|Nickname(s)||Age of Fishes|
|Regional usage||Global (ICS)|
|Time scale(s) used||ICS Time Scale|
|Time span formality||Formal|
|Lower boundary definition||FAD of the Graptolite Monograptus uniformis|
|Lower boundary GSSP|| Klonk, Prague, Czechia |
|Upper boundary definition||FAD of the Conodont Siphonodella sulcata (discovered to have biostratigraphic issues as of 2006).|
|Upper boundary GSSP|| La Serre, Montagne Noire, France |
|Atmospheric and climatic data|
|Mean atmospheric O|
|c. 15 vol %|
(75 % of modern)
|Mean atmospheric CO|
|c. 2200 ppm |
(8 times pre-industrial)
|Mean surface temperature||c. 20 °C|
(6 °C above modern)
|Sea level above present day||Relatively steady around 189m, gradually falling to 120m through period|
The Devonian ( /, -, -/ dih-VOH-nee-ən, də-, deh-) 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. It is named after Devon, England, where rocks from this period were first studied.
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. Various terrestrial arthropods also became well-established.
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.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 severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida.
The palaeogeography was dominated by the supercontinent of Gondwana to the south, the small continent of Siberia to the north, and the early formation of the medium-sized continent of Euramerica in between.
The period is named after Devon, 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 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 supported by George Bellas Greenough. 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, 419.2 Mya, to the beginning of the Carboniferous 358.9 Mya – in North America, at the beginning of the Mississippian subperiod of the Carboniferous.the Devonian extends from the end of the Silurian
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 early fossil discoveries were found. Another common term is "Age of the Fishes",referring to the evolution of several major groups of fish 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 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 fact the climate in the Devonian differed greatly during its epochs 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 had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common.[ citation needed ]
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.
The Early Devonian lasted from to and began with the Lochkovian Stage to , which was followed by the Pragian from to and then by the Emsian, which lasted until the Middle Devonian began, . During this time, the first ammonoids appeared, descending from bactritoid nautiloids. Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to the order Agoniatitida, which in later epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida. This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic Era.
The Middle Devonian comprised two subdivisions: first the Eifelian, which then gave way to the Givetian. 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. 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.
Finally, the Late Devonian started with the Frasnian, to , during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian, .
The Devonian was a relatively warm period, and probably lacked any glaciers. 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. 30 °C (86 °F) in the Early Devonian. CO
2 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 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO
2 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 reefs; microbes 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.
The Devonian Period was a time of great tectonic activity, as Euramerica and Gondwana 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 air-masses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidised iron (hematite) characteristic of drought conditions.
Near the equator, the plate of Euramerica and Gondwana were starting to meet, beginning the early stages of the assembling of Pangaea. This activity further raised the northern Appalachian Mountains and formed the Caledonian Mountains 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, found today in 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 at the beginning of the Carboniferous called the Antler orogeny.
Sea levels were high worldwide, and much of the land lay 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 the Paleo-Tethys Ocean, Proto-Tethys Ocean, Rheic Ocean, and Ural Ocean (which was closed during the collision with Siberia and Baltica).
During the Devonian, Chaitenia, an island arc, accreted to Patagonia.
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederellids, microconchids and corals. Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. 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 first ammonites also appeared during or slightly before the early Devonian Period around 400 Mya.
A now-dry barrier reef, located in present-day Kimberley Basin of northwest Australia, once extended 350 km (220 mi), fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant structures close to 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 stromatoporoids and tabulate and 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.
By the Devonian Period, life was well underway in its colonisation 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, trigonotarbids 8 metres (26 ft) 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 tetrapods 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.and 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
Many 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 axes and terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall. Fossils of 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: lycophytes, horsetails, ferns, and progymnosperms had 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 cladoxylopsids. 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 so many plant groups and growth forms has been called the "Devonian Explosion".
The 'greening' of the continents acted as a carbon sink, and 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).
Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterised a recognisably 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 out a terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.
The Late Devonian extinction 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 "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 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.
The Carboniferous is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period 358.9 million years ago (Mya), to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing", from the Latin carbō ("coal") and ferō, and refers to the many coal beds formed globally during that time.
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 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.
The Permian is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the last period of the Paleozoic Era; the following Triassic Period belongs to the Mesozoic Era. The concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the region of Perm in Russia.
The PaleozoicEra is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from, and is subdivided into six geologic periods : the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic comes after the Neoproterozoic Era of the Proterozoic Eon and is followed by the Mesozoic Era.
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present, and it began with the Cambrian Period when animals first developed hard shells preserved in the fossil record. The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of five largest mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event, which occurred around 372 million years ago, at the boundary between the Frasnian stage and the Famennian stage, the last stage in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct. A second mass extinction, the Hangenberg event, occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period.
Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist Alfred Romer, who first recognised it. Recent discoveries in Scotland are beginning to close this gap in palaeontological knowledge.
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. It lasted from 372.2 million years ago to 358.9 million years ago. It was preceded by the Frasnian Stage and followed by the Tournaisian Stage.
Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Ordovician through the Devonian. They were especially abundant and important reef-formers in the Silurian and most of the Devonian. The group was previously thought to be related to the corals and placed in the phylum Cnidaria. They are now classified in the phylum Porifera, specifically the sclerosponges. There are numerous fossil forms with spherical, branching or encrusting skeletons of laminated calcite with vertical pillars between the laminae. Specimen of its oldest genus, Priscastroma, have been found within the Middle Ordovician Sediments. This same genus has been referred to as the species P. gemina Khrom., and is known to have been known to branch off into two forms, A and B. Form A gave rise to the genus Cystostroma while form B gave rise to the genus Labechia and its descendants. Paleozoic stromatoporoids died out at the Hangenberg Event at the end of the Devonian. Purported Mesozoic stromatoporoids may be unrelated, thus making "stromatoporoids" a polyphyletic group if they are included.
In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.
The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian. It is usually considered the second-largest extinction in the Devonian Period, having occurred approximately 13 million years after the Late Devonian mass extinction at the Frasnian-Famennian boundary. The Hangenberg event was an anoxic event marked by a layer of black shale, and it has been proposed to have been related to a rapid sea-level fall from the last phase of the Devonian Southern Hemisphere glaciation. It has also been suggested to have been linked to an increase in terrestrial plant cover. That would have led to increased nutrient supply in rivers and may have led to eutrophication of semi-restricted epicontinental seas and could have stimulated algal blooms. However, support for a rapid increase in plant cover at the end of the Famennian is lacking. The event is named after the Hangenberg Shale, which is part of a sequence that straddles the Devonian-Carboniferous boundary in the Rhenish Massif of Germany.
Paleontology in Ohio refers to paleontological research occurring within or conducted by people from the U.S. state of Ohio. Ohio is well known for having a great quantity and diversity of fossils preserved in its rocks. The state's fossil record begins early in the Paleozoic era, during the Cambrian period. Ohio was generally covered by seawater from that time on through the rest of the early Paleozoic. Local invertebrates included brachiopods, cephalopods, coral, graptolites, and trilobites. Vertebrates included bony fishes and sharks. The first land plants in the state grew during the Devonian. During the Carboniferous, Ohio became a more terrestrial environment with an increased diversity of plants that formed expansive swampy deltas. Amphibians and reptiles began to inhabit the state at this time, and remained present into the ensuing Permian. A gap in the local rock record spans from this point until the start of the Pleistocene. During the Ice Age, Ohio was home to giant beavers, humans, mammoths, and mastodons. Paleo-Indians collected fossils that were later incorporated into their mounds. Ohio has been the birthplace of many world famous paleontologists, like Charles Schuchert. Many significant fossils curated by museums in Europe and the United States were found in Ohio. Major local fossil discoveries include the 1965 discovery of more than 50,000 Devonian fish fossils in Cuyahoga County. The Ordovician trilobite Isotelus maximus is the Ohio state invertebrate fossil.
Paleontology in Indiana refers to paleontological research occurring within or conducted by people from the U.S. state of Indiana. Indiana's fossil record stretches all the way back to the Precambrian, when the state was inhabited by microbes. More complex organisms came to inhabit the state during the early Paleozoic era. At that time the state was covered by a warm shallow sea that would come to be inhabited by creatures like brachiopods, bryozoans, cephalopods, crinoids, and trilobites. During the Silurian period the state was home to significant reef systems. Indiana became a more terrestrial environment during the Carboniferous, as an expansive river system formed richly vegetated deltas where amphibians lived. There is a gap in the local rock record from the Permian through the Mesozoic. Likewise, little is known about the early to middle Cenozoic era. During the Ice Age however, the state was subject to glacial activity, and home to creatures like short-faced bears, camels, mammoths, and mastodons. After humans came to inhabit the state, Native Americans interpreted the fossil proboscidean remains preserved near Devil's Lake as the bones of water monsters. After the advent of formal scientific investigation one paleontological survey determined that the state was home to nearly 150 different kinds of prehistoric plants.
Paleontology in Pennsylvania refers to paleontological research occurring within or conducted by people from the U.S. state of Pennsylvania. The geologic column of Pennsylvania spans from the Precambrian to Quaternary. During the early part of the Paleozoic, Pennsylvania was submerged by a warm, shallow sea. This sea would come to be inhabited by creatures like brachiopods, bryozoans, crinoids, graptolites, and trilobites. The armored fish Palaeaspis appeared during the Silurian. By the Devonian the state was home to other kinds of fishes. On land, some of the world's oldest tetrapods left behind footprints that would later fossilize. Some of Pennsylvania's most important fossil finds were made in the state's Devonian rocks. Carboniferous Pennsylvania was a swampy environment covered by a wide variety of plants. The latter half of the period was called the Pennsylvanian in honor of the state's rich contemporary rock record. By the end of the Paleozoic the state was no longer so swampy. During the Mesozoic the state was home to dinosaurs and other kinds of reptiles, who left behind fossil footprints. Little is known about the early to mid Cenozoic of Pennsylvania, but during the Ice Age it seemed to have a tundra-like environment. Local Delaware people used to smoke mixtures of fossil bones and tobacco for good luck and to have wishes granted. By the late 1800s Pennsylvania was the site of formal scientific investigation of fossils. Around this time Hadrosaurus foulkii of neighboring New Jersey became the first mounted dinosaur skeleton exhibit at the Academy of Natural Sciences in Philadelphia. The Devonian trilobite Phacops rana is the Pennsylvania state fossil.
Paleontology in Georgia refers to paleontological research occurring within or conducted by people from the U.S. state of Georgia. During the early part of the Paleozoic, Georgia was largely covered by seawater. Although no major Paleozoic discoveries have been uncovered in Georgia, the local fossil record documents a great diversity of ancient life in the state. Inhabitants of Georgia's early Paleozoic sea included corals, stromatolites, and trilobites. During the Carboniferous local sea levels dropped and a vast complex of richly vegetated delta formed in the state. These swampy deltas were home to early tetrapods which left behind footprints that would later fossilize. Little is known of Triassic Georgia and the Jurassic is absent altogether from the state's rock record. During the Cretaceous, however, southern Georgia was covered by a sea that was home to invertebrates and fishes. On land, the tree Araucaria grew, and dinosaurs inhabited the state. Southern Georgia remained submerged by shallow seawater into the ensuing Paleogene and Neogene periods of the Cenozoic era. These seas were home to small coral reefs and a variety of other marine invertebrates. By the Pleistocene the state was mostly dry land covered in forests and grasslands home to mammoths and giant ground sloths. Local coal mining activity has a history of serendipitous Carboniferous-aged fossil discoveries. Another major event in Georgian paleontology was a 1963 discovery of Pleistocene fossils in Bartow County. Shark teeth are the Georgia state fossil.
Paleontology in Wisconsin refers to paleontological research occurring within or conducted by people from the U.S. state of Wisconsin. The state has fossils from the Precambrian, much of the Paleozoic, and the later part of the Cenozoic. Most of the Paleozoic rocks are marine in origin. Because of the thick blanket of Pleistocene glacial sediment that covers the rock strata in most of the state, Wisconsin’s fossil record is relatively sparse. In spite of this, certain Wisconsin paleontological occurrences provide exceptional insights concerning the history and diversity of life on Earth.
Paleontology in Missouri refers to paleontological research occurring within or conducted by people from the U.S. state of Missouri. The geologic column of Missouri spans all of geologic history from the Precambrian to present with the exception of the Permian, Triassic, and Jurassic. Brachiopods are probably the most common fossils in Missouri.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.
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Estimates of oxygen levels during this period suggest that they were unprecedentedly low during the Givetian and Frasnian periods. At the same time, plant diversification was at its most rapid, changing the character of the landscape and contributing, via soils, soluble nutrients, and decaying plant matter, to anoxia in all water systems. The co-occurrence of these global events may explain the evolution of air-breathing adaptations in at least two lobe-finned groups, contributing directly to the rise of the tetrapod stem group.
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