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.[1][2] Overall, 19% of all families and 50% of all genera became extinct.[3] A second, distinct mass extinction, the Hangenberg event, closed the Devonian period.[4]

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.[5] 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.[6] 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.[7]

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,[8] 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.[9][5] This might have been caused by invasions of cosmopolitan species, rather than by any single event.[5] 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.[10]

CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Marine extinction intensity during the Phanerozoic
%
Millions of years ago
Late D
CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Comparison of the three episodes of extinction in the Late Devonian (Late D) to other mass extinction events in Earth's history. Plotted is the extinction intensity, calculated from marine genera.
StromatoporoidSideDevColumbus
Side view of a stromatoporoid showing laminae and pillars; Columbus Limestone (Devonian) of Ohio

The Late Devonian world

Tiktaalik restoration by ObsidianSoul 01
A restored Tiktaalik

During the Late Devonian, the continents were arranged differently from today, with a supercontinent, Gondwana, covering much of the Southern Hemisphere. The continent of Siberia occupied the Northern Hemisphere, while an equatorial continent, Laurussia (formed by the collision of Baltica and Laurentia), was drifting towards Gondwana, closing the Iapetus Ocean. The Caledonian mountains were also growing across what is now the Scottish Highlands and Scandinavia, while the Appalachians rose over America.[13]

The biota was also very different. Plants, which had been on land in forms similar to mosses, liverworts, and lichens since the Ordovician, had just developed roots, seeds, and water transport systems that allowed them to survive away from places that were constantly wet—and consequently built huge forests on the highlands. Several different clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms.[14] Fish were also undergoing a huge radiation, and the first tetrapods, such as Tiktaalik, were beginning to evolve leg-like structures.

Duration and timing of the extinction events

Extinction rates appear to have been higher than the background rate, for an extended interval covering the last 20–25 million years of the Devonian. During this time, about eight to ten distinct events can be seen, of which two stand out as particularly severe.[15] The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.[16] The fossil record of the first 15 million years of the Carboniferous period which followed is largely void of terrestrial animal fossils, likely related to losses during the end-Devonian Hangenberg event. This period is known as Romer's gap.[10][17]

The Kellwasser event

The Kellwasser event, named for its locus typicus, the Kellwassertal in Lower Saxony, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary. Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record. There may in fact have been two closely spaced events here, as shown by the presence of two distinct anoxic shale layers.

The Hangenberg event

The Hangenberg event is found on or just below the Devonian–Carboniferous boundary and marks the last spike in the period of extinction. It is marked by an anoxic black shale layer and an overlying sandstone deposit.[18] Unlike the Kellwasser event, the Hangenberg event affected both marine and terrestrial habitats.[10]

Effects of the events

The extinction events were accompanied by widespread oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter. This, combined with the ability of porous reef rocks to hold oil, has led to Devonian rocks being an important source of oil, especially in the USA.

Biological impact

The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and had a greater effect on shallow warm-water organisms than on cool-water organisms. The most important group to be affected by the Kellwasser event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulate corals. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as oncolites and stromatolites. The collapse of the reef system was so stark that bigger reef-building by new families of carbonate-secreting organisms, the modern scleractinian or "stony" corals, did not recover until the Mesozoic era.

Further taxa to be starkly affected include the brachiopods, trilobites, ammonites, conodonts, and acritarchs. Both graptolites and cystoids disappeared during this event. The surviving taxa show morphological trends through the event. Trilobites evolved smaller eyes in the run-up to the Kellwasser event, with eye size increasing again afterwards. This suggests vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. The brims are thought to have served a respiratory purpose, and the increasing anoxia of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with the oxygen isotope ratio, and thus with the sea water temperature; this may relate to them occupying different trophic levels as nutrient input changed.[19] As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.[2]

The Hangenberg event affected both marine and freshwater communities. This mass extinction affected ammonites and trilobites, as well as jawed vertebrates, including tetrapod ancestors.[10][20] The Hangenberg is linked to the extinction of 44% of high-level vertebrate clades, including all placoderms and most sarcopterygians, and the complete turnover of the vertebrate biota.[10] 97 per cent of vertebrate species disappeared, with only smaller forms surviving. After the event only sharks less than a meter and most fishes and tetrapods less than 10 centimeters remained, and it would take 40 million years before they started to increase in size again.[21] This led to the establishment of the modern vertebrate fauna in the Carboniferous, consisting mostly of actinopterygians, chondrichthyans, and tetrapods. Romer's gap, a 15 million-year hiatus in the early Carboniferous tetrapod record, has been linked to this event.[10] Also, the poor Famennian record for marine invertebrates suggests that some of the losses attributed to the Kellwasser event likely actually occurred during the Hangenberg extinction.[10][22]

Magnitude

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous. A recent survey (McGhee 1996) estimates that 22% of all the 'families' of marine animals (largely invertebrates) were eliminated. The family is a great unit, and to lose so many signifies a deep loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species did not survive into the Carboniferous. These latter estimates[a] need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and sampling biases during the Devonian.

Causes of the extinctions

Since the Kellwasser-related extinctions occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentary record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate.

From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. Evidence exists of widespread anoxia in oceanic bottom waters;[14] the rate of carbon burial shot up,[14] and benthic organisms were devastated, especially in the tropics, and especially reef communities.[14] Good evidence has been found for high-frequency sea-level changes around the Frasnian–Famennian Kellwasser event, with one sea level rise associated with the onset of anoxic deposits.[23] The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall.[18][24]

Possible triggers are as follows:

Bolide impact

Bolide impacts can be dramatic triggers of mass extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover,[2][25] but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the Kellwasser-aged Alamo and the Hangenberg-aged Woodleigh, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction.[1] Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.[26][27]

Plant evolution

During the Devonian, land plants underwent a hugely significant phase of evolution. Their maximum height went from 30 cm at the start of the Devonian, to 30 m[28] at the end of the period. This increase in height was made possible by the evolution of advanced vascular systems, which permitted the growth of complex branching and rooting systems.[14] In conjunction with this, the development of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas.[14] The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian.

Effect on weathering

These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilized a deep layer of soil, which would have been of the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a few centimeters. The mobilization of a large portion of soil had a huge effect: soil promotes weathering, the chemical breakdown of rocks, releasing ions which are nutrients for plants and algae.[14] The relatively sudden input of nutrients into river water may have caused eutrophication and subsequent anoxia. For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposing organisms use up all available oxygen by decaying them, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatolites and (to a lesser degree) corals—organisms which only thrive in low-nutrient conditions. Therefore, the postulated influx of high levels of nutrients may have caused an extinction, just as phosphate run-off from Australian farmers is causing immeasurable damage to the Great Barrier Reef today.[14] Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.[26]

Effect on CO
2

The "greening" of the continents occurred during the Devonian. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced CO2 levels in the atmosphere. Since CO
2
is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (near the Devonian South Pole) suggests widespread glaciation at the end of the Devonian, as a broad continental mass covered the polar region. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. The Hangenberg event has also been linked to glaciation in the tropics equivalent to that of the Pleistocene ice age.[18]

The weathering of silicate rocks also draws down CO2 from the atmosphere. This acted together with the burial of organic matter to decrease atmospheric CO2 concentrations from about 15 to three times present levels. Carbon in the form of plant matter would be produced on prodigious scales, and given the right conditions, could be stored and buried, eventually producing vast coal measures (e.g. in China) which locked the carbon out of the atmosphere and into the lithosphere.[29] This reduction in atmospheric CO
2
would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall),[30] probably fluctuating in intensity alongside the 40ka Milankovic cycle. The continued drawdown of organic carbon eventually pulled the Earth out of its Greenhouse Earth state into the Icehouse that continued throughout the Carboniferous and Permian.

Magmatism

Magmatism was suggested as a cause of the Late Devonian extinction in 2002.[31] The end of the Devonian Period was a time of extremely widespread trap magmatism and rifting in the Russian and Siberian platforms, which were situated above the hot mantle plumes and suggested as a cause of the Frasnian / Famennian and end-Devonian extinctions.[32] The Viluy and Pripyat-Dniepr-Donets large igneous provinces were suggested to correlate with the Frasnian / Famennian extinction and the Kola and Timan-Pechora magmatism was suggested to correspond to the end Devonian-Carboniferous extinction.[32]

Most recently, scientists have confirmed a correlation between Viluy traps (in the Vilyuysk region) on the Siberian Craton and the Kellwasser extinction by Ar/Ar dating.[33][34]

The Viluy Large igneous province covers most of the present day north-eastern margin of the Siberian Platform. The triple-junction rift system was formed during the Devonian Period; the Viluy rift is the western remaining branch of the system and two other branches form the modern margin of the Siberian Platform. Volcanic rocks are covered with post Late Devonian–Early Carboniferous sediments.[35] Volcanic rocks, dyke belts, and sills that cover more than 320 × 103 km2, and a gigantic amount of magmatic material (more than 1000 × 103 km3) formed in the Viluy branch.[35]

Ages show that the two volcanic phase hypotheses are well supported and the weighted mean ages of each volcanic phase are 376.7 ± 3.4 and 364.4 ± 3.4 Ma or 373.4 ± 2.1 and 363.2 ± 2.0 Ma, which the first volcanic phase is in agreement with the age of 372.2 ± 3.2 Ma proposed for the Kellwasser event. However, the second volcanic phase is slightly older than Hangenberg event which place at 358.9 ± 1.2 Ma.[34] Viluy magmatism may have injected enough CO
2
and SO
2
into the atmosphere to have generated a destabilised greenhouse and ecosystem, causing rapid global cooling, sea-level falls and marine anoxia occur during Kellwasser black shale deposition.[36][19][37][38]

Other suggestions

Other mechanisms put forward to explain the extinctions include tectonic-driven climate change, sea-level change, and oceanic overturning. These have all been discounted because they are unable to explain the duration, selectivity, and periodicity of the extinctions.[26]

See also

Notes

  1. ^ The species estimate is the toughest to assess and most likely to be adjusted.

References

  1. ^ a b Racki, 2005
  2. ^ a b c McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0-231-07504-9
  3. ^ "John Baez, Extinction, April 8, 2006".
  4. ^ Caplan and Bustin, 1999
  5. ^ a b c Stigall, 2011
  6. ^ Racki, Grzegorz, "Toward understanding of Late Devonian global events: few answers, many questions" GSA Annual meeting, Seattle 2003 (abstract); McGhee 1996.
  7. ^ Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, Encyclopedia of Global Environmental Change John Wiley & Sons.
  8. ^ Sole, R. V., and Newman, M. Patterns of extinction and biodiversity in the fossil record
  9. ^ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology. 30 (4): 522–542. doi:10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2.
  10. ^ a b c d e f g Sallan and Coates, 2010
  11. ^ Parry, S.F.; Noble S.R.; Crowley Q.G.; Wellman C.H. (2011). "A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications". Journal of the Geological Society. London: Geological Society. 168 (4): 863–872. doi:10.1144/0016-76492010-043.
  12. ^ Kaufmann, B.; Trapp, E.; Mezger, K. (2004). "The numerical age of the Upper Frasnian (Upper Devonian) Kellwasser horizons: A new U-Pb zircon date from Steinbruch Schmidt(Kellerwald, Germany)". The Journal of Geology. 112 (4): 495–501. Bibcode:2004JG....112..495K. doi:10.1086/421077.
  13. ^ McKerrow, W.S.; Mac Niocaill, C.; Dewey, J.F. (2000). "The Caledonian Orogeny redefined". Journal of the Geological Society. 157 (6): 1149–1154. Bibcode:2000JGSoc.157.1149M. doi:10.1144/jgs.157.6.1149.
  14. ^ a b c d e f g h Algeo, T.J.; Scheckler, S. E. (1998). "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events". Philosophical Transactions of the Royal Society B: Biological Sciences. 353 (1365): 113–130. doi:10.1098/rstb.1998.0195. PMC 1692181.
  15. ^ Algeo, T.J., S.E. Scheckler and J. B. Maynard (2001). "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate". In P.G. Gensel; D. Edwards (eds.). Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York. pp. 13–236.CS1 maint: Multiple names: authors list (link)
  16. ^ Streel, M.; Caputo, M.V.; Loboziak, S.; Melo, J.H.G. (2000). "Late Frasnian--Famennian climates based on palynomorph analyses and the question of the Late Devonian glaciations". Earth-Science Reviews. 52 (1–3): 121–173. Bibcode:2000ESRv...52..121S. doi:10.1016/S0012-8252(00)00026-X.
  17. ^ Ward, P. et al. (2006): Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization. Proceedings of the National Academy of Sciences no 103 (45): pp 16818-16822.
  18. ^ a b c Brezinski, D.K.; Cecil, C.B.; Skema, V.W.; Kertis, C.A. (2009). "Evidence for long-term climate change in Upper Devonian strata of the central Appalachians". Palaeogeography, Palaeoclimatology, Palaeoecology. 284 (3–4): 315–325. doi:10.1016/j.palaeo.2009.10.010.
  19. ^ a b Balter, Vincent; Renaud, Sabrina; Girard, Catherine; Joachimski, Michael M. (November 2008). "Record of climate-driven morphological changes in 376 Ma Devonian fossils". Geology. 36 (11): 907. Bibcode:2008Geo....36..907B. doi:10.1130/G24989A.1.
  20. ^ Korn, 2004
  21. ^ "Mass extinctions hit large animals the hardest: Small vertebrates flourish after deadly global catastrophes". Daily Mail. UK.
  22. ^ Foote, 2005
  23. ^ David P. G. Bond; Paul B. Wignalla (2008). "The role of sea-level change and marine anoxia in the Frasnian-Famennian (Late Devonian) mass extinction". Palaeogeography, Palaeoclimatology, Palaeoecology. 263 (3–4): 107–118. doi:10.1016/j.palaeo.2008.02.015.
  24. ^ Algeo et al., 2008
  25. ^ Digby McLaren, 1969;
  26. ^ a b c Algeo, T.J.; Berner, R.A.; Maynard, J.B.; Scheckler, S.E.; Archives, G.S.A.T. (1995). "Late Devonian Oceanic Anoxic Events and Biotic Crises: "Rooted" in the Evolution of Vascular Land Plants?" (PDF). GSA Today. 5 (3).
  27. ^ Wang K, Attrep M, Orth CJ (December 2017). "Global iridium anomaly, mass extinction, and redox change at the Devonian-Carboniferous boundary". Geology. 21 (12): 1071–1074. doi:10.1130/0091-7613(1993)021<1071:giamea>2.3.co;2.
  28. ^ Archaeopterids, see Beck (1981) in Algeo 1998
  29. ^ Carbon locked in Devonian coal, the earliest of Earth's coal deposits, is currently being returned to the atmosphere.
  30. ^ (Caputo 1985; Berner 1992, 1994) in Algeo 1998
  31. ^ Kravchinsky, V.A.; K.M. Konstantinov; V. Courtillot; J.-P. Valet; J.I. Savrasov; S.D. Cherniy; S.G. Mishenin; B.S. Parasotka (2002). "Palaeomagnetism of East Siberian traps and kimberlites: two new poles and palaeogeographic reconstructions at about 360 and 250 Ma". Geophysical Journal International. 148: 1–33. doi:10.1046/j.0956-540x.2001.01548.x.
  32. ^ a b Kravchinsky, V. A. (2012). "Paleozoic large igneous provinces of Northern Eurasia: Correlation with mass extinction events". Global and Planetary Change. 86–87: 31–36. Bibcode:2012GPC....86...31K. doi:10.1016/j.gloplacha.2012.01.007.
  33. ^ Courtillot, V.; et al. (2010). "Preliminary dating of the Viluy traps (Eastern Siberia): Eruption at the time of Late Devonian extinction events?". Earth and Planetary Science Letters. 102 (1–2): 29–59. Bibcode:2010ESRv..102...29K. doi:10.1016/j.earscirev.2010.06.004.
  34. ^ a b Ricci, J.; et al. (2013). "New 40Ar/39Ar and K–Ar ages of the Viluy traps (Eastern Siberia): Further evidence for a relationship with the Frasnian–Famennian mass extinction". Palaeogeography, Palaeoclimatology, Palaeoecology. 386: 531–540. doi:10.1016/j.palaeo.2013.06.020.
  35. ^ a b Kuzmin, M.I.; Yarmolyuk, V.V.; Kravchinsky, V.A. (2010). "Phanerozoic hot spot traces and paleogeographic reconstructions of the Siberian continent based on interaction with the African large low shear velocity province". GEarth-Science Reviews. 148 (1–2): 1–33. Bibcode:2010ESRv..102...29K. doi:10.1016/j.earscirev.2010.06.004.
  36. ^ Bond, D. P. G.; Wignall, P. B. (2014). "Large igneous provinces and mass extinctions: An update". GSA Special Papers. 505: 29–55.
  37. ^ Joachimski, M. M.; et al. (2002). "Conodont apatite δ18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction". Geology. 30 (8): 711. doi:10.1130/0091-7613(2002)030<0711:caosic>2.0.co;2.
  38. ^ Ma, X. P.; et al. (2015). "The Late Devonian Frasnian–Famennian event in South China — Patterns and causes of extinctions, sea level changes, and isotope variations". Palaeogeography, Palaeoclimatology, Palaeoecology. 448: 224–244. doi:10.1016/j.palaeo.2015.10.047.

Sources

  • McGhee, George R. (1 January 1996). The Late Devonian Mass Extinction: The Frasnian/Famennian Crisis. Columbia University Press. p. 9. ISBN 978-0-231-07505-3. Retrieved 23 July 2015.
  • Racki, Grzegorz, "Toward understanding Late Devonian global events: few answers, many questions" in Jeff Over, Jared Morrow, P. Wignall (eds.), Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events, Elsevier, 2005.

External links

Agnatha

Agnatha (Ancient Greek ἀ-γνάθος "no jaws") is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. The group is sister to all vertebrates with jaws, known as gnathostomes.Recent molecular data, both from rRNA and from mtDNA as well as embryological data strongly supports the hypothesis that living agnathans, the cyclostomes, are monophyletic.The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, comprising about 120 species in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata was created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. Lampreys have a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic or cold blooded, with a cartilaginous skeleton, and the heart contains 2 chambers.

While a few scientists still regard the living agnathans as only superficially similar, and argue that many of these similarities are probably shared basal characteristics of ancient vertebrates, recent classification clearly place hagfish (the Myxini or Hyperotreti) with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.

Arthrodira

Arthrodira is an order of extinct armoured, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches.

Greek for "jointed neck", the arthrodires had movable joint between armor surrounding the head and body. Lacking distinct teeth, like all placoderms, they used the sharpened edges of a bony plate as a biting surface. The eye sockets are protected by a bony ring, a feature shared by birds and some ichthyosaurs. Early arthrodires, such as the genus Arctolepis, were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, reaching 1 to as much as 6 m in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.

A common misconception is the arthrodires (along with all other placoderms) were sluggish bottom-dwellers that were outcompeted by more advanced fish. Leading to this misconception is that the arthrodire body plan remained relatively conserved (that is, the majority of arthrodires were bullet- or torpedo-shaped) during the Devonian period, save for increasing in size. However, during their reign, the arthrodires were one of the most diverse and numerically successful, if not the most successful, vertebrate orders of the Devonian, occupying a vast spectrum of roles from apex predator to detritus-nibbling bottom dweller. Despite their success, the arthrodires were one of many groups eliminated by the environmental catastrophes of the Late Devonian extinction, allowing other fish such as sharks to diversify into the vacated ecological niches during the Carboniferous period.

Devonian

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.

The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small continent of Euramerica in between.

Eurypterid

Eurypterids, often informally called sea scorpions, are an extinct group of arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event (or sometime shortly before) 251.9 million years ago.

Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions. Some studies suggest that a dual respiratory system was present, which would have allowed for short periods of time in terrestrial environments. The name Eurypterida comes from the Ancient Greek words εὐρύς (eurús), meaning "broad" or "wide", and πτερόν (pteron), meaning "wing", referring to the pair of wide swimming appendages present in many members of the group.

The eurypterids include the largest known arthopods ever to have lived. The largest, Jaekelopterus, reached 2.5 meters (8.2 ft) in length. Eurypterids were not uniformly large and most species were less than 20 centimeters (8 in) long; the smallest eurypterids, Alkenopterus and Eocarcinosoma, were only 3 centimeters (1.2 in) long. Eurypterid fossils have been recovered from every continent. A majority of fossils are from fossil sites in North America and Europe because the group lived primarily in the waters around and within the ancient supercontinent of Euramerica. Only a handful of eurypterid groups spread beyond the confines of Euramerica and a few genera, such as Adelophthalmus and Pterygotus, achieved a cosmopolitan distribution with fossils being found worldwide.

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.

Evolutionary history of life

The evolutionary history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years (Ga) ago and evidence suggests life emerged prior to 3.7 Ga. (Although there is some evidence of life as early as 4.1 to 4.28 Ga, it remains controversial due to the possible non-biological fomation of the purported fossils.) The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor. Approximately 1 trillion species currently live on Earth of which only 1.75–1.8 million have been named and 1.6 million documented in a central database. These currently living species represent less than one percent of all species that have ever lived on earth.

The earliest evidence of life comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago.Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean Epoch and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants). However the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species. Bilateria, animals with a front and a back, appeared by 555 Ma (million years ago).The earliest complex land plants date back to around 850 Ma, from carbon isotopes in Precambrian rocks, while algae-like multicellular land plants are dated back even to about 1 billion years ago, although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 Ga. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event. (The long causal chain implied seems to involve the success of early tree archaeopteris (1) drew down CO2 levels, leading to global cooling and lowered sea levels, (2) roots of archeopteris fostered soil development which increased rock weathering, and the subsequent nutrient run-off may have triggered algal blooms resulting in anoxic events which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.)

Ediacara biota appear during the Ediacaran period, while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252 Ma. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.

Exshaw Formation

The Exshaw Formation is a stratigraphic unit in the Western Canada Sedimentary Basin. It takes the name from the hamlet of Exshaw, Alberta in the Canadian Rockies, and was first described from outcrops on the banks of Jura Creek north of Exshaw by P.S. Warren in 1937. The formation is of Late Devonian (late Famennian) to Early Mississippian (middle Tournaisian) age as determined by conodont biostratigraphy, and it straddles the Devonian-Carboniferous boundary.The Exshaw strata were deposited in a marine setting during the Hangenberg event, an oceanic anoxic event associated with the Late Devonian extinction. The black shales of the Exshaw Formation are rich in organic matter and are one of the most important petroleum source rocks of the Western Canada Sedimentary Basin.

Extinction (disambiguation)

Extinction is in biology and palaeontology, the end of a species or other taxon.

Extinction may also refer to:

Extinction (peerage), in the United Kingdom, happens when all possible heirs of a peer have died out

Flood basalt

A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa (meaning "stairs"), due to the characteristic stairstep geomorphology of many associated landscapes. Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large volcanic provinces, plateaus, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group. Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.

Gondwana

Gondwana ( ), (or Gondwanaland), was a supercontinent that existed from the Neoproterozoic (about 550 million years ago) until the Jurassic (about 180 million years ago).

It was formed by the accretion of several cratons. Eventually, Gondwana became the largest piece of continental crust of the Paleozoic Era, covering an area of about 100,000,000 km2 (39,000,000 sq mi). During the Carboniferous Period, it merged with Laurussia to form a larger supercontinent called Pangaea. Gondwana (and Pangaea) gradually broke up during the Mesozoic Era. The remnants of Gondwana make up about two thirds of today's continental area, including South America, Africa, Antarctica, Australia, and the Indian Subcontinent.

The formation of Gondwana began c. 800 to 650 Ma with the East African Orogeny, the collision of India and Madagascar with East Africa,and was completed c. 600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa and the addition of Australia and Antarctica, respectively.

Goniatite

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. Goniatites (goniatitida) survived the Late Devonian extinction to flourish during the Carboniferous and Permian only to become extinct at the end of the Permian some 139 million years later.

Hangenberg event

The Hangenberg event is a bioevent that occurred at the end of the Famennian stage (late Devonian) associated with the Late Devonian extinction (roughly 358.9 ± 0.4 million years ago); it was an anoxic event marked by a black shale. It has been proposed that this was related to a rapid sea-level fall due to the last phase of the Devonian Southern Hemisphere glaciation. It has also been suggested that it was linked to an increase in terrestrial plant cover, leading to increased nutrient supply in rivers. This may have led to eutrophication of semi-restricted epicontinental seas and could have stimulated algal blooms.It is named from the Hangenberg Shale, part of a sequence that straddles the Devonian-Carboniferous boundary, from the Rhenish Massif in Germany.Following the extinction, vertebrates experienced reduced body size for the following 36 million years, at least in part because smaller taxa diversified more successfully.

Ordovician–Silurian extinction events

The Ordovician–Silurian extinction events, when combined, are the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. This event greatly affected marine communities, which caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. The Ordovician–Silurian extinction occurred during the Hirnantian stage of the Ordovician Period and the subsequent Rhuddanian stage of the Silurian Period. The last event is dated in the interval of 455–430 Ma ago, i.e., lasting from the Middle Ordovician to Early Silurian, thus including the extinction period. This event was the first of the big five Phanerozoic events and was the first to significantly affect animal-based communities.Almost all major taxonomic groups were affected during this extinction event. Extinction was global during this period, eliminating 49-60% of marine genera and nearly 85% of marine species.Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. Before the late Ordovician cooling, temperatures were relatively warm and it is the suddenness of the climate changes and the elimination of habitats due to sea-level fall that are believed to have precipitated the extinctions. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling were likely driving agents for the Ordovician mass extinction.

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.

Styggforsen

Styggforsen is a waterfall and a nature reserve in Dalarna County, Sweden. It is part of the European Union-wide Natura 2000 network.

Timeline of plant evolution

This article attempts to place key plant innovations in a geological context. It concerns itself only with novel adaptations and events that had a major ecological significance, not those that are of solely anthropological interest. The timeline displays a graphical representation of the adaptations; the text attempts to explain the nature and robustness of the evidence.

Plant evolution is an aspect of the study of biological evolution, predominantly involving evolution of plants suited to live on land, greening of various land masses by the filling of their niches with land plants, and diversification of groups of land plants.

Woodleigh crater

Woodleigh is a large meteorite impact crater (astrobleme) in Western Australia, centred on Woodleigh Station east of Shark Bay, Gascoyne region. A team of four scientists at the Geological Survey of Western Australia and the Australian National University, led by Arthur J. Mory, announced the discovery in the 15 April 2000 issue of Earth and Planetary Science Letters.

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See also

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