The first fish to have jaws were shark-like creatures that lived about 440 million years ago. Climatius (klime-att-ee-uss), sometimes called “spiny shark”, was an early jawed fish. The jaws probably developed from the set of small bones that supported the gills, with which fish breathe oxygen from the water.
from "100 Things you need to know about Prehistoric Life" by Rupert Matthews.
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