The Ordovician spans 41.2 million years, from 485.4 million to 443.8 million years ago. While there was no plant or animal life on the land, invertebrates and arthropods, dominated the oceans. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared.

The Devonian period was 60 million long and spanned between 419 million and 358 million years ago. During this time, plants began to spread across dry land, and fish reached substantial diversity, leading the Devonian to often be dubbed the "Age of Fishes."