Cambrian
The first fish and the first vertebrates, were the ostracoderms, which appeared in the Cambrian, about 510 million years ago, and became extinct near the end of the Devonian, about 377 million years ago. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were often less than 30 cm (1 ft) long. The ostracoderms are placed in the class Agnatha along with the living jawless fishes, the lampreys and hagfishes, which are believed to be descended from the ostracoderms, as are all jawed fishes, or gnathostomes. Paired fins, or limbs, first evolved within this group.
Read more about this topic: Prehistoric Fish