The earliest representative of lungfish is the bipterygian fish of the Middle Devonian. Based on this, lungfish were relatively abundant from the Late Devonian to the Carboniferous, but today only a few highly specialized representatives live in the equatorial regions of Africa, Australia, and South America.
Dipterus
The Australian lungfish is the most primitive of the three regional lungfish species. They live in a river in Queensland, and during the dry season when the river's water level drops, they live in isolated small pools, coming to the surface to breathe air using their single lungs, which are covered with many blood vessels. However, this fish cannot live away from the water.
African and South American lungfish can survive for months even after their rivers have completely dried up. When the dry season arrives, these lungfish burrow into the mud and enclose themselves, leaving only one or a few small holes for ventilation. Unlike the Australian lungfish, these two species have a pair of lungs.
The ability of lungfish to breathe air naturally leads us to think that they may be an intermediate transitional link between fish and terrestrial vertebrates. In particular, the paired fins of the Australian lungfish have evolved to resemble slender legs, and they can even use these paired fins to move their bodies on the bottom of rivers or ponds as if walking. Such body structure and behavior vividly reflect the early form of terrestrial tetrapod vertebrates.
Compared to other fish families, lungfish have always been a small and inconspicuous family. From the main branch of this evolutionary lineage developed the genus *Ceratodon*, which was once widely distributed in most of the world's continental waters during the Triassic period and throughout the subsequent Mesozoic era. Modern Australian lungfish are direct descendants of *Ceratodon*.
African and South American lungfish are offshoots that diverged from the main evolutionary branch of the subclass Lungfish. The paired fins of African lungfish have degenerated into long, thin, whip-like structures, while the paired fins of South American lungfish have also shrunk significantly, becoming rather small appendages.
lungfish
The distribution of lungfish across the Southern Hemisphere continents has led some geohistorians to believe that it proves the close connections between the continents of the Southern Hemisphere in the past. However, other scientists argue that since various lungfish species once roamed the world throughout geological history, the current distribution of lungfish merely represents the remnants of a once vast lungfish habitat in the modern era.
The existence of lungfish provides a reference for understanding the transition from fish to primitive amphibians. However, the evidence to say that lungfish are the ancestors of amphibians is still insufficient. For example, their skulls are not very ossified, which is quite different from the solid, bony skulls of amphibians. Moreover, the paired fins of lungfish are highly specialized and appear too delicate, making it difficult to associate them with the strong limbs of amphibians.
