The Devonian is a Geologic Period that extends from about 345 to 400 million Years before the present. As with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by 5-10 million years. The Devonian is named for England's Devonshire area where Devonian Outcrops are common. The Devonian follows the Silurian Period and precedes the Mississippian subperiod of the Carboniferous Period.
The Devonian is usually broken into Lower, Middle, and Upper subdivisions. The Faunal stages from youngest to oldest are:
Famennian/Chautauquan/Canadaway/Conneaut/Conneautan/Conewango/Conewangan (Upper) Frasnian/Senecan/Sonyea/Sonyean/West Falls (Upper) Cazenovia/Cazenovian (Middle) Givetian/Erian/Senecan/Tioughniogan/Tioughnioga/Taghanic/Taghanican/Genesee/Geneseean (Middle) Eifelian/Southwood (Middle) Helderberg (Lower) Emsian/Sawkill/Deer Park (Lower) Pragian/Siegenian (Lower) Lochkovian/Gedinnian (Lower)
Devonian rocks are oil and gas producers in some areas.
Sea levels in the Devonian were generally high. Fish were abundant and diverse. The first sharks appeared early in the period. Bony fish, many of substantial size appeared shortly thereafter. Small land plants appeared early in the period. By the end of the period forests of primitive plants existed. Both vertebrates and arthropods were solidly established on the land. Marine faunas continued to be dominated by bryozoa, brachiopods and corals. Trilobites were still fairly common, but less diverse than in earlier periods.
The southern continents remained tied together into the Supercontinent Gondwana. In equatorial regions, North America and Europe formed a continent. The remainder of modern Eurasia lay in the Northern Hemisphere.