Cephalaspis

Cephalaspis was a fish from the early Devonian period.

Description
Cephalaspis was a member of the Osteostraci, one of the most advanced sorts of Paleozoic armoured jawless fish. She was a scavenger and bottom-feeder, unable to bite, instead feeding on worms, algae, and small shellfish on the sea-floor; she would have become prey for the large arthropods of her time, including the thunder-scorpion Brontoscorpio and other arthropods such as the eurypterids. She lived during the Lower Devonian period, roughly 400 million years ago. Adaptations

These fish also had an early warning system: special sensors on their skin detected even the tiniest vibrations in the water. But when they’re in danger and when running from it, with their defensive headgear, Cephalaspis couldn’t swim fast for long. They had to rest frequently, or they’d tire completely very quickly.

Spawning
When breading season came, Cephalaspis congregated to head for the one place they could escape the scorpions: fresh water, inland, with their convoy plowing upriver, away from the sea. They returned to the spawning grounds where they hatched, using memories. Their toughened head had a vital weapon, one of the first complex brains, which is much more developed than their rival's who had no memory at all.

Water Dwellers
Cephalaspis appeared in the first episode. Several were seen swimming away from a Brontoscorpio. One was feeding until a Brontoscorpio appeared. It accelerated when it detected a Pterygotus underneath the seabed. Later, a large shoal of Cephalaspis was seen swimming to their spawning grounds further inland. In the process, several were killed by a group of Brontoscorpio. One then evolved into Hynerpeton.

Errors

 * This is no evidence that Cephalaspis migrated to the same spawning grounds each year.
 * Cephalaspis never evolved into Hynerpeton.