The Devonian is a geologic period and system of the Paleozoic, spanning 60 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 first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (tetrapods) began adapting to walking on land, as their strong pectoral and pelvic fins gradually evolved into legs. In the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician.
The first ammonites, species 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.
419.2–358.9 million years ago
|Mean atmospheric O
2 content over period duration
|c. 15 vol %|
(75 % of modern level)
|Mean atmospheric CO
2 content over period duration
|c. 2200 ppm|
(8 times pre-industrial level)
|Mean surface temperature over period duration||c. 20 °C|
(6 °C above modern level)
|Sea level (above present day)||Relatively steady around 189m, gradually falling to 120m through period|
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 (Ogg, 2004), the Devonian extends from the end of the Silurian 419.2 Mya, to the beginning of the Carboniferous 358.9 Mya (in North America, the beginning of the Mississippian subperiod of the Carboniferous).
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 but 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.
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 Lochkovian stage, which lasted until the Pragian. It spanned from to , and was followed 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.to and began with the
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. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian. CO2 levels dropped steeply throughout the Devonian period as the burial of the newly evolved forests drew carbon out of the atmosphere 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 CO2 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 and
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).
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 a thousand kilometres, 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.
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 and myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as Climactichnites suggest that land arthropods may have appeared as early as the Cambrian). 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.
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. By far the largest land organism during 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 8 metres tall, and towered over the low, carpet-like vegetation. 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, from the genus Wattieza, appeared in the Late Devonian around 385 Mya. In the Late Devonian, the tree-like ancestral Progymnosperm Archaeopteris which had conifer-like true wood and fern-like foliage and the cladoxylopsids grew. (See also: lignin.) 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 period. 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.
A major extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage (the Frasnian-Famennian boundary), about 372.2 Mya, when all the fossil agnathan fishes, save for the psammosteid heterostraci, suddenly disappeared. A second strong pulse closed the Devonian period. The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota, and was more drastic than the familiar extinction event that closed the Cretaceous.
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, conodonts, and acritarchs, as well as jawless fish, and all 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 Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns, and shrimp. Most decapods are scavengers. The order is estimated to contain nearly 15,000 species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp (about 3000 species) and Anomura including hermit crabs, porcelain crabs, squat lobsters (about 2500 species) making up the bulk of the remainder. The earliest fossil decapod is the Devonian Palaeopalaemon.Devonian Gardens (Calgary)
Devonian Gardens is a large indoor park and botanical garden located in the downtown core of Calgary, Alberta, Canada. In 2012 a major $37-million renovation was completed.Located on the Stephen Avenue pedestrian mall (8 Avenue SW) between 2 Street SW and 3 Street SW, the park is completely enclosed with glass and covers 2.5 acres (10,000 m2) (one full city block) on the top floor of The Core Shopping Centre (formerly TD Square). It is maintained by The City of Calgary Parks.
The gardens include a living wall, koi ponds, fountains, a children's play area, and over 550 trees, as well as meeting/function space for special events. A full service restaurant is also planned.Elpistostegalia
Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik.Evolution of fish
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 earliest jawed vertebrates probably developed during the late Ordovician period. They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as placoderms, which evolved from the ostracoderms; and the Acanthodii (or spiny sharks). The jawed fish that are still extant in modern days also appeared during the late Silurian: the Chondrichthyes (or cartilaginous fish) and the Osteichthyes (or bony fish). The bony fish evolved into two separate groups: the Actinopterygii (or ray-finned fish) and Sarcopterygii (which includes the lobe-finned fish).
During the Devonian period a great increase in fish variety occurred, especially among the ostracoderms and placoderms, and also among the lobe-finned fish and early sharks. This has led to the Devonian being known as the age of fishes. It was from the lobe-finned fish that the tetrapods evolved, the four-limbed vertebrates, represented today by amphibians, reptiles, mammals, and birds. Transitional tetrapods first appeared during the early Devonian, and by the late Devonian the first tetrapods appeared. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth; but it is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. Fish do not represent a monophyletic group, but a paraphyletic one, as they exclude the tetrapods.Fish, like many other organisms, have been greatly affected by extinction events throughout natural history. The Ordovician–Silurian extinction events led to the loss of many species. The late Devonian extinction led to the extinction of the ostracoderms and placoderms by the end of the Devonian, as well as other fish. The spiny sharks became extinct at the Permian–Triassic extinction event; the conodonts became extinct at the Triassic–Jurassic extinction event. The Cretaceous–Paleogene extinction event, and the present day Holocene extinction, have also affected fish variety and fish stocks.Famennian
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.
It was during this age that tetrapods first appeared. In the seas, a novel major group of ammonoid cephalopods called clymeniids appeared, underwent tremendous diversification and spread worldwide, then just as suddenly went extinct.
The beginning of the Famennian is marked by a major extinction event, the Kellwasser Event, and the end with a smaller but still quite severe extinction event, the Hangenberg Event.
North American subdivisions of the Famennian include the Chautauquan, Canadaway, Conneaut, Conneautan, Conewango and Conewangan.Frasnian
The Frasnian is one of two faunal stages in the Late Devonian period. It lasted from 382.7 million years ago to 372.2 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage.
Major reef-building was under way during the Frasnian stage, particularly in western Canada and Australia. On land, the first forests were taking shape. In North America, the Antler and Taconic orogenies peaked, which were contemporary with the Bretonic phase of the Variscan orogeny in Europe.
The Frasnian coincides with the second half of the "charcoal gap" in the fossil record, a time when atmospheric oxygen levels were below 13%, the minimum necessary to sustain wildfires.North American subdivisions of the Frasnian include
West Falls Group
The Givetian is one of two faunal stages in the Middle Devonian period. It lasted from 387.7 million years ago to 382.7 million years ago. It was preceded by the Eifelian stage and followed by the Frasnian stage. It is named after the town of Givet in France.Hexapoda
The subphylum Hexapoda (from the Greek for six legs) constitutes the largest number of species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs (six legs). Most other arthropods have more than three pairs of legs.Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of life on Earth. A major extinction, the Kellwasser event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage (the Frasnian–Famennian boundary), about 376–360 million years ago. Overall, 19% of all families and 50% of all genera became extinct. A second, distinct mass extinction, the Hangenberg event, closed the Devonian period.Although it is clear that there was a massive loss of biodiversity in the Late Devonian, the timespan of this event is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian. Nor is it clear whether there were two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses during an interval of some three million years. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.By the Late Devonian, the land had been colonized by plants and insects. In the oceans were massive reefs built by corals and stromatoporoids. Euramerica and Gondwana were beginning to converge into what would become Pangaea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the reef-building organisms almost completely disappeared. The causes of these extinctions are unclear. Leading hypotheses include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested, such as the Siljan Ring event in Sweden. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than by any single event. Surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs or other aspects of the Kellwasser event, while agnathans were in decline long before the end of the Frasnian.Marcellus Formation
The Marcellus Formation (also classified as the Marcellus Subgroup of the Hamilton Group, Marcellus Member of the Romney Formation, or simply the Marcellus Shale) is a Middle Devonian age unit of sedimentary rock found in eastern North America. Named for a distinctive outcrop near the village of Marcellus, New York, in the United States,
it extends throughout much of the Appalachian Basin.The unit name usage by the U.S. Geological Survey (USGS) includes Marcellus Shale and Marcellus Formation. The unit was first described and named as the "Marcellus shales" by J. Hall in 1839.Stratigraphically, the Marcellus is the lowest unit of the Devonian age Hamilton Group, and is divided into several sub-units. Although black shale is the dominant lithology, it also contains lighter shales and interbedded limestone layers due to sea level variation during its deposition almost 400 million years ago. The black shale was deposited in relatively deep water devoid of oxygen, and is only sparsely fossiliferous. Most fossils are contained in the limestone members, and the fossil record in these layers provides important paleontological insights on faunal turnovers.
The shale contains largely untapped natural gas reserves, and its proximity to the high-demand markets along the East Coast of the United States makes it an attractive target for energy development and export.
The black shales also contain iron ore that was used in the early economic development of the region, and uranium and pyrite which are environmental hazards. The fissile shales are also easily eroded, presenting additional civil and environmental engineering challenges.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 extends northwards into Greenland and Svalbard. In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. For convenience the short version of the term, ORS is often used in literature on the subject. The term was coined to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.Paleozoic
The Paleozoic (or Palaeozoic) Era ( ; from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 251.902 million years ago, and is subdivided into six geologic periods (from oldest to youngest): 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 Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, synapsids and diapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms. Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated diapsids and synapsids were dominant and the first modern plants (conifers) appeared.
The Paleozoic Era ended with the largest extinction event in the history of Earth, the Permian–Triassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic Era to recover. Recovery of life in the sea may have been much faster.Rhipidistia
The Rhipidistia, also known as dipnotetrapodomorphs (formally Dipnotetrapodomorpha) are a clade of lobe-finned fishes which include the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a definition that is now obsolete. However as cladistic understanding of the vertebrates has improved over the last few decades a monophyletic Rhipidistia is now understood to include the whole of Tetrapoda and the lungfishes.
Rhipidistia includes porolepiformes and dipnoi. Extensive fossilization of lungfishes has contributed to many evolutionary studies of this group. Evolution of autostylic jaw suspension, in which the palatoquadrate bone fuses to the cranium, is unique to this group.
The precise time at which the choana evolved is debated, with some considering early rhipidistians as the first choanates.Silurian
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 several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out.
A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life also began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes, streams, and coastlines, and terrestrial arthropods are also first found on land during the Silurian. However, terrestrial life would not greatly diversify and affect the landscape until the Devonian.Stegocephalia
Stegocephalia is a name used for four-limbed stem-tetrapods, and their amphibian-grade descendants, and in phylogenetic nomenclature for all tetrapods. The term was coined in 1868 by American palaentologist Edward Drinker Cope and comes from Greek στεγοκεφαλια - "roofed head", and refer to the copious amounts of dermal armour some of the larger primitive forms evidently had. Canadian paleontologist Michel Laurin gave the group its first formal phylogenetic definition, roughly including all vertebrates with toes rather than fins.Tetrapod
Tetrapods ()(from Greek: τετρα- "four" and πούς "foot") are four-limbed (with a few exceptions, such as snakes) animals constituting the superclass Tetrapoda. It includes existing and extinct amphibians, reptiles (including dinosaurs and birds), and mammals. Tetrapods evolved from a group of animals known as the Tetrapodomorpha which, in turn, evolved from ancient Sarcopterygii (lobe-finned fishes) around 390 million years ago in the middle Devonian period; their forms were transitional between lobe-finned fishes and the four-limbed tetrapods. The first tetrapods (from a traditional, apomorphy-based perspective) appeared by the late Devonian, 367.5 million years ago; the specific aquatic ancestors of the tetrapods, and the process by which they colonized Earth's land after emerging from water remains unclear. The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known. The first tetrapods were primarily aquatic. Modern amphibians, which evolved from earlier groups, are generally semiaquatic; the first stage of their lives is as fish-like tadpoles, and later stages are partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes, most of those are terrestrial tetrapods whose branch evolved from earlier tetrapods about 340 million years ago (crown amniotes evolved 318 million years ago). The key innovation in amniotes over amphibians is laying of eggs on land or having further evolved to retain the fertilized egg(s) within the mother.
Amniote tetrapods began to dominate and drove most amphibian tetrapods to extinction. One group of amniotes diverged into the reptiles, which includes lepidosaurs, dinosaurs (which includes birds), crocodilians, turtles, and extinct relatives; while another group of amniotes diverged into the mammals and their extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from among the dinosaurs, and bats from among the mammals.
Some tetrapods, such as the snakes, have lost some or all of their limbs through further speciation and evolution; some have only concealed vestigial bones as a remnant of the limbs of their distant ancestors. Others, such as amphibians, returned to partially or fully aquatic lives, the first during the Carboniferous period; others as recently as the Cenozoic.Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors. These include the structure of the jaw and teeth for feeding on land, limb girdles and extremities for land locomotion, lungs for respiration in air, a heart for circulation, and eyes and ears for seeing and hearing in air.Tetrapodomorpha
The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advanced forms transitional between fish and the early labyrinthodonts, such as Tiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish, half-tetrapods, in appearance and limb morphology. The Tetrapodomorpha contains the crown group tetrapods (the last common ancestor of living tetrapods and all of its descendants) and several groups of early stem tetrapods, which includes several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda are the Stem Tetrapoda, a paraphyletic unit encompassing the fish to tetrapod transition.
Among the characters defining tetrapodomorphs are modifications to the fins, notably a humerus with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Another key trait is the internal nostril or choana. Most fish have two pairs of nostrils, one on either side of the head for incoming water (incurrent nostrils) and another pair for outgoing water (excurrent nostrils). Early tetrapodomorphs such as Kenichthys had excurrent nostrils that had migrated to the edge of the mouth. In later tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana.Tetrapodomorph fossils are known from the early Devonian onwards, and include Osteolepis, Panderichthys, Kenichthys and Tungsenia.
(541.0 Mya–2.5 Gya)
|Archean eon (2.5–4 Gya)|
|Hadean eon (4–4.6 Gya)|