During the Ordovician and Silurian periods, fish were beginning to evolve. By the Devonian, they were well established in the sea and were spreading into rivers and lakes.
The earliest fish had no jaws - just suckers. Later, jaws and armour developed. Some fish had skeletons of bone. Others had skeletons of horny gristle. Many even developed lungs and muscular fins and could spend some time out of the water. These were the ancestors of all the animals with back-bones (vertebrates), which lived on the land.
(A) SHARKS
Early sharks, such as Cladoselache, looked very much like modern types.
(B) FIERCE JAWS
Fish with complex jaws, scales, and plates, such as Coccosteus, soon evolved.
(C) JAWLESS FISH
The earliest fish, like Cephalaspis, had no jaws. These were bottom-dwelling fish that sucked in food.
(D) ARMOUR
Bothriolepiswas an early form of fish. The head and front half were protected by armoured plates.
(E) LOBEFINS
Some fish, like Holoptychius, developed pairs of muscular fins that looked like limbs.
(F) SPINY SHARKS
The 'spiny sharks,' like Climatius, had rows of tiny fins along the belly.
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