The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya (million years ago), to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya (ICS, 2004). It is named after Devon, England, where rocks from this period were first studied.

The Devonian period experienced the first significant adaptive radiation of terrestrial life. Since large vertebrate terrestrial herbivores had not yet appeared, free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the "Age of Fish". The first ray-finned and lobe-finned bony fish appeared. The ancestors of all tetrapods, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik) as they became adapted to walking on land. 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 severely affected marine life.