Devonian

The Devonian (/dɪˈv.ni.ən, də-, dɛ-/ dih-VOH-nee-ən, də-, deh-)[9][10] is a geologic period and system of the Paleozoic, spanning 60.3 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya.[11] It is named after Devon, England, where rocks from this period were first studied.

Devonian
419.2 ± 3.2 – 358.9 ± 0.4 Ma
Late Devonian world map
Chronology
Etymology
Name formalityFormal
Nickname(s)Age of Fishes
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitPeriod
Stratigraphic unitSystem
Time span formalityFormal
Lower boundary definitionFAD of the Graptolite Monograptus uniformis
Lower boundary GSSPKlonk, Czech Republic
49.8550°N 13.7920°E / 49.8550; 13.7920
GSSP ratified1972[5]
Upper boundary definitionFAD of the Conodont Siphonodella sulcata (discovered to have biostratigraphic issues as of 2006).[6]
Upper boundary GSSPLa Serre, Montagne Noire, France
43.5555°N 3.3573°E / 43.5555; 3.3573
GSSP ratified1990[7]
Atmospheric and climatic data
Sea level above present dayRelatively steady around 189 m, gradually falling to 120 m through period[8]

The first significant adaptive radiation of life on dry land occurred during the Devonian. 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. The arthropod groups of myriapods, arachnids and hexapods also became well-established early on in this period, after having started their expansion to land at least from the Ordovician period.

Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fishes. The placoderms began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (tetrapods) began adapting to walk on land, as their strong pectoral and pelvic fins gradually evolved into legs, though they were not fully established until the Late Carboniferous.[12] In the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician.

The first ammonites, a subclass of molluscs, appeared. Trilobites, the mollusc-like brachiopods, and the great coral reefs were still common. The Late Devonian extinction which started about 375 million years ago[13] severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida.

Devonian palaeogeography was dominated by the supercontinent of Gondwana 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 Pangaea in the Late Paleozoic.


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