Sarcopterygii - lobe-finned fishes

The Sarcopterygii, or lobe-finned fishes, is a clade containing the coelacanths, lungfishes, tetrapods, and their fossil relatives, including the osteolepiformes and panderichthyids. They are the sister group to the ray-finned fishes (Actinopterygii), together forming the bony fishes (Osteichthyes).

Sarcopterygians are characterised by their fleshy pectoral and pelvic (paired) fins that articulate with the pectoral (shoulder) and pelvic (hip) girdles via a single bone. This is apparent in the coelacanths and lungfishes, which are more intuitively fish-like. These lobe-fins gave rise to the paired limbs of tetrapods, with the single bones representing the humerus (forelimb) and femur (hindlimb).

The once-diverse coelacanths (Actinistia) are now represented by just two species in a single genus -Latimeria chalumnae and L. menadoensis.

The lungfishes (Dipnoi) are also a small relict of a once-diverse assemblage, with only six extant species in three genera - Protopterus (4x species), Lepidosiren paradoxa, and Neoceratodus forsteri.

The palaeontological record makes clear that the terrestrial verterbates evolved from lobe-finned fishes nearly 400 million years ago during the Devonian, and are therefore members of the Sarcopterygii. The only terrestrial vertebrates still living today are the tetrapods, which originated around 350 million years ago and are defined as that group which comprises the common ancestor of the living amphibians and amniotes plus all its descendants. The vertebrate conquest of the land was a major evolutionary transition that required many morphological and physiological changes away from a fish-like form, and has given rise to around 21100 living species and probably many more extinct forms. As such, the extant tetrapods are considered in depth in the following ZooMoodle webpages.

While some molecular data have proposed a sister group relationship between lungfishes and coelacanths to the exclusion of tetrapods (e.g., nuclear 28S rRNA gene; Zardoya & Meyer 1996), and certain studies have been unable to statistically reject the placement of the coelacanths as the closest living relative of the tetrapods (Zardoya & Meyer 1997a, Zardoya et al. 1998), most morphological, palaeontological, and molecular evidence (e.g., combined mitochondrial protein coding genes) supports the lungfishes as the closest living relatives to the tetrapods, to the exclusion of the Actinistia (Meyer & Zardoya, 2003). This is further supported by a unique deletion in the gene encoding RAG2 that is uniquely shared between tetrapods and lungfishes (Venkatesh et al. 2001). This prevailing view is depicted in the following phylogeny:

Distribution and Habitat:

Fossils of Coelacanths have been discovered on every continent, evidence of their previous distribution (Helfman et al., 2009). But modern coelacanths have a limited geographic distribution. Previously thought extinct, Latimeria chalumnae has been indentified since 1938 in the waters of the West Indo Pacific Ocean near the Comoros Island and eastern coast of southern Africa (Bone et Moore, 2008). Latimeria menadoensis has been sighted only in Sulawesi, Indonesia since its discovery in 1999 (Pouyaud et al., 1999). Coelacanths live in the benthic zone, between 200 - 300m along steep underwater slopes and shelves. (Helfman et al., 2009). They aggregate in submarine caves during the day and emerge to feed during the night (Boine and Moore, 2005).

The Dipnoi have a wider distribution - Central and South Africa (Protopterus), Amazon and Paron riverbasins of South America (Lepidosiren), and Queensland, Australia (Neoceratodus) (Bone and Moore, 2005; Helfman et al. 2009). They inhabit freshwater streams, rivers and swamps. During drought, Protopterus and Lepidosiren will burrow into the earth, breath intermittently with their lung and can remain in this state of torpidity for months to avoid desiccation (Helfman et al, 2009).Neoceratodus is unable to estivate and lives only in deep rivers where there is no risk of drought.

Conservation Status (IUCN):

Latimeria chalumnae is classified by the IUCN as critically endangered. Latimeria menadoensis isvulnerable, but there have only ever been three sighting of this species so it is difficult to ascertain populations levels (IUCN, 2008). Both species are often by-catch of deep sea trawlers and shark nets. Low fecundity and slow growth rates put the Coelacanths at risk of extinction and even small depletions in population size can take decades to recover.

The Dipnoi are not considered at risk of extinction because of their wide distribution. But they do face anthropogenic threats such as habitat loss and degradation - contruction of dams impedes flooding, the spread of agriculture is reducing wetland habitats and both practices produce harmful pollution (IUCN, 2008).

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