Lungfishes are sarcopterygian fish belonging to the order Dipnoi. Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Today, they live only in Africa, South America, and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests that advanced lungfish had a cosmopolitan freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of both Pangea and subsequently Gondwana and Laurasia.
All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition. Basal lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish taxa, including all modern species, show a significant reduction in marginal bones and a cartilaginous braincase. The bones of the skull roof in primitive lungfish are covered in a mineralized tissue called cosmine, but in post-devonian lungfishes, the skull roof is subdermal and the cosmine covering is lost. All modern lungfish show significant reductions and fusions of the bones of the skull roof, and the specific bones of the skull roof show no homology to the skull roof bones of actinopterygiians or tetrapods.
The dentition of lungfish is conspicuously different from that of any other vertebrate group. Odontodes on the palate and lower jaws develop in a series of rows to form a fan-shaped occlusion surface. These odontodes then wear to form a uniform crushing surface. In several groups, including the modern lepidosireniformes, these ridges have been modified to form occluding blades.
The modern lungfishes have a number of larval features, which suggest paedomorphosis. They also demonstrate the largest genome among the vertebrates.
Modern lungfish all have an elongate body with fleshy paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal, and anal fin of most fishes.
African and South American lungfish are capable of surviving seasonal desiccation of habitats by burrowing into mud and estivating throughout the dry season. Changes in physiology allow the lungfish to slow its metabolism to greater than 1/60th of the normal metabolic rate, and protein waste is converted from ammonia to less-toxic urea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water). Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. It has been proposed both that burrowing is plesiomorphic for lungfish as well as that gnathorhizids are directly ancestral to modern Lepidosireniformes, but it is possible that the similarity is simply due to convergent evolution or parallel evolution.
Taxonomy of lungfishes can be viewed in terms of taxonomic position of lungfishes within Osteichthyes and taxonomic relationships within lungfishes.
The former is well-resolved. In a cladogram of osteichthyan relationships, lungfishes clade closest with Powichthyes, and then with the Porolepiformes. Together, these taxa form the Dipnomorpha. The dipnomorpha form a sister group to the tetrapodomorpha. Together, these are known as the Rhipidistia, and form a sister group to the coelacanths.
The latter is significantly more difficult to resolve. While Devonian lungfish retain enough ossification of the endocranium to determine relationships, post-Devonian lungfish are represented entirely by skull roofs and teeth, as the rest of the skull is cartilaginous. Additionally, many of the taxa that have been identified may not be monophyletic. Current phylogenetic studies support the following relationships of major lungfish taxa:
Subclass DIPNOI ,--†Family Diabolichthyidae | ,--†Family Uranolophidae | | __,--†Family Speonesydrionidae '-|-| '--†Family Dipnorhynchidae | ,--†Family Stomiahykidae '---| ___ ,--†Family Chirodipteridae | '-|--†Family Holodontidae |------†Family Dipteridae | __,--†Family [[Fleurantiidae] '-| '--†Family Rhynchodipteridae '--†Family Phaneropleuridae | ,--†Family Ctenodontidae '-| ,--†Family Sagenodontidae '-|--†Family Gnathorhizidae '--Order Ceratodontiformes |--†Family Asiatoceratodontidae |--†Family Ptychoceratodontidae |--Family Ceratodontidae | '--†Genus Ceratodus | '--†Genus Metaceratodus '--Family Neoceratodontidae | '--†Genus Mioceratodus | '--Genus Neoceratodus - Queensland Lungfish '--Order Lepidosireniformes '--Family Lepidosirenidae - South American lungfish '--Family Protopteridae - African Lungfish
- Lungfish information site
- Dipnoiformes at Palaeos.com
- Dipnoi at the University of California Museum of Paleontology
Ahlberg, PE, Smith, MM, and Johanson, Z, (2006). Developmental plasticity and disparity in early dipnoan (lungfish) dentitions. Evolution and Development 8(4):331-349.
Schultze, HP, and Chorn, J., (1997). The Permo-Carboniferous genus Sagenodus and the beginning of modern lungfish. Contributions to Zoology 61(7):9-70.