The Devonian ( ) is a
geologic period and system of the
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
era, spanning 60.3 million years from the end of the
Silurian, million years ago (Mya), to the beginning of the
Carboniferous, Mya.
It is named after
Devon
Devon ( , historically known as Devonshire , ) is a ceremonial and non-metropolitan county in South West England. The most populous settlement in Devon is the city of Plymouth, followed by Devon's county town, the city of Exeter. Devo ...
, England, where rocks from this period were first studied.
The first significant
adaptive radiation
In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic int ...
of life on dry land occurred during the Devonian.
Free-sporing vascular plants began to spread across dry land, forming extensive
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
s 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
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
groups of
myriapods
Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
The fossil record of myriapods reaches back into the late Silurian, ...
,
arachnids and
hexapods also became well-established early in this period, after starting their expansion to land at least from the
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
period.
Fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fishes. The
placoderms
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were ...
began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s) 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
Late may refer to:
* LATE, an acronym which could stand for:
** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia
** Local-authority trading enterprise, a New Zealand business law
** Local average treatment effect, ...
. In the oceans, primitive
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s became more numerous than in the Silurian and
Late Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
...
.
The first
ammonites
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...
, a subclass of molluscs, appeared.
Trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
s, the mollusc-like
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s, and the great
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
C ...
s were still common. The
Late Devonian extinction
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...
which started about 375 million years ago severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order
Proetida
Proetida is an order of trilobite that lived from the Ordovician to the Permian. It was the last order of trilobite to go extinct, finally dying out in the Permian-Triassic extinction event.
Description
These typically small trilobites resem ...
.
Devonian
palaeogeography
Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term pal ...
was dominated by the
supercontinent
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leav ...
of
Gondwana to the south, the small continent of
Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...
to the north, and the medium-sized continent of
Laurussia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
to the east. Major tectonic events include the closure of the
Rheic Ocean
The Rheic Ocean was an ocean which separated two major palaeocontinents, Gondwana and Laurussia (Laurentia- Baltica-Avalonia). One of the principal oceans of the Palaeozoic, its sutures today stretch from Mexico to Turkey and its closure result ...
, the separation of
South China from Gondwana, and the resulting expansion of the
Paleo-Tethys Ocean
The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; exi ...
. 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
The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.
Nomenclature
The name ''Variscan'', comes f ...
in Europe. These early collisions preceded the formation of
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
in the
Late Paleozoic.
History
![Lummaton Quarry 1](https://upload.wikimedia.org/wikipedia/commons/7/76/Lummaton_Quarry_1.JPG)
The period is named after
Devon
Devon ( , historically known as Devonshire , ) is a ceremonial and non-metropolitan county in South West England. The most populous settlement in Devon is the city of Plymouth, followed by Devon's county town, the city of Exeter. Devo ...
, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was eventually resolved by the definition of the Devonian Period in the geological timescale.
The Great Devonian Controversy
The Great Devonian Controversy began in 1834 when Roderick Murchison disagreed with Henry De la Beche as to the dating of certain petrified plants found in coals in the Greywacke stratum in North Devon, England. De La Beche was claiming that s ...
was a long period of vigorous argument and counter-argument between the main protagonists of
Roderick Murchison with
Adam Sedgwick against
Henry De la Beche
Sir Henry Thomas De la Beche KCB, FRS (10 February 179613 April 1855) was an English geologist and palaeontologist, the first director of the Geological Survey of Great Britain, who helped pioneer early geological survey methods. He was the ...
supported by
George Bellas Greenough
George Bellas Greenough FRS FGS (18 January 1778 – 2 April 1855) was a pioneering English geologist. He is best known as a synthesizer of geology rather than as an original researcher.
Trained as a lawyer, he was a talented speaker and his ...
. Murchison and Sedgwick won the debate and named the period they proposed as the Devonian System.
While the
rock beds that define the start and end of the Devonian Period are well identified, the exact dates are uncertain. According to the
International Commission on Stratigraphy
The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigr ...
, the Devonian extends from the end of the Silurian
Mya, to the beginning of the
Carboniferous Mya – in
North America, at the beginning of the
Mississippian subperiod of the Carboniferous.
In 19th 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
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian, Breconian, and Farlovian stages, the latter three of which are placed in the Devonian.
The Devonian has also erroneously been characterised as a "greenhouse age", due to
sampling bias: most of the early Devonian-age discoveries came from the
strata of
western Europe
Western Europe is the western region of Europe. The region's countries and territories vary depending on context.
The concept of "the West" appeared in Europe in juxtaposition to "the East" and originally applied to the ancient Mediterranean ...
and eastern
North America, which at the time straddled the
Equator as part of the supercontinent of Euramerica where
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
signatures of widespread reefs indicate tropical
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
s that were warm and moderately humid. In fact the climate in the Devonian differed greatly during its
epochs
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
and between geographic regions. For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, Australia, North America, and China, but Africa and
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
had a warm
temperate climate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and
temperate climate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
s were more common.
Subdivisions
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those
epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
s are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.
;Early Devonian
The Early Devonian lasted from and began with the
Lochkovian
The Lochkovian is one of three faunal stages in the Early Devonian Epoch. It lasted from 419.2 ± 3.2 million years ago to 410.8 ± 2.8 million years ago. It marked the beginning of the Devonian Period, and was followed by the Pragian Stage. It is ...
Stage , which was followed by the
Pragian from and then by the
Emsian
The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...
, which lasted until the Middle Devonian began, .
During this time, the first
ammonoids
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
appeared, descending from
bactritoid nautiloids
Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species ...
. Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to the order
Agoniatitida
Agoniatitida, also known as the Anarcestida, is the ancestral order within the cephalopod subclass Ammonoidea originating from bactritoid nautiloids, that lived in what would become Africa, Asia, Australia, Europe, and North America during t ...
, which in later epochs evolved to new ammonoid orders, for example
Goniatitida
Goniatids, informally goniatites, are ammonoid cephalopods that form the order Goniatitida, derived from the more primitive Agoniatitida during the Middle Devonian some 390 million years ago (around Eifelian stage). Goniatites (goniatitids) sur ...
and
Clymeniida
Clymeniida is an order of ammonoid cephalopods from the Upper Devonian characterized by having an unusual dorsal siphuncle. They measured about in diameter and are restricted to Europe, North Africa, and possibly Australia.
Morphologic cha ...
. This class of
cephalopod molluscs would dominate the marine fauna until the beginning of the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
Era.
;Middle Devonian
The Middle Devonian comprised two subdivisions: first the
Eifelian
The Eifelian is the first of two faunal stages in the Middle Devonian Epoch. It lasted from 393.3 ± 1.2 million years ago to 387.7 ± 0.8 million years ago. It was preceded by the Emsian Stage and followed by the Givetian Stage.
North American ...
, which then gave way to the
Givetian
The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian
The Frasnian is one of two faunal stages in the ...
. During this time the jawless
agnathan fishes began to decline in diversity in freshwater and marine environments partly due to drastic environmental changes and partly due to the increasing competition, predation, and diversity of
jawed fishes
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs, and the ability to crawl out of the water and onto the land for short periods of time.
;Late Devonian
Finally, the Late Devonian started with the
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage.
Major reef-building was under way during th ...
, , during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing
Famennian
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used b ...
subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian, .
[
]
Climate
The Devonian was a relatively warm period, and probably lacked any glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
s for much of the period. The temperature gradient from the equator to the poles was not as large as it is today. The weather was also very arid, mostly along the equator where it was the driest. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of in the Early Devonian. levels dropped steeply throughout the Devonian Period. The newly evolved forests drew carbon out of the atmosphere, which were then buried into sediments. This may be reflected by a Mid-Devonian cooling of around . The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to a Late Devonian warming. The climate would have affected the dominant organisms in reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...
s; microbes
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids. At the terminus of the Devonian, Earth rapidly cooled into an icehouse, marking the beginning of the Late Palaeozoic Ice Age.
Paleogeography
The Devonian world involved many continents and ocean basins of various sizes. The largest continent, Gondwana, was located entirely within the Southern Hemisphere. It corresponds to modern day South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
, Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, Australia, Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
, and India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, as well as minor components of North America and Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
. The second-largest continent, Laurussia, was northwest of Gondwana, and corresponds to much of modern-day North America and Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...
. Various smaller continents, microcontinents, and terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...
s were present east of Laurussia and north of Gondwana, corresponding to parts of Europe and Asia. The Devonian Period was a time of great tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
activity, as the major continents of Laurussia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
and Gondwana drew closer together.
Sea levels were high worldwide, and much of the land lay under shallow seas, where tropical reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...
organisms lived. The enormous "world ocean", Panthalassa
Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek "all" and "sea"), was the superocean that surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During th ...
, occupied much of the Northern Hemisphere as well as wide swathes east of Gondwana and west of Laurussia. Other minor oceans were the Paleo-Tethys Ocean
The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; exi ...
and Rheic Ocean
The Rheic Ocean was an ocean which separated two major palaeocontinents, Gondwana and Laurussia (Laurentia- Baltica-Avalonia). One of the principal oceans of the Palaeozoic, its sutures today stretch from Mexico to Turkey and its closure result ...
.
Laurussia
By the early Devonian, the continent Laurussia (also known as Euramerica) was fully formed through the collision of the continents Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...
(modern day North America) and Baltica
Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains.
The thick core of Baltica, the East European Craton, ...
(modern day northern and eastern Europe). The tectonic effects of this collision continued into the Devonian, producing a string of mountain ranges along the southeastern coast of the continent. In present-day eastern North America, the Acadian Orogeny continued to raise the Appalachian Mountains
The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. They ...
. Further east, the collision also extended the rise of the Caledonian Mountains of Great Britain
Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...
and Scandinavia
Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and Swe ...
. As the Caledonian Orogeny wound down in the later part of the period, orogenic collapse
In geology, orogenic collapse is the thinning and lateral spread of thickened Crust (geology), crust. It is a broad term referring to processes which distribute material from regions of high gravitational energy, gravitational potential energy to r ...
facilitated a cluster of granite intrusions in Scotland.
Most of Laurussia was located south of the equator, but in the Devonian it moved northwards and began to rotate counterclockwise towards its modern position. While the most northern parts of the continent (such as Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
and Ellesmere Island) established tropical conditions, most of the continent was located within the natural dry zone along the Tropic of Capricorn
The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reac ...
, which (as nowadays) is a result of the convergence of two great air-masses, the Hadley cell
The Hadley cell, named after George Hadley, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics ...
and the Ferrel cell
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, bu ...
. In these near-deserts, the Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
sedimentary beds formed, made red by the oxidised iron ( hematite) characteristic of drought conditions. The abundance of red sandstone on continental land also lends Laurussia the name "the Old Red Continent". For much of the Devonian, the majority of western Laurussia (North America) was covered by subtropical inland seas
An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland se ...
which hosted a diverse ecosystem of reefs and marine life. Devonian marine deposits are particularly prevalent in the midwestern
The Midwestern United States, also referred to as the Midwest or the American Midwest, is one of four census regions of the United States Census Bureau (also known as "Region 2"). It occupies the northern central part of the United States. I ...
and northeastern
The points of the compass are a set of horizontal, radially arrayed compass directions (or azimuths) used in navigation and cartography. A compass rose is primarily composed of four cardinal directions—north, east, south, and west—each se ...
United States. Devonian reefs also extended along the southeast edge of Laurussia, a coastline now corresponding to southern England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
, Belgium
Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to th ...
, and other mid-latitude areas of Europe.
In the Early and Middle Devonian, the west coast of Laurussia was a passive margin with broad coastal waters, deep silty embayments, river deltas and estuaries, found today in Idaho
Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...
and Nevada
Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
. In the Late Devonian, an approaching volcanic island arc
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...
reached the steep slope of the continental shelf and began to uplift deep water deposits. This minor collision sparked the start of a mountain-building episode called the Antler orogeny
The Antler orogeny was a tectonic event that began in the early Late Devonian with widespread effects continuing into the Mississippian and early Pennsylvanian. Most of the evidence for this event is in Nevada but the limits of its reach are un ...
, which extended into the Carboniferous. Mountain building could also be found in the far northeastern extent of the continent, as minor tropical island arcs and detached Baltic terranes re-join the continent. Deformed remnants of these mountains can still be found on Ellesmere Island and Svalbard. Many of the Devonian collisions in Laurussia produce both mountain chains and foreland basins, which are frequently fossiliferous.
Gondwana
Gondwana was by far the largest continent on the planet. It was completely south of the equator, although the northeastern sector (now Australia) did reach tropical latitudes. The southwestern sector (now South America) was located to the far south, with Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
situated near the South Pole
The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
. The northwestern edge of Gondwana was an active margin
Active may refer to:
Music
* ''Active'' (album), a 1992 album by Casiopea
* Active Records, a record label
Ships
* ''Active'' (ship), several commercial ships by that name
* HMS ''Active'', the name of various ships of the British Royal ...
for much of the Devonian, and saw the accretion of many smaller land masses and island arcs. These include Chilenia, Cuyania
The Precordillera Terrane or Cuyania was an ancient microcontinent or terrane whose history affected many of the older rocks of Cuyo in Argentina. It was separated by oceanic crust from the Chilenia terrane which accreted into it at ~420-390 Ma w ...
, and Chaitenia, which now form much of Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
and Patagonia
Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...
. These collisions were associated with volcanic activity
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
and plutons
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
, but by the Late Devonian the tectonic situation had relaxed and much of South America was covered by shallow seas. These south polar seas hosted a distinctive brachiopod fauna, the Malvinokaffric Realm, which extended eastward to marginal areas now equivalent to South Africa and Antarctica. Malvinokaffric faunas even managed to approach the South Pole via a tongue of Panthalassa which extended into the Paraná Basin
The Paraná Basin ( pt, Bacia do Paraná, es, Cuenca del Paraná) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande d ...
.
The northern rim of Gondwana was mostly a passive margin, hosting extensive marine deposits in areas such as northwest Africa and Tibet
Tibet (; ''Böd''; ) is a region in East Asia, covering much of the Tibetan Plateau and spanning about . It is the traditional homeland of the Tibetan people. Also resident on the plateau are some other ethnic groups such as Monpa, Taman ...
. The eastern margin, though warmer than the west, was equally active. Numerous mountain building events and granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...
and kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of S ...
intrusions affected areas equivalent to modern day eastern Australia, Tasmania
)
, nickname =
, image_map = Tasmania in Australia.svg
, map_caption = Location of Tasmania in AustraliaCoordinates:
, subdivision_type = Country
, subdi ...
, and Antarctica.
Asian terranes
Several island microcontinents (which would later coalesce into modern day Asia) stretched over a low-latitude archipelago to the north of Gondwana. They were separated from the southern continent by an oceanic basin: the Paleo-Tethys Ocean, Paleo-Tethys. Although the western Paleo-Tethys Ocean had existed since the Cambrian, the eastern part only began to rift apart as late as the Silurian. This process accelerated in the Devonian. The eastern branch of the Paleo-Tethys was fully opened when South China Craton, South China and Annamia block, Annamia (a terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...
equivalent to most of Indochina), together as a unified continent, detached from the northeastern sector of Gondwana. Nevertheless, they remained close enough to Gondwana that their Devonian fossils were more closely related to Australian species than to north Asian species. Other Asian terranes remained attached to Gondwana, including Sibumasu (western Indochina), Tibet, and the rest of the Cimmeria (continent), Cimmerian blocks.
While the South China-Annamia continent was the newest addition to the Asian microcontinents, it was not the first. North China Craton, North China and the Tarim Block (now northwesternmost China) were located westward and continued to drift northwards, powering over older oceanic crust in the process. Further west was a small ocean (the Turkestan Ocean), followed by the larger microcontinents of Kazakhstania, Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...
, and Amurian Plate, Amuria. Kazakhstania was a volcanically active region during the Devonian, as it continued to assimilate smaller island arcs.
Siberia was located just north of the equator as the largest landmass in the Northern Hemisphere. At the beginning of the Devonian, Siberia was inverted (upside down) relative to its modern orientation. Later in the period it moved northwards and began to twist clockwise, though it was not near its modern location. Siberia approached the eastern edge of Laurussia as the Devonian progressed, but it was still separated by a seaway, the Ural Ocean. Although Siberia's margins were generally tectonically stable and ecologically productive, rifting and deep mantle plumes impacted the continent with flood basalts during the Late Devonian. The Altai-Sayan region was shaken by volcanism in the Early and Middle Devonian, while Late Devonian magmatism was magnified further to produce the Vilyuy Traps, flood basalts which may have contributed to the Late Devonian Mass Extinction. The last major round of volcanism, the Yakutsk Large Igneous Province, continued into the Carboniferous to produce extensive kimberlite deposits.
Similar volcanic activity also affected the nearby microcontinent of Amuria (now Manchuria, Mongolia and their vicinities). Though certainly close to Siberia in the Devonian, the precise location of Amuria is uncertain due to contradictory Paleomagnetism, paleomagnetic data.
Closure of the Rheic Ocean
The Rheic Ocean, which separated Laurussia from Gondwana, was wide at the start of the Devonian, having formed after the drift of Avalonia away from Gondwana. It steadily shrunk as the period continued, as the two major continents approached near the equator in the early stages of the assembly of Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
. The closure of the Rheic Ocean began in the Devonian and continued into the Carboniferous. As the ocean narrowed, endemic marine faunas of Gondwana and Laurussia combined into a single tropical fauna. The history of the western Rheic Ocean is a subject of debate, but there is good evidence that Rheic oceanic crust experienced intense subduction and metamorphism under Mexico and Central America.
The closure of the eastern part of the Rheic Ocean is associated with the assemblage of central and southern Europe. In the early Paleozoic, much of Europe was still attached to Gondwana, including the terranes of Iberia, Armorican terrane, Armorica (France), Palaeo-Adria (the western Mediterranean area), Bohemia, Franconia, and Saxothuringian Zone, Saxothuringia. These continental blocks, collectively known as the Armorican Terrane Assemblage, split away from Gondwana in the Silurian and drifted towards Laurussia through the Devonian. Their collision with Laurussia leads to the beginning of the Variscan Orogeny
The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.
Nomenclature
The name ''Variscan'', comes f ...
, a major mountain-building event which would escalate further in the Late Paleozoic. Franconia and Saxothuringia collided with Laurussia near the end of the Early Devonian, pinching out the easternmost Rheic Ocean. The rest of the Armorican terranes followed, and by the end of the Devonian they were fully connected with Laurussia. This sequence of rifting and collision events led to the successive creation and destruction of several small seaways, including the Rheno-Hercynian, Saxo-Thuringian, and Galicia-Moldanubian oceans. Their sediments were eventually compressed and completely buried as Gondwana fully collided with Laurussia in the Carboniferous.
Life
Marine biota
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s, the enigmatic hederellids, microconchida, microconchids and corals. Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. Bivalvia, Bivalves became commonplace in deep water and outer shelf environments. The first ammonites also appeared during or slightly before the early Devonian Period around 400 Mya. Bactritida, Bactritoids make their first appearance in the Early Devonian as well; their radiation, along with that of ammonoids, has been attributed by some authors to increased environmental stress resulting from decreasing oxygen levels in the deeper parts of the water column. Among vertebrates, jawless armored fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water. Armored placoderms were numerous during the lower stages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (Chondrichthyes) and bony fishes (Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of shark, ''Cladoselache'', appeared in the oceans during the Devonian Period. The great diversity of fish around at the time has led to the Devonian being given the name "The Age of Fish" in popular culture.
The Devonian saw significant expansion in the diversity of nektonic marine life driven by the abundance of planktonic microorganisms in the free water column as well as high ecological competition in benthic habitats, which were extremely saturated; this diversification has been labeled the Devonian Nekton Revolution by many researchers. However, other researchers have questioned whether this revolution existed at all; a 2018 study found that although the proportion of biodiversity constituted by nekton increased across the boundary between the Silurian and Devonian, it decreased across the span of the Devonian, particularly during the Pragian, and that the overall diversity of nektonic taxa did not increase significantly during the Devonian compared to during other geologic periods, and was in fact higher during the intervals spanning from the Wenlock to the Lochkovian and from the Carboniferous to the Permian. The study's authors instead attribute the increased overall diversity of nekton in the Devonian to a broader, gradual trend of nektonic diversification across the entire Palaeozoic.
Reefs
A now-dry barrier reef, located in present-day Kimberley (Western Australia), Kimberley Basin of northwest Australia, once extended , fringing a Devonian continent. Reefs are generally built by various carbonate-secreting organisms that can erect wave-resistant structures near sea level. Although modern reefs are constructed mainly by corals and calcareous algae, Devonian reefs were either microbial reefs built up mostly by autotrophic cyanobacteria or coral-stromatoporoid reefs built up by coral-like Stromatoporoidea, stromatoporoids and tabulate and Rugosa, rugose corals. Microbial reefs dominated under the warmer conditions of the early and late Devonian, while coral-stromatoporoid reefs dominated during the cooler middle Devonian.
Terrestrial biota
By the Devonian Period, life was well underway in its colonization of the land. The moss forests and bacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stable soils and harbored arthropods like mites, scorpions, Trigonotarbida, trigonotarbids and Myriapoda, myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as ''Protichnites'' suggest that amphibious arthropods may have appeared as early as the Cambrian). By far the largest land organism at the beginning of this period was the enigmatic ''Prototaxites'', which was possibly the fruiting body of an enormous fungus, rolled liverwort mat, or another organism of uncertain affinities that stood more than tall, and towered over the low, carpet-like vegetation during the early part of the Devonian. Also, the first possible fossils of insects appeared around 416 Mya, in the Early Devonian. Evidence for the earliest tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s takes the form of trace fossils in shallow lagoon environments within a marine carbonate platform/shelf during the Middle Devonian, although these traces have been questioned and an interpretation as fish feeding traces (''Piscichnus'') has been advanced.
The greening of land
Many List of Early Devonian land plants, Early Devonian plants did not have true roots or leaves like extant plants, although vascular tissue is observed in many of those plants. Some of the early land plants such as ''Drepanophycus'' likely spread by vegetative growth and spores. The earliest land plants such as ''Cooksonia'' consisted of leafless, Dichotomous branching, dichotomous axes and terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall. Fossils of ''Armoricaphyton, Armoricaphyton chateaupannense'', about 400 million years old, represent the oldest known plants with woody tissue. By the Middle Devonian, shrub-like forests of primitive plants existed: Lycopodiophyta, lycophytes, Equisetales, horsetails, ferns, and progymnosperms evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest-known trees appeared in the Middle Devonian. These included a lineage of lycopods and another arborescent, woody vascular plant, the Cladoxylopsida, cladoxylopsids and progymnosperm ''Archaeopteris''. These tracheophytes were able to grow to large size on dry land because they had evolved the ability to biosynthesize lignin, which gave them physical rigidity and improved the effectiveness of their vascular system while giving them resistance to pathogens and herbivores. 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 many plant groups and growth forms has been referred to as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution.
The 'greening' of the continents acted as a carbon sink, and Atmosphere of Earth, atmospheric concentrations of carbon dioxide may have dropped. This may have cooled the climate and led to a massive extinction event. (''See'' Late Devonian extinction
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...
).
Animals and the first soils
Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed plants that characterized a recognizably modern world had its genesis in the Late Devonian Epoch. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition. The rapid evolution of a terrestrial ecosystem that contained copious animals opened the way for the first vertebrates to seek terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.
Gallery
Image:D_Terrelli.png, ''Dunkleosteus'', one of the largest armoured fish ever to roam the planet, lived during the Late Devonian
Image:Eastmanosteus pustulosus.jpg, Lower jaw of ''Eastmanosteus pustulosus'' from the Middle Devonian of Wisconsin
Image:Onychodus.jpg, Tooth of the lobe-finned fish ''Onychodus'' from the Middle Devonian of Wisconsin
Image:Devonianfishes ntm 1905 smit 1929.gif, Early shark ''Cladoselache'', several Sarcopterygii, lobe-finned fishes, including ''Eusthenopteron'' that was an early marine tetrapod, and the placoderm ''Bothriolepis'' in a painting from 1905
Image:Melocrinites nodosus spinosus.jpg, ''Melocrinites nodosus spinosus'', a spiny, stalked crinoid from the Middle Devonian of Wisconsin
Image:PhacopidDevonian.jpg, Enrolled phacopid trilobite from the Devonian of Ohio
Image:AuloporaDevonianSilicaShale.jpg, The common tabulate coral ''Aulopora'' from the Middle Devonian of Ohio – view of colony encrusting a brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
valve
Image:Tropidoleptus carinatus.jpg, ''Tropidoleptus carinatus'', an orthid brachiopod from the Middle Devonian of New York
Image:Pleurodictyum americanum Kashong.jpg, ''Pleurodictyum, Pleurodictyum americanum'', Kashong Shale, Middle Devonian of New York
Image:HederelloidSEM.jpg, SEM image of a hederelloid from the Devonian of Michigan (largest tube diameter is 0.75 mm)
Image:HederellaOH3.jpg, Devonian spiriferid brachiopod from Ohio which served as a host substrate for a colony of hederelloids
Late Devonian extinction
The Late Devonian extinction
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...
is not a single event, but rather is a series of pulsed extinctions at the Givetian-Frasnian boundary, the Frasnian-Famennian boundary, and the Devonian-Carboniferous boundary. Together, these are considered one of the "Extinction event#The "Big Five" mass extinctions, Big Five" mass extinctions in Earth's history. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef systems.
Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, and acritarchs, and the world saw the disappearance of an estimated 96% of vertebrates like conodonts and Osteichthyes, bony fishes, and all of the ostracoderms and placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event (there is a counterargument that the Devonian extinctions nearly wiped out the tetrapods).
The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative. Canadian paleontologist Digby McLaren suggested in 1969 that the Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see the Alamo bolide impact), little evidence supports the existence of a large enough Devonian crater.
See also
* . One of the largest exposed Devonian fossil beds in the world.
*
*
* ''(with link directory)''
* , a Devonian trilobite
;Categories:
*
Notes
References
External links
*
* – site introduces the Devonian
*
*
*
*
*
{{Authority control
Devonian,
Geological periods
Geology of Devon, .