The Paleozoic era of Devonian period is known as 'age of fishes' because there was great development of fishes on the earth.
The Devonian period begun from 419.2 million years ago to 358.9 million years ago. This era was known as the age of fishes because in this era different varieties of fishes were spawned and the most formidable ones among them were the armored placoderms which are the group of fishes that appeared during the Silurian period with powerful jaws and blade like plates that acted as teeth.