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Paleozoic

"Primitive period" redirects here. For the use in mathematics, seePeriodic function.

ThePaleozoic(/ˌpæli.əˈzoʊ.ɪk,-i.oʊ-,ˌpeɪ-/PAL-ee-ə-ZOH-ik,-⁠ee-oh-,PAY-;[1]orPalaeozoic)Erais the first of threegeological erasof thePhanerozoicEon. Beginning 538.8 million years ago (Ma), it succeeds theNeoproterozoic(the last era of theProterozoicEon) and ends 251.9 Ma at the start of theMesozoicEra.[2]The Paleozoic is subdivided into sixgeologic periods(from oldest to youngest):

Paleozoic
538.8 ± 0.2 – 251.9 ± 0.024Ma
Chronology
Etymology
Name formalityFormal
Alternate spelling(s)Palaeozoic
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEra
Stratigraphic unitErathem
Lower boundary definitionAppearance of theIchnofossilTreptichnus pedum
Lower boundary GSSPFortune Head section,Newfoundland,Canada
47°04′34″N55°49′52″W/ 47.0762°N 55.8310°W/47.0762; -55.8310
Lower GSSP ratified1992
Upper boundary definitionFirst appearance of the ConodontHindeodus parvus.
Upper boundary GSSPMeishan,Zhe gian g,China
31°04′47″N119°42′21″E/ 31.0798°N 119.7058°E/31.0798; 119.7058
Upper GSSP ratified2001
  1. Cambrian
  2. Ordovician
  3. Silurian
  4. Devonian
  5. Carboniferous
  6. Permian

Some geological timescales divide the Paleozoic informally into early and late sub-eras: the Early Paleozoic consisting of the Cambrian, Ordovician and Silurian; the Late Paleozoic consisting of the Devonian, Carboniferous and Permian.[3]

The namePaleozoicwas first used byAdam Sedgwick(1785–1873) in 1838[4]to describe the Cambrian and Ordovician periods. It was redefined byJohn Phillips(1800–1874) in 1840 to cover the Cambrian to Permian periods.[5]It is derived from theGreekpalaiós(παλαιός, "old" ) andzōḗ(ζωή, "life" ) meaning "ancient life".[6]

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 theCambrian explosion,in which most modernphylafirst appeared.Arthropods,molluscs,fish,amphibians,reptiles,andsynapsidsall evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, greatforestsof primitive plants covered the continents, many of which formed thecoalbeds ofEuropeand easternNorth America.Towards the end of the era, large, sophisticated synapsids and diapsids were dominant and the first modern plants (conifers) appeared.

The Paleozoic Era ended with the largestextinction eventof thePhanerozoic Eon,[a]thePermian–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.[7] Recovery of life in the sea may have been much faster.[8]

Boundaries

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The base of the Paleozoic is one of the major divisions in geological time representing the divide between the Proterozoic and Phanerozoic eons, the Paleozoic and Neoproterozoic eras and theEdiacaranand Cambrian periods.[9]When Adam Sedgwick named the Paleozoic in 1835, he defined the base as the first appearance of complex life in the rock record as shown by the presence oftrilobite-dominated fauna.[4]Since then evidence of complex life in older rock sequences has increased and by the second half of the 20th century, the first appearance ofsmall shelly fauna(SSF), also known as early skeletal fossils, were considered markers for the base of the Paleozoic. However, whilst SSF are well preserved incarbonatesediments, the majority of Ediacaran to Cambrian rock sequences are composed ofsiliciclasticrocks where skeletal fossils are rarely preserved.[9]This led theInternational Commission on Stratigraphy(ICS) to usetrace fossilsas an indicator of complex life.[10]Unlike later in the fossil record, Cambrian trace fossils are preserved in a wide range of sediments and environments, which aids correlation between different sites around the world. Trace fossils reflect the complexity of the body plan of the organism that made them. Ediacaran trace fossils are simple, sub-horizontal feeding traces. As more complex organisms evolved, their more complex behaviour was reflected in greater diversity and complexity of the trace fossils they left behind.[9]After two decades of deliberation, the ICS choseFortune Head,Burin Peninsula, Newfoundland as the basal Cambrian Global Stratotype Section and Point (GSSP) at the base of theTreptichnus pedumassemblage of trace fossils and immediately above the last occurrence of the Ediacaran problematica fossilsHarlaniellapodolicaandPalaeopsacichnus.[10]The base of the Phanerozoic, Paleozoic and Cambrian is dated at 538.8+/-0.2 Ma and now lies below both the first appearance of trilobites and SSF.[9][10]

The boundary between the Paleozoic and Mesozoic eras and the Permian and Triassic periods is marked by the first occurrence of theconodontHindeodus parvus.This is the firstbiostratigraphicevent found worldwide that is associated with the beginning of the recovery following the end-Permian mass extinctionsand environmental changes. In non-marine strata, the equivalent level is marked by the disappearance of the PermianDicynodontetrapods.[11]This means events previously considered to mark the Permian-Triassic boundary, such as the eruption of theSiberian Trapsflood basalts,the onset of greenhouse climate,ocean anoxiaandacidificationand the resulting mass extinction are now regarded as being of latest Permian in age.[11]The GSSP is nearMeishan,Zhe gian g Province, southern China.Radiometric datingof volcanic clay layers just above and below the boundary confine its age to a narrow range of 251.902+/-0.024 Ma.[11]

Geology

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The beginning of the Paleozoic Era witnessed the breakup of the supercontinent ofPannotia[12][13] and ended while the supercontinentPangaeawas assembling.[14] The breakup of Pannotia began with the opening of theIapetus Oceanand other Cambrian seas and coincided with a dramatic rise in sea level.[15] Paleoclimaticstudies and evidence ofglaciersindicate thatCentral Africawas most likely in the polar regions during the early Paleozoic. The breakup of Pannotia was followed by the assembly of the huge continentGondwana(510million years ago). By the mid-Paleozoic, the collision of North America and Europe produced the Acadian-Caledonian uplifts, and a subducting plate uplifted easternAustralia.By the late Paleozoic, continental collisions formed the supercontinent of Pangaea and created great mountain chains, including theAppalachians,Caledonides,Ural Mountains,and mountains ofTasmania.[14]

Cambrian Period

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Main article:Cambrian
Trilobites

The Cambrian spanned from 539–485 million years ago and is the first period of the Paleozoic Era of the Phanerozoic. The Cambrian marked a boom in evolution in an event known as theCambrian explosionin which the largest number of creatures evolved in any single period of the history of the Earth. Creatures likealgaeevolved, but the most ubiquitous of that period were the armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the supercontinent Pannotia begins to break up, most of which later became the supercontinent Gondwana.[16]

Ordovician Period

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Main article:Ordovician
Cephalaspis(a jawless fish)

The Ordovician spanned from 485–444 million years ago. The Ordovician was a time in Earth's history in which many of thebiological classesstill prevalent today evolved, such as primitive fish, cephalopods, and coral. The most common forms of life, however, were trilobites, snails and shellfish. The first arthropods went ashore to colonize the empty continent of Gondwana. By the end of the Ordovician, Gondwana was at the south pole, early North America had collided with Europe, closing the intervening ocean. Glaciation of Africa resulted in a major drop in sea level, killing off all life that had established along coastal Gondwana. Glaciation may have caused theOrdovician–Silurian extinction events,in which 60% of marine invertebrates and 25% of families became extinct, and is considered the first Phanerozoic mass extinction event, and the second deadliest.[a][17]

Silurian Period

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Main article:Silurian

The Silurian spanned from 444–419 million years ago. The Silurian saw the rejuvenation of life as the Earth recovered from the previous glaciation. This period saw the mass evolution of fish, as jawless fish became more numerous, jawed fish evolved, and the first freshwater fish evolved, though arthropods, such assea scorpions,were stillapex predators.Fully terrestrial life evolved, including early arachnids, fungi, and centipedes. The evolution ofvascular plants(Cooksonia) allowed plants to gain a foothold on land. These early plants were the forerunners of all plant life on land. During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, Siberia), Laurentia (North America), Baltica (Northern Europe), and Avalonia (Western Europe). The recent rise in sea levels allowed many new species to thrive in water.[18]

Devonian Period

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Main article:Devonian
Eogyrinus(an amphibian) of the Carboniferous

The Devonian spanned from 419–359 million years ago. Also known as "The Age of the Fish", the Devonian featured a huge diversification of fish, including armored fish likeDunkleosteusand lobe-finned fish which eventually evolved into the first tetrapods. On land, plant groups diversified rapidly in an event known as theDevonian explosionwhen plants madelignin,leading to taller growth and vascular tissue; the first trees and seeds evolved. These new habitats led to greater arthropod diversification. The first amphibians appeared and fish occupied the top of the food chain. Earth's second Phanerozoic mass extinction event (a group of several smaller extinction events), theLate Devonian extinction,ended 70% of existing species.[a][19]

Carboniferous Period

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Main article:Carboniferous

The Carboniferous is named after the large coal deposits laid down during the period. It spanned from 359–299 million years ago. During this time, average global temperatures were exceedingly high; the early Carboniferous averaged at about 20 degrees Celsius (but cooled to 10 °C during the Middle Carboniferous).[20]An important evolutionary development of the time was the evolution ofamniotic eggs,which allowed amphibians to move farther inland and remain the dominant vertebrates for the duration of this period. Also, the first reptiles andsynapsidsevolved in the swamps. Throughout the Carboniferous, there was a cooling trend, which led to the Permo-Carboniferous glaciation or theCarboniferous Rainforest Collapse.Gondwanawas glaciated as much of it was situated around the south pole.[21]

Permian Period

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Main article:Permian
Synapsid:Dimetrodon grandis

The Permian spanned from 299–252 million years ago and was the last period of the Paleozoic Era. At the beginning of this period, all continents joined together to form the supercontinent Pangaea, which was encircled by one ocean calledPanthalassa.The land mass was very dry during this time, with harsh seasons, as the climate of the interior of Pangaea was not regulated by large bodies of water.Diapsidsandsynapsidsflourished in the new dry climate. Creatures such asDimetrodonandEdaphosaurusruled the new continent. The first conifers evolved, and dominated the terrestrial landscape. Near the end of the Permian, however, Pangaea grew drier. The interior was desert, and new taxa such asScutosaurusandGorgonopsidsfilled it. Eventually they disappeared, along with 95% of all life on Earth, in a cataclysm known as "The Great Dying",the third and most severe Phanerozoic mass extinction.[a][22][23]

Climate

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Life in the early Paleozoic
Swamp forest in the Carboniferous

The early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustainedsea level risein the Phanerozoic got underway. However, as if to offset this trend, Gondwana moved south, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over theSouth Pole.

The early Paleozoic climate was strongly zonal, with the result that the "climate", in an abstract sense, became warmer, but the living space of most organisms of the time – the continental shelf marine environment – became steadily colder. However,Baltica(Northern Europe and Russia) andLaurentia(eastern North America and Greenland) remained in the tropical zone, while China and Australia lay in waters which were at least temperate. The early Paleozoic ended, rather abruptly, with the short, but apparently severe, late Ordovician ice age. This cold spell caused the second-greatestmass extinctionof the Phanerozoic Eon.[24][a]Over time, the warmer weather moved into the Paleozoic Era.

The Ordovician and Silurian were warm greenhouse periods, with the highest sea levels of the Paleozoic (200 m above today's); the warm climate was interrupted only by a30million yearcool period, theEarly Palaeozoic Icehouse,culminating in theHirnantianglaciation,445million years agoat the end of the Ordovician.[25]

The middle Paleozoic was a time of considerable stability. Sea levels had dropped coincident with the ice age, but slowly recovered over the course of the Silurian and Devonian. The slow merger of Baltica and Laurentia, and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow sea floor. As plants took hold on the continental margins,oxygenlevels increased andcarbon dioxidedropped, although much less dramatically. The north–south temperature gradient also seems to have moderated, ormetazoan lifesimply became hardier, or both. At any event, the far southern continental margins ofAntarcticaand West Gondwana became increasingly less barren. The Devonian ended with a series ofturnover pulseswhich killed off much of middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.

There are many unanswered questions about the late Paleozoic. TheMississippian(early Carboniferous Period) began with a spike in atmospheric oxygen, while carbon dioxide plummeted to new lows. This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous. These were far more severe than the brief Late Ordovician ice age; but, this time, the effects on world biota were inconsequential. By theCisuralianEpoch, both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangaea created huge arid inland areas subject to temperature extremes. TheLopingianEpoch is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction.

Flora

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An artist's impression of early land plants

While macroscopic plant life appeared early in the Paleozoic Era and possibly late in the Neoproterozoic Era of the earlier eon, plants mostly remained aquatic until the Silurian Period, about 420 million years ago, when they began to transition onto dry land. Terrestrial flora reached its climax in the Carboniferous, when toweringlycopsidrainforests dominated the tropical belt ofEuramerica.Climate change caused theCarboniferous Rainforest Collapsewhich fragmented this habitat, diminishing the diversity of plant life in the late Carboniferous and Permian periods.[26]

Fauna

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A noteworthy feature of Paleozoic life is the sudden appearance of nearly all of theinvertebrateanimal phyla in great abundance at the beginning of the Cambrian. The first vertebrates appeared in the form of primitive fish, which greatly diversified in the Silurian and Devonian Periods. The first animals to venture onto dry land were the arthropods. Some fish had lungs, and powerful bony fins that in the late Devonian, 367.5 million years ago, allowed them to crawl onto land. The bones in their fins eventually evolved into legs and they became the first tetrapods,390million years ago,and began to develop lungs. Amphibians were the dominant tetrapods until the mid-Carboniferous, when climate change greatly reduced their diversity. Later, reptiles prospered and continued to increase in number and variety by the late Permian period.[26]

The Palaeozoic marine fauna was notably lacking in predators relative to the present day. Predators made up about 4% of the fauna in Palaeozoic assemblages while making up 17% of temperate Cenozoic assemblages and 31% of tropical ones. Infaunal animals made up 4% of soft substrate Palaeozoic communities but about 47% of Cenozoic communities. Additionally, the Palaeozoic had very few facultatively motile animals that could easily adjust to disturbance, with such creatures composing 1% of its assemblages in contrast to 50% in Cenozoic faunal assemblages. Non-motile animals untethered to the substrate, extremely rare in the Cenozoic, were abundant in the Palaeozoic.[27]

Microbiota

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Palaeozoicphytoplanktonoverall were both nutrient-poor themselves and adapted to nutrient-poor environmental conditions. This phytoplankton nutrient poverty has been cited as an explanation for the Palaeozoic's relatively low biodiversity.[28]

See also

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  • Geologic time scale– System that relates geologic strata to time
  • Precambrian– History of Earth 4600–539 million years ago
  • Cenozoic– Third era of the Phanerozoic Eon
  • Mesozoic– Second era of the Phanerozoic Eon: ~252–66 million years ago
  • Phanerozoic– Fourth and current eon of the geological timescale

Footnotes

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  1. ^abcde The list of the "big 5"mass extinctionsonly counts extinctions in thePhanerozoic Eon,since up tothe end oftheProterozoic Eon,life wasall soft-bodied.The meagrefossil traces of earlier lifemake it essentially impossible to identifyspeciesorgenera,and it is the disappearance of large proportions of existing genera from the fossil record that is the standard for comparingextinction eventsof the Phanerozoic "big 5". The one knownextinction eventin theeonsbefore the Phanerozoic was theOxygen Catastrophe,or theGreat Oxygenation Event,when the previouslyanoxicseas were poisoned withoxygenbynewly photosynthesizing bacteria.By some estimates, that event killed almost all life on the Earth, and might qualify as the "greatest ever" mass extinction, if its consequences for soft-bodied genera could be measured. Further, there might have beenotherextinction events in theprecambrianeons, whose traces in thegeologic record(if any) are less obvious than theOxygenation Event.

References

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  6. ^Harper, Douglas."Paleozoic".Online Etymology Dictionary.
  7. ^ Sahney, S. & Benton, M.J. (2008)."Recovery from the most profound mass extinction of all time".Proceedings of the Royal Society B: Biological Sciences.275(1636): 759–65.doi:10.1098/rspb.2007.1370.PMC2596898.PMID18198148.
  8. ^ "Dead-ammonite bounce".Science & technology.The Economist.5 July 2010.
  9. ^abcdGeyer, Gerd; Landing, Ed (2016-11-02)."The Precambrian–Phanerozoic and Ediacaran–Cambrian boundaries: a historical approach to a dilemma".Geological Society, London, Special Publications.448(1): 311–349.doi:10.1144/sp448.10.ISSN0305-8719.
  10. ^abcPeng, S. C.; Babcock, L. E.; Ahlberg, P. (2020-01-01), Gradstein, Felix M.; Ogg, James G.; Schmitz, Mark D.; Ogg, Gabi M. (eds.),"Chapter 19 – The Cambrian Period",Geologic Time Scale 2020,Elsevier, pp. 565–629,ISBN978-0-12-824360-2,retrieved2023-08-24
  11. ^abcOgg, J. G.; Chen, Z. -Q.; Orchard, M. J.; Jiang, H. S. (2020-01-01), Gradstein, Felix M.; Ogg, James G.; Schmitz, Mark D.; Ogg, Gabi M. (eds.),"Chapter 25 – The Triassic Period",Geologic Time Scale 2020,Elsevier, pp. 903–953,ISBN978-0-12-824360-2,retrieved2023-08-24
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  15. ^ Dalziel, I.W. (1997). "Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation".Geological Society of America Bulletin.109(1): 16–42.Bibcode:1997GSAB..109...16D.doi:10.1130/0016-7606(1997)109<0016:ONPGAT>2.3.CO;2.
  16. ^"Cambrian".ucmp.berkeley.edu.Berkeley, CA:University of California Museum of Paleontology.Archivedfrom the original on 2012-05-15.Retrieved2015-04-26.
  17. ^"Ordovician".ucmp.berkeley.edu.Berkeley, CA:University of California Museum of Paleontology.Archivedfrom the original on 2015-05-02.Retrieved2015-04-26.
  18. ^"Silurian".ucmp.berkeley.edu.Berkeley, CA:University of California Museum of Paleontology.Archivedfrom the original on 2017-06-16.Retrieved2015-04-26.
  19. ^"Devonian".ucmp.berkeley.edu.Berkeley, CA:University of California Museum of Paleontology.Archivedfrom the original on 2012-05-11.Retrieved2015-04-26.
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  22. ^"The Great Dying".nhm.ac.uk.London, UK:Natural History Museum.Archived fromthe originalon 2015-04-20.
  23. ^"Permian Era".ucmp.berkeley.edu.Berkeley, CA:University of California Museum of Paleontology.Archivedfrom the original on 2017-07-04.Retrieved2015-05-24.
  24. ^Saupe, Erin E.; Qiao, Huijie; Donnadieu, Yannick; Farnsworth, Alexander; Kennedy-Asser, Alan T.; Ladant, Jean-Baptiste; Lunt, Daniel J.; Pohl, Alexandre; Valdes, Paul; Finnegan, Seth (16 December 2019)."Extinction intensity during Ordovician and Cenozoic glaciations explained by cooling and palaeogeography".Nature Geoscience.13(1): 65–70.doi:10.1038/s41561-019-0504-6.hdl:1983/c88c3d46-e95d-43e6-aeaf-685580089635.S2CID209381464.Retrieved22 October2022.
  25. ^ Munnecke, A.; Calner, M.;Harper, D.A.T.;Servais, T. (2010). "Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis".Palaeogeography, Palaeoclimatology, Palaeoecology.296(3–4): 389–413.Bibcode:2010PPP...296..389M.doi:10.1016/j.palaeo.2010.08.001.
  26. ^abSahney, S.; Benton, M.J. & Falcon-Lang, H.J. (2010)."Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica"(PDF abstract).Geology.38(12): 1079–1082.Bibcode:2010Geo....38.1079S.doi:10.1130/G31182.1.Archivedfrom the original on 2011-10-11.Retrieved2012-02-17.
  27. ^Bush, Andrew M.; Bambach, Richard K.; Daley, Gwen M. (January 2007)."Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Paleozoic and late Cenozoic".Paleobiology.33(1): 76–97.doi:10.1666/06013.1.ISSN0094-8373.Retrieved10 December2023.
  28. ^Martin, Ronald E.; Quigg, Antonietta; Podkovyrov, Victor (27 February 2008)."Marine biodiversification in response to evolving phytoplankton stoichiometry".Palaeogeography, Palaeoclimatology, Palaeoecology.258(4): 277–291.doi:10.1016/j.palaeo.2007.11.003.ISSN0031-0182.Retrieved30 September2023.

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