Fossil taxa, see text Lungfish are freshwater vertebrates belonging to the class Dipnoi.

The mouths of lungfish typically bear tooth plates, which are used to crush hard shelled organisms.

Today there are only six known species of lungfish, living in Africa, South America, and Australia, though they were formerly globally distributed.

The fossil record of the group extends into the Early Devonian, over 410 million years ago.

The earliest known members of the group were marine, while almost all post-Carboniferous representatives inhabit freshwater environments.

All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition.

Basal ("primitive") lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish groups, including all modern species, show a significant reduction in the 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 lies beneath the skin and the cosmine covering is lost.

During the breeding season, the South American lungfish develops a pair of feathery appendages that are actually highly modified pelvic fins.

"Odontodes" on the palate and lower jaws develop in a series of rows to form a fan-shaped occlusion surface.

In several groups, including the modern lepidosireniformes, these ridges have been modified to form occluding blades.

In contrast, the lungs of lungfish are subdivided into numerous smaller air sacs, maximizing the surface area available for gas exchange.

When a lungfish is obtaining oxygen from its gills, its circulatory system is configured similarly to the common fish.

Blood flow through the secondary lamellae is countercurrent to the water, maintaining a more constant concentration gradient.

Air flow through the mouth is tidal, and through the lungs it is bidirectional and observes "uniform pool" diffusion of oxygen.

Lungfish are omnivorous, feeding on fish, insects, crustaceans, worms, mollusks, amphibians and plant matter.

Changes in physiology allow it to slow its metabolism to as little as one sixtieth 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).

[17] Fossil records of this group date back 380 million years, around the time when the higher vertebrate classes were beginning to evolve.

[18] Fossils of lungfish belonging to the genus Neoceratodus have been uncovered in northern New South Wales, indicating that the Queensland lungfish has existed in Australia for at least 100 million years, making it a living fossil and one of the oldest living vertebrate genera on the planet.

[24] They have the lowest aquatic respiration of all extant lungfish species,[25] and their gills are greatly reduced and essentially non-functional in the adults.

These fish have a yellowish gray or pinkish toned ground color with dark slate-gray splotches, creating a marbling or leopard effect over the body and fins.

[30][31] It generally reaches only 44 centimetres (17 inches) long, making it the smallest extant lungfish in the world.