Devonian landscape. The fourth period of the Paleozoic Era, encompassing an interval of geologic time between 418 and 362 million years before present based on radiometric data. The Devonian System encompasses all rocks deposited or formed during the Devonian Period. The Devonian is customarily divided into Lower, Middle, and Upper series and their corresponding epochs. The great diversification of fish in the Devonian has led to the term "Age of Fishes" for the period. The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago, severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida. Illustration originally captioned: Devonian landscape by Franz Unger, 1916. This image has been color-enhanced.