Phanerozoic

The Phanerozoic^[1] (British English Phanærozoic) is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present,^[4] and goes back to the period when diverse hard-shelled animals first appeared. Its name derives from the Ancient Greek words φανερός (phanerós) and ζωή (zo̱í̱), meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The time before the Phanerozoic, called the Precambrian supereon, is now divided into the Hadean, Archaean and Proterozoic eons. Plant life also appeared from early in the Phanerozoic eon.

The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the emergence and development of complex plants; the evolution of fish; the emergence of insects and tetrapods; and the development of modern fauna. During this time span tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea, which then separated into the current continental landmasses.

Proterozoic-Phanerozoic boundary

The Proterozoic-Phanerozoic boundary is at 541 million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils. But several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef building animals (archaeocyatha) such as corals and others appear; at the first appearance of a complex feeding burrow called Treptichnus pedum; or at the first appearance of a group of small, generally disarticulated, armored forms termed 'the small shelly fauna'. The three different dividing points are within a few million years of each other.

In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature.

Eras of the Phanerozoic

The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, and consisting of 12 periods: the Cambrian, the Ordovician, the Silurian, the Devonian, the Carboniferous, the Permian, the Triassic, the Jurassic, the Cretaceous, the Paleogene, the Neogene, and the Quaternary. The Paleozoic features the rise of fish, amphibians and reptiles. The Mesozoic is ruled by the reptiles, and features the evolution of mammals, birds and more famously, dinosaurs. The Cenozoic is the time of the mammals, and more recently, humans.

Paleozoic era

The Paleozoic is a time in earth's history when complex life forms evolve, take their first breath of oxygen on dry land, and when the forerunners of all life on earth begin to diversify. There are six periods in the Paleozoic era: the Cambrian, the Ordovician, the Silurian, the Devonian, the Carboniferous and the Permian.^[5]

Cambrian

Trilobites

The Cambrian spans from 540 million years to 485 million years ago and is the first period of the Paleozoic and of the Phanerozoic eon. The Cambrian sparks a boom in evolution in an event known as the Cambrian Explosion in which the largest number of creatures evolve in the history of Earth during one period. Plants like algae evolve, the fauna is dominated by armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the super-continent Pannotia begins to break up, most of which becomes the super-continent Gondwana.^[6]

Ordovician

Cephalaspis, a jawless fish

The Ordovician spans from 485 million years to 440 million years ago. The Ordovician is a time in Earth's history in which many species still prevalent today evolved, such as primitive fish, cephalopods, and coral. The most common forms of life, however, were trilobites, snails and shellfish. More importantly, the first arthropods went ashore to colonize the empty continent of Gondwana. By the end of the period, Gondwana was at the south pole, and early North America had collided with Europe, closing the Atlantic Ocean. Glaciation of Africa resulted in a major drop in sea level, killing off all life that staked a claim along coastal Gondwana. Glaciation caused a snowball Earth, leading to the Ordovician-Silurian extinction, when 60% of marine invertebrates and 25% of families went extinct. This is considered the first mass extinction and the second deadliest extinction in the history of Earth.^[7]

Silurian

The Silurian spans from 440 million years to 415 million years ago. The Silurian saw the healing of the earth that recovered from the Snowball Earth. This period saw the mass evolution of fish, as jaw-less fish became more numerous, jawed fish evolved, and the first freshwater fish evolved, though arthropods, such as sea scorpions, were still apex predators. Fully terrestrial life evolved, which included early arachnids, fungi, and centipedes. Also, the evolution of vascular plants (Cooksonia) allowed plants to gain a foothold on land. These early terrestrial plants are the forerunners of all plant life on land. During this time, there are four continents: Gondwana (Africa, South America, Australia, Antarctica, India), Laurentia (North America with parts of Europe), Baltica (the rest of Europe), and Siberia (Northern Asia). The recent rise in sea levels provided many new species to thrive in water.^[8]

Devonian

Eogyrinus (an amphibian) of the Carboniferous

The Devonian spans from 415 million years to 360 million years ago. Also known as "The Age of the Fish", the Devonian features a huge diversification in fish, including armored fish like Dunkleosteus and lobe-finned fish which eventually evolved into the first tetrapods. On land, plant groups diversified incredibly in an event known as the Devonian Explosion where the first trees evolved, as well as seeds. This event also diversified arthropod life. The first amphibians also evolved, and the fish were now at the top of the food chain. Near the end of the Devonian, 70% of all species went extinct in an event known as the Late Devonian extinction, which is the second mass extinction known to have happened.^[9]

Carboniferous

Dimetrodon

The Carboniferous spans from 360 million to 300 million years ago. During this time, average global temperatures were exceedingly high: the early Carboniferous averaged at about 20 degrees Celsius (but cooled down to 10 degrees during the Middle Carboniferous).^[10] Tropical swamps dominated the earth, and the large amounts of trees created much of the carbon for the coal that is used today (hence the name Carboniferous). Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to head farther inland and remain the dominant vertebrae throughout the period. Also, the first reptiles and synapsids evolved in the swamps. Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of Gondwana as much of it was situated around the south pole, in an event known as the Permo-Carboniferous glaciation or the Carboniferous Rainforest Collapse.^[11]

Permian

The Permian spans from 300 million to 250 million years ago and was the last period of the Paleozoic. At the beginning, all continents clamped together to form the super-continent Pangaea, surrounded by one ocean called Panthalassa. The earth was very dry during this time, with harsh seasons, as the climate of the interior of Pangaea wasn't regulated by large bodies of water. Reptiles and synapsids flourished in the new dry climate. Creatures such as Dimetrodon and Edaphosaurus ruled the new continent. The first conifers evolved, then dominated the terrestrial landscape. Nearing the end of the period, Scutosaurus and gorgonopsids filled the empty desert. Eventually, they disappeared, along with 95% of all life on earth in an event simply known as "the Great Dying", the world's third mass extinction event.^[12][13]

Mesozoic era

Also known as "the Age of the dinosaurs", the Mesozoic features the rise of reptiles on their 150 million year conquest to rule the earth from the seas, the land, and even in the air. There are 3 periods in the Mesozoic: the Triassic, the Jurassic, and the Cretaceous.

Triassic

The Triassic ranges from 250 million to 200 million years ago. The Triassic is a desolate transitional state in Earth's history between the Permian Extinction and the lush Jurassic Period. It has three major epochs: the Early Triassic, the Middle Triassic and the Late Triassic.^[14]

The Early Triassic lasted between 250 million to 247 million years ago and was dominated by deserts as Pangaea had not yet broken up, thus the interior was nothing but arid. The Earth had just witnessed a massive die-off in which 95% of all life went extinct. The most common life on earth were Lystrosaurus, labyrinthodonts, and Euparkeria along with many other creatures that managed to survive the Great Dying. Temnospondyli evolved during this time and would be the dominant predator for much of the Triassic.^[15]

Plateosaurus (a prosauropod)

The Middle Triassic spans from 247 million to 237 million years ago. The Middle Triassic featured the beginnings of the breakup of Pangaea, and the beginning of the Tethys Sea. The ecosystem had recovered from the devastation of the Great Dying. Phytoplankton, coral, and crustaceans all had recovered, and the reptiles began to get bigger and bigger. New aquatic reptiles evolved such as ichthyosaurs and nothosaurs. Meanwhile, on land, pine forests flourished, bringing along mosquitoes and fruit flies. The first ancient crocodilians evolved, which sparked competition with the large amphibians that had since rule the freshwater world.^[16]

The Late Triassic spans from 237 million to 200 million years ago. Following the bloom of the Middle Triassic, the Late Triassic featured frequent heat spells, as well as moderate precipitation (10-20 inches per year). The recent warming led to a boom of reptilian evolution on land as the first true dinosaurs evolve, as well as pterosaurs. All this climactic change, however, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which all archosaurs (excluding ancient crocodiles), synapsids, and almost all large amphibians went extinct, as well as 34% of marine life in the fourth mass extinction event of the world. The cause is debatable.^[17][18]

Jurassic

Rhamphorhynchus

The Jurassic ranges from 200 million years to 145 million years ago and features 3 major epochs: The Early Jurassic, the Middle Jurassic, and the Late Jurassic.^[19]

The Early Jurassic spans from 200 million years to 175 million years ago.^[19] The climate was much more humid than the Triassic, and as a result, the world was very tropical. In the oceans, plesiosaurs, ichthyosaurs and ammonites filled the waters as the dominant creatures of the seas. On land, dinosaurs and other reptiles staked their claim as the dominant race of the land, with species such as Dilophosaurus at the top. The first true crocodiles evolved, pushing out the large amphibians to near extinction. All-in-all, reptiles rise to rule the world. Meanwhile, the first true mammals evolve, but never exceed the height of a shrew.^[20]

The Middle Jurassic spans from 175 million to 163 million years ago.^[19] During this epoch, reptiles flourished as huge herds of sauropods, such as Brachiosaurus and Diplodicus, filled the fern prairies of the Middle Jurassic. Many other predators rose as well, such as Allosaurus. Conifer forests made up a large portion of the forests. In the oceans, plesiosaurs were quite common, and ichthyosaurs were flourishing. This epoch was the peak of the reptiles.^[21]

(Inaccurately portrayed) Stegosaurus

The Late Jurassic spans from 163 million to 145 million years ago.^[19] The Late Jurassic featured a massive extinction of sauropods and ichthyosaurs due to the separation of Pangaea into Laurasia and Gondwana in an extinction known as the Jurassic-Cretaceous extinction. Sea levels rose, destroying fern prairies and creating shallows in its wake. Ichthyosaurs went extinct whereas sauropods, as a whole, did not die out in the Jurassic; in fact, some species, like Titanosaurus, lived up to the K-T extinction.^[22] The increase in sea-levels opened up the Atlantic sea way which would continue to get larger over time. The divided world would give opportunity for the diversification of new dinosaurs.

Cretaceous

The Cretaceous is the longest period in the Mesozoic, but has only two periods: the Early Cretaceous, and the Late Cretaceous.^[23]

Tylosaurus (a mosasaur) hunting Xiphactinus

The Early Cretaceous spans from 145 million to 100 million years ago.^[23] The Early Cretaceous saw the expansion of seaways, and as a result, the decline and extinction of sauropods (except in South America). Many coastal shallows were created, and that caused ichthyosaurs to die out. Mosasaurs evolved to replace them as head of the seas. Some island-hopping dinosaurs, like Eustreptospondylus, evolved to cope with the coastal shallows and small islands of ancient Europe. Other dinosaurs rose up to fill the empty space that the Jurassic-Cretaceous extinction left behind, such as Carcharodontosaurus and Spinosaurus. Of the most successful would be the Iguanodon which spread to every continent. Seasons came back into effect and the poles got seasonally colder, but dinosaurs still inhabited this area like the Leaellynasaura which inhabited the polar forests year-round, and many dinosaurs migrated there during summer like Muttaburrasaurus. Since it was too cold for crocodiles, it was the last stronghold for large amphibians, like Koolasuchus. Pterosaurs got larger as species like Tapejara and Ornithocheirus evolved. More importantly, the first true birds evolved which sparked competition between them and the pterosaurs.

The Late Cretaceous spans from 100 million to 65 million years ago.^[23] The Late Cretaceous featured a cooling trend that would continue on in the Cenozoic period. Eventually, tropics were restricted to the equator and areas beyond the tropic lines featured extreme seasonal changes in weather. Dinosaurs still thrived as new species such as Tyrannosaurus, Ankylosaurus, Triceratops and hadrosaurs dominated the food web. Pterosaurs, however, were going into a decline as birds took to the skies. The last pterosaur to die off was Quetzalcoatlus. Marsupials evolved within the large conifer forests as scavengers. In the oceans, Mosasaurs ruled the seas to fill the role of the ichthyosaurs, and huge plesiosaurs, such as Elasmosaurus, evolved. Also, the first flowering plants evolved. At the end of the Cretaceous, the Deccan Traps and other volcanic eruptions were poisoning the atmosphere. As this was continuing, it is thought that a large meteor smashed into earth, creating the Chicxulub Crater in an event known as the K-T Extinction, the fifth and most recent mass extinction event, in which 75% of life on earth went extinct, including all non-avian dinosaurs. Everything over 10 kilograms went extinct. The age of the dinosaurs was officially over.^[24][25]

Cenozoic era

The Cenozoic features the rise of mammals on their conquest to rule the land, as the dinosaurs have now left a huge opening as top dog. There are three division of the Cenozoic: the Paleogene, the Neogene and Quaternary.

Paleogene

The Paleogene spans from the extinction of the dinosaurs, some 65 million years ago, to the dawn of the Neogene twenty three million years ago. It features three epochs: the Paleocene, Eocene and Oligocene.

Basilosaurus (a whale, despite the name)

The Paleocene ranged from 65 million to 55 million years ago. The Paleocene is a transitional point between the devastation that is the K-T extinction, to the rich jungles environment that is the Early Eocene. The Early Paleocene saw the recovery of the earth. The continents began to take their modern shape, but all continents (and India) were separated from each other. Afro-Eurasia is separated by the Tethys Sea, and the Americas are separated by the strait of Panama, as the isthmus has not yet formed. This epoch features a general warming trend, with jungles eventually reaching the poles. The oceans were dominated by sharks as the large reptiles that had once ruled went extinct. Archaic mammals filled the world such as creodonts and early primates that evolved during the Mesozoic, and as a result, there was nothing over 10 kilograms. Mammals are still quite small.^[26]

The Eocene epoch ranged from 55 million years to 33 million years ago. In the Early Eocene, life was small and living in cramped jungles, much like the Paleocene. There was nothing over the weight of 10 kilograms.^[27] Among them were early primates, whales and horses along with many other early forms of mammals. At the top of the food chains were huge birds, such as Gastornis. It is the only time in recorded history that birds ruled the world (excluding their ancestors, the dinosaurs). The temperature was 30 degrees Celsius with little temperature gradient from pole to pole. In the Mid Eocene, the circum-Antarctic current between Australia and Antarctica formed which disrupted ocean currents worldwide and as a result caused a global cooling effect, shrinking the jungles. This allowed mammals to grow to mammoth proportions, such as whales which are, by now, almost fully aquatic. Mammals like Andrewsarchus were now at the top of the food-chain and sharks were replaced by whales such as Basilosaurus as rulers of the seas. The Late Eocene saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass.^[28][29]

The Oligocene epoch spans from 33 million to 23 million years ago. The Oligocene feature the expansion of grass which had led to many new species to evolve, including the first elephants, cats, dogs, marsupials and many other species still prevalent today. Many other species of plants evolved in this period too, such as the evergreen trees. A cooling period was still in effect and seasonal rains were as well. Mammals still continued to grow larger and larger. Paraceratherium, the largest land mammal to ever live evolved during this period, along with many other perissodactyls in an event known as the Grand Coupure.^[30]

Neogene

Animals of the Miocene (Chalicotherium, Hyenadon, entelodont)

The Neogene spans from 23 million to 3 million years ago, and is the shortest geological period in the Phanerozoic. It features two epochs: the Miocene and the Pliocene.^[31]

The Miocene spans from 23 to 5 million years ago and is a period in which grass spreads further across, effectively dominating a large portion of the world, diminishing forests in the process. Kelp forests evolved, leading to new species such as sea otters to evolve. During this time, perissodactyls thrived, and evolved into many different varieties. Alongside them were the apes, which evolved into a staggering 30 species. Overall, arid and mountainous land dominated most of the world, as did grazers. The Tethys Sea finally closed with the creation of the Arabian Peninsula and in its wake left the Black, Red, Mediterranean and Caspian Seas. This only increased aridity. Many new plants evolved, and 95% of modern seed plants evolved in the mid-Miocene.^[32]

The Pliocene ranges from 5 to 2 million years ago. The Pliocene features dramatic climactic changes, which ultimately leads to modern species and plants. The most dramatic are the formation of Panama, and the accumulation of ice at the poles, leading to a massive die-off, India and Asia collide forming the Himalayas, the Rockies and Appalachian mountain ranges were formed, and the Mediterranean Sea dried up for the next several million years. Along with these major geological events, Australopithecus evolves in Africa, beginning the human branch. Also, with the isthmus of Panama, animals migrate across North and South America, wreaking havoc on the local ecology. Climactic changes bring along savannas that are still continuing to spread across the world, Indian monsoons, deserts in East Asia, and the beginnings of the Sahara desert. The earth's continents and seas move into their present shapes, and the world map hasn't changed much since.^[33][34]

Quaternary

The Quaternary ranges from 3 million years ago to the present day, and features modern animals and dramatic climate changes. It consists of two epochs: the Pleistocene and the Holocene.

Mega-fauna of the Pleistocene (mammoths, cave lions, woolly rhino, Megaloceros, American horses)

The Pleistocene epoch lasted from 3 million to 12,000 years ago. This epoch featured the ice ages that resulted from the cooling effect that started in the Mid-Eocene. As the ice progressively migrated towards the equator, the areas north and south of the tropic lines featured intense winters yet mild summers. Meanwhile, Africa experienced terrible droughts, which resulted in the creation of the Sahara, Namib, and Kalahari deserts. To cope, many animals evolved including mammoths, giant ground sloths, dire wolves and most famously Homo sapiens. 100,000 years ago marked the end of one of the worst droughts of Africa, and the expansion of primitive man. As the Pleistocene epoch drew to a close, one of the largest die-outs caused many mega-fauna to die off, including the last hominid species (excluding Homo sapiens). All continents were affected, but Africa less so.^[35]

The Holocene ranges from 12,000 years ago to present day. Also known as "the Age of Man", the Holocene features the rise of humans on their path to sentience. All recorded history and "the history of the world" lie within the boundaries of the Holocene epoch.^[36] Human activity, however, is being blamed for a die-out that has been going on since 10,000 B.C.E. commonly referred to as "the Sixth Extinction" with an estimated extinction rate of 140,000 species per year.^[37]

Biodiversity

During the Phanerozoic, biodiversity shows a steady but not monotonic increase from near zero to several thousands of genera.

It has been demonstrated that changes in biodiversity through the Phanerozoic correlate much better with the hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) or a negative feedback arising from resource limitation, or both. The hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the world population growth arises from a second-order positive feedback between the population size and the rate of technological growth.^[38] The character of biodiversity growth in the Phanerozoic can be similarly accounted for by a feedback between the diversity and community structure complexity. It is suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend with cyclical and stochastic dynamics.^[38]

See also

• Paleoclimatology

Notes

References

• Markov, Alexander V.; Korotayev, Andrey V. (2007). "Phanerozoic marine biodiversity follows a hyperbolic trend". Palaeoworld 16 (4): 311–318. doi:10.1016/j.palwor.2007.01.002. 
• Miller, K. G.; Kominz, M. A.; Browning, J. V.; Wright, J. D.; Mountain, G. S.; Katz, M. E.; Sugarman, P. J.; Cramer, B. S.; Christie-Blick, N; Pekar, S. F.; et al. (2005). "The Phanerozoic record of global sea-level change". Science 310 (5752): 1293–1298. Bibcode:2005Sci...310.1293M. doi:10.1126/science.1116412. PMID 16311326. 

External links

• Media related to Phanerozoic at Wikimedia Commons

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