Lobe-finned fish breathed with gills. Unlike other fish, they also breathed
with lungs. These were the predecessors of modern lungfish. An abundance of
food swept up onto the beaches during high tide might have enticed these fish
to come ashore. Fierce competition in the ocean for scarce food supplies pro-
vided an extraordinary evolutionary incentive for any animal that could find
food on land.Their descendants became the first advanced animals to populate
the continents.
By the middle Devonian, stiff competition in the sea encouraged
crossopterygians to make short forays onto shore to prey on abundant crus-
taceans and insects. The crossopterygians were lobe-finned fish with heavy,
enamel-like scales. Their fin bones were attached to the skeleton in such a
manner to form primitive limbs.The crossopterygians strengthened their lobe
fins, which eventually evolved into legs, by digging in the sand for food and
shelter.They eventually ventured farther inland, though not too distant from
accessible sources of water such as swamps or streams. Primitive Devonian fish,
similar to today’s lungfish, crawled on their bellies from one pool to another,
pushing themselves along with their fins (Fig. 102).
Links between fish and terrestrial vertebrates were the Devonian lung-
fish.Their descendants still live today in Africa, Australia, and South America,
which comprised the supercontinent Gondwana. Modern lungfish live in
African swamps that seasonally dry out, forcing the fish to hole up for long
stretches until the rains return. They burrow into the moist sand, leaving an
air hole to the surface, and live in suspended animation, breathing with prim-
itive lungs. In this manner, they can survive out of water for several months or
even a year or more if necessary.When the rainy season returns, the pond fills
again, and the fish come back to life, breathing normally with their gills.
In Florida, a walking catfish originating from Asia will leave its drying
pond and travel by pushing itself along with its tail and fins, sometimes a con-Figure 102Air-breathing
fish traveled overland to
new water holes.
Historical Geology