Devonian

System (period)		Series (epoch)	Stage (age)	Lower boundary, Ma
Carboniferous	Mis.	Lower	Tournaisian	358.9±0.4
Devonian		Upper (Late)	Famennian	372.2±1.6
			Frasnian	382.7±1.6
		Middle	Givetian	387.7±0.8
			Eifelian	393.3±1.2
		Lower (Early)	Emsian	407.6±2.6
			Pragian	410.8±2.8
			Lochkovian	419.2±3.2
Silurian		Pridoli		older
Subdivisions and "golden spikes" according to IUGS as of September 2023

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The Devonian is the fourth period (system in stratigraphy) of the Paleozoic era. It extends from the end of the Pridoli epoch of the Silurian period 419.2 ± 3.2 Ma to the beginning of the Tournaisian age of the Carboniferous (Mississippian) 358.9 ± 0.4 Ma. It is named after Devon, England, where rocks from this period were first studied.

During the Devonian period the fish like Eusthenopteron evolved legs and started to walk on land as tetrapodomorphs like Ichthyostega, and the first arthropods like insects and spiders appeared. The first seed-bearing plants (seed ferns) spread across dry land, forming the first forests in Earth history. In the oceans, fish diversified into the first sharks, and the first lobe-finned and bony fish. The first ammonite mollusks appeared, and trilobites, the mollusc-like brachiopods, as well as great coral reefs were still common. The Late Devonian extinction severely affected marine life. The paleogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small supercontinent of Euramerica in the middle.

Naming

The period is named after Devon, England where Devonian outcrops are common. While the rock beds that define the start and end of the period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian Period 419.2 million years ago (Mya), to the beginning of the Carboniferous Period 358.9 Mya (and the beginning of the Mississippian subperiod of the Carboniferous) (IUGS 2023).

The Devonian is also known as the Age of Fishes, but the term is out of favor. While fish underwent a major radiation it was only one of several major evolutionary landmarks during the period, and other lifeforms were more common.

The Devonian has also erroneously been characterized as a Greenhouse Age, due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid.

In nineteenth-century texts the Devonian has been called the Old Red Age, after the red and brown terrestrial deposits known in the United Kingdom as the "Old Red Sandstone" in which these early discoveries were found.

Definition

The Global Stratotype Section and Point (GSSP) of the Devonian system as well as the Lower Devonian series and the Lochkovian stage was ratified in 1972. The lower boundary was fixed within Bed 20 in Klonk, Czech Republic, and the first appearance of the graptolite species Monograptus uniformis is related to it.

Devonian subdivisions

The Devonian system is broken into Lower, Middle, and Upper series, and each of them includes two or three stages. In geochronology, all series correspond to the same epochs, and all stages correspond to the same ages. The subdivisions of the Devonian from upper/younger to lower/oldest are:

• Upper Devonian series (corresponds to the Late Devonian epoch)
  • Famennian
  • Frasnian
• Middle Devonian series/epoch
  • Givetian
  • Eifelian
• Lower Devonian series (corresponds to the Early Devonian epoch)
  • Emsian
  • Pragian
  • Lochkovian

Smaller units like groups or horizons are used in several regions but they cannot be correlated worldwide.

Devonian palaeogeography

The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together. The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn, which is formed as much in Paleozoic times as nowadays by the convergence of two great airmasses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidized iron (hematite) characteristic of drought conditions.

Near the equator, Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountains and forming the Caledonides in Great Britain and Scandinavia. The west coast of Devonian North America, by contrast, was a passive margin with deep silty embayments, river deltas and estuaries, in today's Idaho and Nevada; an approaching volcanic island arc reached the steep slope of the continental shelf in Late Devonian times and began to uplift deep water deposits, a collision that was the prelude to the mountain-building episode of Mississippian times called the Antler orogeny.

The southern continents remained tied together in the supercontinent of Gondwana. The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas, where tropical reef organisms lived.

The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean, and Ural Ocean (which was closed during the collision with Siberia and Baltica).

Devonian biota

Marine biota

Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods and corals. Lily-like crinoids were abundant, and trilobites were still fairly common, but less diverse than in earlier periods. The ostracoderms were joined in the mid-Devonian by the first jawed fishes, the great armored placoderms, as well as the first sharks and ray-finned fish. The first shark, the Cladoselache, appeared in the oceans during the Devonian period. They became abundant and diverse. In the late Devonian the lobe-finned fish appeared, giving rise to the first tetrapods.

Reefs

A great barrier reef, now left high and dry in the Kimberley Basin of northwest Australia, once extended a thousand kilometers, fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant frameworks close to sea level. The main contributors of the Devonian reefs were unlike modern reefs, which are constructed mainly by corals and calcareous algae. They were composed of calcareous algae and coral-like stromatoporoids, and tabulate and rugose corals, in that order of importance.

Terrestrial biota

By the Devonian Period, life was well underway in its colonization of the land. The bacterial and algal mats were joined early in the period by primitive plants that created the first recognizable soils and harbored some arthropods like mites, scorpions and myriapods. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.

By the Late Devonian, forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-like ancestral fern Archaeopteris, grew as a large tree with true wood. These are the oldest known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and plants that characterizes a recognizably modern world had its genesis in the late Devonian, with first evidence of insect-spore interactions (spores became larger and some with spines and hooks for dispersal). Insects also became winged for flight at this time. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition.

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event. see Late Devonian extinction.

Also in the Devonian, both vertebrates and arthropods were solidly established on the land.

Fungal life

While the marine fungi remained in the oceans and diversified into nature's recyclers, land fungi were also recycling the land, decomposing both plants' and animals' dead bodies. Also, due to more terrestrial plants and animals appearance, fungi flourished.

Gallery

Devonian/Gallery

Further reading

• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

See also

• Geologic timescale
• Phacops rana: a Devonian trilobite.

External links

• UC Berkeley site introduces the Devonian.
• 'Devonian Times;' life and ecology.
• Devonian reef system in northwest Australia.
• "International Commission on Stratigraphy (ICS)". Geologic Time Scale 2004. http://www.stratigraphy.org/. Retrieved Sept 19.