85% of the Devonian earth was covered in ocean water. The oceans and fresh water areas were filled with primative fish. The Devonian is known as the age of the fishes. The oceans of this time were filled with boney fish, lobe fin fish, sharks, ray fin fish, and a wide assortment of invertebrates like coral and cephalopods.
There were two continents in the Devonian, Euramerica and Gonduana. On land there were massive forests of vascular plants filled with land invertabrates like millipedes, centipeds, aractnids, and dragonfly-like animals. The first vertibrates came to land some time in the middle Devonian.
The Devonian extinction was caused by volcanic eruptions and a loss of dissolved oxygen in the oceans. The mass extinction wiped out 80% of all life and was followed by a mass cooling event probably because of ash blocking out the sun. The loss of oxygen in the oceans was thought to have been caused by massive algae blooms dieing out and removing oxygen.
The effects of the Devonian were massive. In the devonian the ancestors to all land vertibrates were born. The first forest formed. The devonian gave way to some of the most successful lineages such as ferns, nautilus, and scorpians. This time period lead to the extinction of the giant fungus of the Silurian Period.