The evolution of lunglike sacs in fishes appears to have been a response to the inadequacy of gills for respira-
tion in oxygen-poor waters, but it also set the stage for the invasion of the land. Some early ray-finned fishes probably used their lungs to supplement their gills when oxygen levels in the water were low, as lung-fishes do today. This ability would also have allowed them to leave the water temporarily and breathe air when pursued by predators unable to do so. But with their unjointed fins, these fishes could only flop around on land, as most fish out of water do today. Changes in the structure of the fins allowed these fishes to move on land.
The lobe-finned fishes (class Actinistia) were the first lineage to evolve jointed fins. Lobe-fins flourished from the Devonian period until about 65 million years ago, when they were thought to have become extinct. However, in 1938, a liv ing lobe-fin was caught by commercial fishermen off South Africa. Since that time, several dozen specimens of this extraordinary fish, Latimeria chalumnae, have been collected. La-timeria, a predator on other fish, reaches a length of about 1.8 meters and weighs up to 82 kilograms (Figure 34.14a). Its skeleton is mostly composed of cartilage, not bone. A second species, L. menadoensis, was discovered in 1998 off the Indonesian island of Sulawesi.
Lungfishes (class Dipnoi) were important predators in shallow-water habitats in the Devonian, but most lineages died out. The three surviving species live in stagnant swamps and muddy waters in the Southern Hemisphere, one each in South America, Africa, and Australia (Figure 34.14b). Lung-fishes have both gills and lungs. When ponds dry up, they can burrow deep into the mud and survive for many months in an inactive state.
It is believed that descendants of some lungfishes began to use terrestrial food sources, became more fully adapted to life on land, and eventually evolved to become the tetrapods— the four-legged amphibians, reptiles, birds, and mammals.
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