coelacanthimorpha

Order Coelacanthiformes (coelacanths)

Family Miguashaiidae †

Family Diplocericidae †

Family Hadronectoridae †

Family Rhabdodermatidae †

Family Laugiidae †

Family Whiteiidae †

Family Caelacanthidae †

Family Mawsoniidae †

Family Latimeriidae (coelacanths or gombessas)

Genus Latimeria

Species Latimeria chalumnae

Species Latimeria menadoesnis

Genus Holophagus †

Genus Libys †

Genus Macropoma †

Genus Undina †

• Subclass Dipnotetrapodomorpha

• Unranked 1a. Onychodontida

Order Onychodontiformes †

• Unranked 1b. Rhipidistia

• Unranked 2a. Dipnomorpha

• Superorder Porolepimorpha †

Order Porolepiformes †

Dipnoi (Lungfishes)

• Superorder Dipterimorpha †

Family Diabolepididae †

Family Uranolophidae †

Family Dipnorhynchidae †

Family Chirodipteridae †

Family Stomiahykidae †

Family Dipteridae †

Family Rhynchodipteridae †

Family Fleurantiidae †

Family Phaneropleuridae †

Family Ctenodontidae †

Family Fleurantiidae †

• Superorder Ceratodontimorpha

Order Ceratodontiformes (living lungfishes)

Suborder Ceratodontoidei

Family Ceratodontidae (Australian lungfishes)

Genus Neoceratodus

Species Neoceratodus forsteri

Suborder Lepidosirenoidei

Family Lepidosirenidae (South American lungfishes)

Genus Lepidosiren

Species Lepidosiren paradoxa

Family Protopteridae (African lungfishes)

Genus Protopterus

Species Protopterus aethiopicus

Species Protopterus amphibius

Species Protopterus annectens

Species Protopterus dolloi

• Unranked 2b. Tetrapodomorpha

• Unranked 3a. Rhizodontimorpha (Rhizodontida) †

Order Rhizodontiformes †

Family Rhizodontidae †

• Unranked 3b. Osteolepidimorpha †

• Unranked 4a. Unnamed Ostelepidiformes and Elipistostegalia and Tetrapoda

Order Ostelepidiformes †

• Unranked 4b. Unnamed Elipistostegalia + Tetrapoda

• Infraclass Elipistostegalia †
• Tetrapoda (tetrapods)

See also

• Actinopterygii—ray-finned fishes

References

ISBN links support NWE through referral fees

• Clack, J. A. 2002. Gaining Ground: The Origin and Evolution of Tetrapods. Bloomington, Ind: Indiana University Press. ISBN 0253340543.
• Nelson, J. S. 2006. Fishes of the World, 4th edition. Hoboken, NJ: John Wiley & Sons. ISBN 0471250317.
• Rosen, D. E., P. I. Forey, B. G. Gardiner, and C. Patterson. 1981. Lungfishes, tetrapods, paleontology, and plesiomorphy. Bull. Am. Mus. Nat. Hist. 167(4): 159-276.

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