TheProterozoic–Phanerozoic boundary is at 538.8 million years ago.[1] In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan)fossils, buttrace fossils of several hundred groups (taxa) of complex soft-bodied metazoa from the precedingEdiacaran period of the Proterozoic eon, known as theAvalon Explosion, have been identified since the systematic study of those forms started in the 1950s.[8][9] The transition from thelargely sessile Precambrian biota to the active mobile Cambrian biota occurred early in the Phanerozoic.
The Paleozoic is a time in Earth's history when active complex life forms evolved, took their first foothold on dry land, and when the forerunners of all multicellular life on Earth began to diversify. There are six periods in the Paleozoic era:Cambrian,Ordovician,Silurian,Devonian,Carboniferous andPermian.[10]
The Cambrian is the first period of the Paleozoic Era and ran from 539 million to 485 million years ago. The Cambrian sparked arapid expansion in the diversity of animals, in an event known as theCambrian explosion, during which the greatest number of animalbody plans evolved in a single period in the history of Earth. Complexalgae evolved, and the fauna was dominated by armoured arthropods (such astrilobites andradiodontids) and to a lesser extent shelledcephalopods (such asorthocones). Almost allphyla of marine animals evolved in this period. During this time, the super-continentPannotia began to break up, most of which later recombined into the super-continentGondwana.[11]
By the end of the Ordovician, Gondwana had moved from the equator to theSouth Pole, andLaurentia had collided withBaltica, closing theIapetus Ocean. The glaciation of Gondwana resulted in a major drop in sea level, killing off all life that had established along its coast. Glaciation caused anicehouse Earth, leading to theOrdovician–Silurian extinction, during which 60% of marine invertebrates and 25% of families became extinct. Though one of the deadliest mass extinctions in earth's history, the O–S extinction did not cause profound ecological changes between the periods.[13]
The Silurian spans from 444 million to 419 million years ago, which saw a warming from an icehouse Earth. This period saw themass diversification of fish, as jawless fish became more numerous, and earlyjawed fish andfreshwater species appeared in the fossil record. Arthropods remained abundant, and some groups, such aseurypterids, becameapex predators in the ocean. Fully terrestrial life established itself on land, including earlyfungi,arachnids,hexapods andmyriapods. The evolution ofvascular plants (mainlyspore-producingferns such asCooksonia) allowed land plants to gain a foothold further inland as well. During this time, there were four continents:Gondwana (Africa, South America, Australia, Antarctica, India),Laurentia (North America with parts of Europe),Baltica (the rest of Europe), andSiberia (Northern Asia).[14]
The Carboniferous spans from 359 million to 299 million years ago.[16] Tropicalswamps dominated the Earth, and the large amounts of treessequestered much of the carbon that becamecoal deposits (hence the name Carboniferous and the term "coal forest"). About 90% of all coal beds were deposited in the Carboniferous and Permian periods, which represent just 2% of the Earth's geologic history.[17] The highoxygen levels caused by these wetland rainforests allowed arthropods, normally limited in size bytheir respiratory systems, to proliferate and increase in size. Tetrapods also diversified during the Carboniferous assemiaquaticamphibians such as thetemnospondyls, and one lineage developedextraembryonic membranes that allowed theireggs to survive outside of the water. These tetrapods, theamniotes, included the firstsauropsids (which evolved thereptiles,dinosaurs andbirds) andsynapsids (the ancestors ofmammal). Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation ofGondwana as much of it was situated around theSouth Pole. This event was known as thePermo-Carboniferous Glaciation and resulted in a major loss of coal forests, known as theCarboniferous rainforest collapse.[18]
The Permian spans from 299 million to 251 million years ago and was the last period of the Paleozoic era. At its beginning, all landmasses came together to form the supercontinentPangaea, surrounded by one expansive ocean calledPanthalassa. The Earth was relatively dry compared to the Carboniferous, withharsh seasons, as the climate of the interior of Pangaea was not moderated by large bodies of water.Amniotes still flourished and diversified in the new dry climate, particularlysynapsids such asDimetrodon,Edaphosaurus andtherapsids, which gave rise to the ancestors of modern mammals. The firstconifers evolved during this period, then dominated the terrestrial landscape. The Permian ended with at least onemass extinction, an event sometimes known as "the Great Dying", caused by largefloods of lava (theSiberian Traps in Russia and theEmeishan Traps in China). This extinction was the largest in Earth's history and led to the loss of 95% of all species of life.[19][20]
The Mesozoic ranges from 252 million to 66 million years ago. Also referred to as the Age of Reptiles, Age of Dinosaurs or Age of Conifers,[21] the Mesozoic featured the first time the sauropsids ascended to ecological dominance over the synapsids, as well as the diversification of many modernray-finned fish,insects,molluscs (particularly thecoleoids), tetrapods and plants. The Mesozoic is subdivided into three periods: the Triassic, Jurassic and Cretaceous.
The Triassic ranges from 252 million to 201 million years ago. The Triassic is mostly a transitional recovery period between the desolate aftermath of thePermian Extinction and the lush Jurassic Period. It has three major epochs:Early Triassic,Middle Triassic, andLate Triassic.[22]
The Early Triassic lasted between 252 million to 247 million years ago,[23] and was a hot and arid epoch in the aftermath of the Permian Extinction. Many tetrapods during this epoch represented adisaster fauna, a group of survivor animals with low diversity and cosmopolitanism (wide geographic ranges).[24]Temnospondyli recovered first and evolved into large aquatic predators during the Triassic.[25][26] Other reptiles also diversified rapidly, with aquatic reptiles such asichthyosaurs andsauropterygians proliferating in the seas. On land, the first truearchosaurs appeared, includingpseudosuchians (crocodile relatives) andavemetatarsalians (bird/dinosaur relatives).
The Middle Triassic spans from 247 million to 237 million years ago.[23] The Middle Triassic featured the beginnings of the break-up ofPangaea as rifting commenced in north Pangaea. The northern part of theTethys Ocean, thePaleotethys Ocean, had become a passive basin, but a spreading center was active in the southern part of the Tethys Ocean, the Neotethys Ocean.[27]Phytoplankton, coral,crustaceans and many other marine invertebrates recovered from the Permian extinction by the end of the Middle Triassic.[28] Meanwhile, on land, reptiles continued to diversify, conifer forests flourished,[29] as well as the first flies.[30][31][32]
The Late Triassic spans from 237 million to 201 million years ago.[23] Following the bloom of the Middle Triassic, the Late Triassic was initially warm and arid with a strongmonsoon climate and with most precipitation limited to coastal regions and high latitudes.[33] This changed late in theCarnian period with a2 million years-long wet season which transformed the arid continental interior into lushalluvial forests. The first truedinosaurs appeared early in the Late Triassic,[34] andpterosaurs evolved a bit later.[35][36][37] Other large reptilian competitors to the dinosaurs were wiped out by theTriassic–Jurassic extinction event, in which mostarchosaurs (excludingcrocodylomorphs, pterosaurs and dinosaurs), mosttherapsids (exceptcynodonts) and almost all large amphibians became extinct, as well as 34% of marine life in the fourth mass extinction event. The cause of the extinction is debated, but likely resulted from eruptions of theCAMPlarge igneous province.[38]
The Early Jurassic epoch spans from 201 million to 174 million years ago.[39] The climate was much more humid than during the Triassic, and as a result, the world was warm and partially tropical,[40][41] though possibly with short colder intervals.[42]Plesiosaurs, ichthyosaurs andammonites dominated the seas,[43] while dinosaurs, pterysaurs and other reptiles dominated the land,[43] with species such asDilophosaurus at the apex.[44]Crocodylomorphs evolved into aquatic forms, pushing the remaining large amphibians to near extinction.[45][43] True mammals were present during the Jurassic[46] but remained small, with average body masses of less than 10 kilograms (22 lb) until the end of the Cretaceous.[47][48]
The Middle and Late Jurassic Epochs span from 174 million to 145 million years ago.[39] Conifersavannahs made up a large portion of the world's forests.[49][50] In the oceans, plesiosaurs were quite common, andichthyosaurs were flourishing.[51] The Late Jurassic Epoch spans from 163 million to 145 million years ago.[39] The Late Jurassic featured a severe extinction ofsauropods in northern continents, alongside many ichthyosaurs. However, the Jurassic-Cretaceous boundary did not strongly impact most forms of life.[52]
The Cretaceous is the Phanerozoic's longest period and the last period of the Mesozoic. It spans from 145 million to 66 million years ago, and is divided into two epochs:Early Cretaceous, andLate Cretaceous.[53]
The Early Cretaceous Epoch spans from 145 million to 100 million years ago.[53] Dinosaurs continued to be abundant, with groups such astyrannosauroids,avialans (birds),marginocephalians, andornithopods seeing early glimpses of later success. Other tetrapods, such asstegosaurs and ichthyosaurs, declined significantly, and sauropods were restricted to southern continents.
The Late Cretaceous Epoch spans from 100 million to 66 million years ago.[53] The Late Cretaceous featured a cooling trend that would continue into theCenozoic Era. Eventually, the tropical climate was restricted to the equator and areas beyond the tropic lines featured more seasonal climates. Dinosaurs still thrived as new species such asTyrannosaurus,Ankylosaurus,Triceratops andhadrosaurs dominated the food web. Whether or notpterosaurs went into a decline as birds radiated is debated; however, many families survived until the end of the Cretaceous, alongside new forms such as the giganticQuetzalcoatlus.[54] Mammals diversified despite their small sizes, withmetatherians (marsupials and kin) andeutherians (placentals and kin) coming into their own. In the oceans,mosasaurs diversified to fill the role of the now-extinct ichthyosaurs, alongside huge plesiosaurs such asElasmosaurus. Also, the first flowering plants evolved. At the end of the Cretaceous, theDeccan Traps and other volcanic eruptions were poisoning the atmosphere. As this was continued, it is thought that a largemeteor smashed into Earth, creating theChicxulub Crater and causing the event known as theK–Pg extinction, the fifth and most recent mass extinction event, during which 75% of life on Earth became extinct, including all non-avian dinosaurs. Every living thing with a body mass over 10 kilograms became extinct, and the Age of Dinosaurs came to an end.[55][56]
The Cenozoic featured the rise of mammals and birds as the dominant class of animals, as the end of the Age of Dinosaurs left significant openniches. There are three divisions of the Cenozoic: Paleogene, Neogene and Quaternary.
The Paleogene spans from the extinction of the non-avian dinosaurs, some 66 million years ago, to the dawn of the Neogene 23 million years ago. It features threeepochs:Paleocene,Eocene andOligocene.
Basilosaurus was an early cetacean, related to modern whales
The Paleocene Epoch began with the K–Pg extinction event, and the early part of the Paleocene saw the recovery of the Earth from that event. The continents began to take their modern shapes, but most continents (and India) remained separated from each other: Africa andEurasia were separated by theTethys Sea, and theAmericas were separated by thePanamanic Seaway (as theIsthmus of Panama had not yet formed). This epoch featured a general warming trend that peaked at thePaleocene-Eocene Thermal Maximum, and the earliest modernjungles expanded, eventually reaching the poles. The oceans were dominated by sharks, as the large reptiles that had once ruled had become extinct. Mammals diversified rapidly, but most remained small. The largest tetrapod carnivores during the Paleocene were reptiles, includingcrocodyliforms,choristoderans andsnakes.Titanoboa, the largest known snake, lived in South America during the Paleocene.
The Eocene Epoch ranged from 56 million to 34 million years ago. In the early Eocene, most land mammals were small and living in cramped jungles, much like the Paleocene. Among them were earlyprimates,whales andhorses along with many other early forms of mammals. The climate was warm and humid, with little temperature gradient from pole to pole. In the Middle Eocene Epoch, theAntarctic Circumpolar Current formed when South America and Australia both separated from Antarctica to open up theDrake Passage andTasmanian Passage, disrupting ocean currents worldwide, resulting in global cooling and causing the jungles to shrink. More modern forms of mammals continued to diversify with the cooling climate even as more archaic forms died out. By the end of the Eocene, whales such asBasilosaurus had become fully aquatic. The late Eocene Epoch saw the rebirth of seasons, which caused the expansion of savanna-like areas with the earliest substantialgrasslands.[57][58] At the transition between the Eocene and Oligocene epochs there was a significantextinction event, the cause of which is debated.
The Oligocene Epoch spans from 34 million to 23 million years ago. The Oligocene was an important transitional period between the tropical world of the Eocene and more modern ecosystems. This period featured a global expansion of grass which led to many new species taking advantage, including the firstelephants,felines,canines,marsupials and many other species still prevalent today. Many other species of plants evolved during this epoch also, such as the evergreen trees. The long term cooling continued and seasonal rain patterns established. Mammals continued to grow larger.Paraceratherium, one of the largest land mammals to ever live, evolved during this epoch, along with many otherperissodactyls.
The Neogene spans from 23.03 million to 2.58 million years ago. It features two epochs: theMiocene and thePliocene.[59]
The Miocene spans from 23.03 million to 5.333 million years ago and is a period in whichgrass spread further across, effectively dominating a large portion of the world, diminishing forests in the process.Kelp forests evolved, leading to the evolution of new species such assea otters. During this time,perissodactyls thrived, and evolved into many different varieties. Alongside them were theapes, which evolved into 30 species. Overall, arid and mountainous land dominated most of the world, as did grazers. The Tethys Sea finally closed with the creation of theArabian Peninsula and in its wake left theBlack,Red,Mediterranean andCaspian seas. This only increased aridity. Many new plants evolved, and 95% of modernseed plants evolved in the mid-Miocene.[60]
The Pliocene lasted from 5.333 million to 2.58 million years ago. The Pliocene featured dramatic climatic changes, which ultimately led to modern species and plants. The Mediterranean Sea dried up for hundreds of thousand years in theMessinian salinity crisis. Along with these major geological events, Africa saw the appearance ofAustralopithecus, the ancestor ofHomo. The Isthmus of Panama formed, andanimals migrated between North and South America, wreaking havoc on the local ecology. Climatic changes brought savannas that are still continuing to spread across the world,Indian monsoons, deserts in East Asia, and the beginnings of theSahara Desert. The Earth's continents and seas moved into their present shapes. The world map has not changed much since, save for changes brought about by theQuaternary glaciation such asLake Agassiz (precursor of theGreat Lakes).[61][62]
The Quaternary spans from 2.58 million years ago to present day, and is the shortest geological period in thePhanerozoic Eon. It features modern animals, and dramatic changes in the climate. It is divided into two epochs: thePleistocene and theHolocene.
It has been demonstrated that changes in biodiversity through the Phanerozoic correlate much better with thehyperbolic model (widely used indemography andmacrosociology) than withexponential andlogistic models (traditionally used inpopulation biology and extensively applied tofossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-orderpositive feedback (more ancestors, more descendants) or anegative feedback that arises from resource limitation, or both. The hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of thehuman population growth arises from quadratic positive feedback, caused by the interaction of the population size and the rate of technological growth.[67] The character of biodiversity growth in the Phanerozoic Eon can be similarly accounted for by a feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the superposition on the hyperbolic trend of cyclical and random dynamics.[67]
Across the Phanerozoic, the dominant driver of long-term climatic change was the concentration of carbon dioxide in the atmosphere,[68] though some studies have suggested a decoupling of carbon dioxide and palaeotemperature, particularly during cold intervals of the Phanerozoic.[69] Phanerozoic carbon dioxide concentrations have been governed partially by a 26 million year oceanic crustal cycle.[70] Since the Devonian, large swings in carbon dioxide of 2,000 ppm or more were uncommon over short timescales.[71] Variations in global temperature were limited by negative feedbacks in thephosphorus cycle, wherein increasedphosphorus input into the ocean would increase surficial biological productivity that would in turn enhance iron redox cycling and thus remove phosphorus from seawater; this maintained a relatively stable rate of removal of carbon from the atmosphere and ocean via organic carbon burial.[72] The climate also controlled the availability of phosphate through its regulation of rates of continental and seafloor weathering.[73] Major global temperature variations of >7 °C during the Phanerozoic were strongly associated with mass extinctions.[74]
Global map reconstruction showing continents at 500 million years agoGlobal maps showing continental movement from 250 million years ago to present day.
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