Devonian

EtymologyName formalityFormalNickname(s)Age of FishesUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionFAD of the GraptoliteLower boundary GSSPKlonk, Czech Republic
49°51′18″N13°47′31″E / 49.8550°N 13.7920°E / 49.8550; 13.7920Lower GSSP ratified1972Upper boundary definitionFAD of the ConodontSiphonodella sulcata (discovered to have biostratigraphic issues as of 2006).Upper boundary GSSPLa Serre, Montagne Noire, France
43°33′20″N3°21′26″E / 43.5555°N 3.3573°E / 43.5555; 3.3573Upper GSSP ratified1990Atmospheric and climatic dataSea level above present dayRelatively steady around 189 m, gradually falling to 120 m through period

The Devonian ( ) is a geologic period and system of the Paleozoicera during the Phanerozoiceon, spanning 60.7 million years from the end of the preceding Silurian period at 419.62 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.86 Ma. It is the fourth period of both the Paleozoic and the Phanerozoic. It is named after Devon, South West England, where rocks from this period were first studied.

The first significant evolutionary radiation of life on land occurred during the Devonian, as free-sporingland plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants (pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revolution. The earliest land animals, predominantly arthropods such as myriapods, arachnids and hexapods, also became well-established early in this period, after beginning their colonization of land at least from the Ordovician Period.

Fish, especially jawed fish, reached substantial diversity during this time, leading the Devonian to be called the Age of Fishes. The armoredplacoderms began dominating almost every known aquatic environment. In the oceans, cartilaginous fishes such as primitive sharks became more numerous than in the Silurian and Late Ordovician. Tetrapodomorphs, which include the ancestors of all four-limbed vertebrates (i.e. tetrapods), began diverging from freshwaterlobe-finned fish as their more robust and muscled pectoral and pelvic fins gradually evolved into forelimbs and hindlimbs, though they were not fully established for life on land until the Late Carboniferous.

The first ammonites, a subclass of cephalopodmolluscs, appeared. Trilobites, brachiopods and the great coral reefs were still common during the Devonian. The Late Devonian extinction, which started about 375 Ma, severely affected marine life, killing off most of the reef systems, most of the jawless fishes, the placoderms, and nearly all trilobites save for a few species of the order Proetida. The subsequent end-Devonian extinction, which occurred at around 359 Ma, further impacted the ecosystems and completed the extinction of all calcitesponge reefs and placoderms.

Devonian palaeogeography was dominated by the supercontinentGondwana to the south, the small continent of Siberia to the north, and the medium-sized continent of Laurussia to the east. Major tectonic events include the closure of the Rheic Ocean, the separation of South China from Gondwana, and the resulting expansion of the Paleo-Tethys Ocean. The Devonian experienced several major mountain-building events as Laurussia and Gondwana approached; these include the Acadian Orogeny in North America and the beginning of the Variscan Orogeny in Europe. These early collisions preceded the formation of the single supercontinent Pangaea in the Late Paleozoic.