Australia: The Land Where Time Began
Mass Extinctions Extinction event periodicity
The known mass extinctions occurred at the end of a number of the periods during the history of life on Earth, beginning with the Precambrian.
There was at least 1 mass extinction event during this period that covered the time between 4.6 Ga (billion years ago) to 523 Ma (million years ago), occurring near the end of the period. During this period all organisms were soft-bodied, the first hard-shelled forms didn't evolve until the the Cambrian. The first mass extinction event is thought to have occurred about 650 Ma. From what is known of the fossil record it is believed the stromatolites and acritarchs (microfossils with organic walls that were common in the fossil record up until the Early Palaeozoic) were badly affected by this event. At about 600 Ma there was a period of severe glaciation, conditions so extreme that most of even the microorganisms were wiped out. As after all mass extinctions there was a period of explosive radiation as the surviving organisms rapidly evolved to fill the vacated niches.
The reality of the Vendian period is more uncertain than that with the Precambrian. Following the Precambrian event many soft-bodied forms evolved resembling modern jelly fish, sea pens, segmented worms, etc. Some believe that these organisms were wiped out at the close of the Vendian, though others believe they merely gave rise the forms of the following period, the Cambrian.
Cambrian 523-510 Ma
Epeiric seas (epicontinental seas, seas that covered continental areas during times of marine transgression) were widespread during the Cambrian. It was at this time that the shelled forms evolved. See Cambrian Fauna. The oldest trilobites, the olnellids, and the archaeocyathids dropped out of the fossil record during this period. The other 3 known extinction events occurred around the Cambrian-Ordovician boundary, affecting, among others, the trilobites, brachiopods and conodonts.
There are 2 proposed theories for the cause of the Cambrian mass extinction events, the Glacial Cooling Hypothesis and the Oxygen Depletion Hypothesis.
Glacial Cooling Hypothesis
James F Miller proposed, based on the evidence of Early Ordovician glacial deposits found in South America, that a period of continental glaciation occurred at the Cambrian-Ordovician boundary when the climate of the Earth cooled. He suggested that at this time the cold conditions wiped out much of the Cambrian fauna, mostly the warm water organisms, because they were cold intolerant. As a result of this cooling much of the ocean water was locked up as glaciers, lowering the sea level and causing the retreat of epicontinental seas. The loss of habitat and the increased competition among the displaced fauna led to the extinction of many of them - a mass extinction.
Oxygen Depletion Hypothesis
This hypothesis also involves a period of world-wide glaciation. As cool water from deeper parts of the ocean spread to upper levels and onto the land in the epicontinental seas. In this case it is the resulting oxygen depletion as well as the cooling that killed the organisms that were cold-intolerant and/or those that couldn't tolerate lower oxygen levels. The cooling would also result in increased stratification of the water column in the oceans, many of the organisms would be unable to tolerate the large shifts in temperature and oxygen concentrations. This hypothesis needs more testing of the proposed mechanisms.
Ordovician - 439 million years ago
Lost: 25 (27) % of marine families and 60 (57) % or marine genera
Many groups present in the Cambrian continued into the Ordovician and many new types appeared during the period. Some of the new forms were cephalopods, rugose and tabulate corals, bryozoans, crinoids, graptolites, and bivalves. The increased diversity since the end of the Cambrian led to more complex communities
At the end of the Ordovician the second most severe extinction event of marine extinctions occurred. Many groups of conodonts, graptolites and trilobites also went extinct. 1/3 of all brachiopod and bryozoan families were lost as well as many of the reef-building organisms.
It is widely believed among the scientists in the field that glaciation of Gondwana was the main reason of the extinction event. The finding of glacial deposits in the Sahara Desert combined with the geomagnetic evidence that had been accumulating suggest that Gondwana was at the North Pole during the Ordovician. The resulting continent-wide glaciation led to global cooling. As well as the direct effect of this cooling there was the effect of the lower sea levels because of the water being locked up in the glaciers. This lowering deprived the oceans of much of their most complex and diverse ecosystems on the continental shelves.
Devonian - About 364 million years ago
Lost: 22 % of marine families, 57 % of marine genera, 70 % of all known species. Terrestrial life is not well known at this time.
This extinction event appears to have consisted of a number of pulses that together covered about 20 million years.
As usual after an extinction event the surviving fauna underwent a rapid diversification to fill the vacated niches, in this case continuing throughout the Silurian and Devonian. During the period the sharks, bony fish and ammonoids appeared in the fossil record. Stromatoporoids and corals dominated the oceans, building some of the largest reef complexes. At this time amphibians and insects moved onto the land and the first land plants eventually formed forests.
The mass extinction event occurred towards the end of the Devonian, at the Frasnian - Famennian boundary. As in most other mass extinction events the marine fauns bore the brunt of the extinctions, and on the land the flora were barely affected. The stromatoporoids and rugose and tabulate corals, being reef builders were among the key organisms to be affected as the reefs they built provided a habitat for many other organisms . This extinction event affected the reef-builders so severely that the reef building was not common until the appearance of the of modern corals, scleractinian corals during the Mesozoic. 70 % of marine invertebrates failed to survived until the Carboniferous.
Among the organisms most affected by the extinction event were placoderms, brachiopods, trilobites, conodonts and acritarchs.
It has been noted that most of the groups that went extinct in the Devonian were warm water species. This has led to the suggestion that global cooling was the cause of the extinction event. Glacial deposits found in Brazil seem to point towards this extinction event being similar to the event of the Ordovician, again glaciation of Gondwana was implicated, and lowering of sea levels.
It has also been suggested the the extinction event may have been triggered by a meteorite impact. This theory needs a lot more evidence before it can be widely accepted.
It has been suggested that it was actually caused by forests taking so much CO2 out of the atmosphere that the global temperature dropped to the point where widespread glaciation occurred.
Lost : 22 % of marine families & 52 % of marine genera. The effect on vertebrates is uncertain.
This has been suggested to have been caused by effects on global climate by the huge amounts of lava brought to the surface as the Atlantic Ocean opened up. The Central Atlantic Magmatic Province covers an area of 4.2 million square kilometres. The vast scale of this volcanic activity leads some the suggest it must have had a global-scale impact on the climate. The extinction event appears to have occurred in possibly 2 phases. The first was about 225 million years ago in the Carnian, and this event might possibly have been composed of 2 phases. The later event occurred about 213 million years ago in the last 2 million years of the Triassic, in the Norian. This was a transitional time for the vegetation. The Dicroidium dominated flora, that had dominated the lowlands of Gondwana during most of the Triassic, was replaced by conifers and other gymnosperms. The reason for the transition to the new flora are uncertain. The Jurassic fauna that arose after the Triassic extinction event was very different from the preceding fauna. The Therapsids and amphibians were gone and the dinosaurs were in their first bloom, as they diversified to dominate the land and sea until the end of the Cretaceous.
During the Triassic the ancestral mammals, the therapsids, outnumbered the ancestral dinosaurs, the archosaurs. After the Triassic extinction event the tables were turned, allowing the dinosaurs to dominate for the next 135 million years. Some groups that lost out in this extinction event were conodonts, conularids and the placodonts in the oceans and on land the thecodonts, rhynchosaurs and most labyrinthodonts.
See 10, 11, 12
2 extinction events have been postulated for the Jurassic. The first recognised was found in European strata of the Pleinsbachian age. This event affected shallow water species, including the elimination of 80 % of marine bivalve species.
The second event occurred near the close of the Jurassic, some of the most severely affected organisms were bivalves, marine reptiles and ammonoids, though on the land some dinosaurs such as stegosaurs and most of the sauropods disappeared from the fossil record by the time the beginning of the Cretaceous. This event is so little understood that the causes have yet to be hypothesised.
Lost : 85 % of all species. 16 % of marine families, 47 % of marine genera, 18 % of terrestrial vertebrate families.
After this extinction event the surviving species flourished, but it was some time before the diversity increased as the survivors radiated to fill abandoned niches. It was at this time that the dinosaurs, mammals, pterosaurs, frogs and turtles made their appearance. A bit later, in the Jurassic, the birds appear in the fossil record. The gymnosperms were dominant until the Cretaceous when the angiosperms evolved.
The marine reptiles, rudist bivalves, ammonoids, belmnoids, scleractinian corals, bivalves and brachiopods underwent an extensive radiation. Other groups continued but did not radiate as much, gastropods, bryozoans, crinoids, sponges and sea urchins.
This was the second most severe, and possible the best-known mass extinction, probably because the dinosaurs were included among the causalities. Among the groups lost were, as well as the dinosaurs, the pterosaurs, marine reptiles, ammonoids, belmnoids and rudist bivalves, and many plant species, though the ferns and angiosperms were less affected. Other groups survived, but were greatly depleted, planktonic foraminifera, calcareous nannoplankton, dinoflagellates, diatoms, molluscs, crinoids, brachiopods, fish. Most mammals, birds, turtles, crocodiles, lizards, snakes and amphibians were largely unaffected.
Impact Theory (5)
The best-known proposed mechanism for the mass extinction at the close of the Cretaceous is the impact of an asteroid at Chicxulub on the Yucatan Peninsula of Mexico. There is no doubt that this impact occurred 65 million years ago, and that it had a massive effect on life on Earth. The debate about the cause of the mass extinction centres on whether it caused the extinction or was part of the cause, possibly as the final blow in a sequence of events that occurred around the same time.
A presence of a layer of iridium that is found around the world in marine and terrestrial sediments dating from the K-T boundary leaves little doubt that some world-wide event/events occurred that was either of volcanic or impact origin, or both at precisely ( geologically speaking) the right time to have a huge effect on the life around the world. Iridium is found in the mantle and in extra-terrestrial bodies such as asteroids. In the iridium layer there are also found spherules, small droplets of basalt, that are believed to have come from the impact of a very large body, probably an asteroid. Other evidence is found in the iridium layer in the form of tiny grains of shocked quartz having features indicating the high pressure generated in an impact.The volcanic eruption theory
According to this the extinction event was caused by the massive eruptions in the Deccan Traps that occurred at the close of the Cretaceous, as India passed over a hot spot in the Indian Ocean. The lava produced by these eruptions covered an area of 100,000 square kilometres to a depth of 150 m. Such enormous eruptions would have produced vast amounts of ash that could account for a 'nuclear winter' conditions with or without the assistance of the known impact. Such large eruptions would also have produced the iridium and possibly also the shocked quartz and spherules as a result of explosive eruptions.
Both the Indian eruptions and the impact are known to have occurred. Maybe the eruptions set the scene for the extinctions that were finished off by the impact. It would depend on whether the eruptions occurred before or after the impact. Some believe it was the impact that triggered the eruptions.
Whatever the sequence of events each event would have exacerbated the effects of the other.
In the Early Oligocene yet another global cooling period triggered this extinction event. This cooling led to drastic changes in climate and vegetation that severely affected land animals that depended on the vegetation. Vegetation zones changed and tropical forests were replaced by temperate grasslands and savannahs. As the forests declined the forest-dwelling browsers and their predators declined, and as the grasslands spread hoofed grazers came to dominate. Some of the major mammal groups that disappeared were the mesonychids and creodonts.
Since the beginning of the Miocene 6 extinction events have been identified, the first 9 million years ago and the last 11,000 years ago, The last event was restricted to large land mammals. At this time 39 genera disappeared, including sabre-tooths, woolly mammoths, mastodons, large ground sloths, short-faced bears and dire wolves. The main theories regarding the causes of this latest extinction event are global cooling and the appearance and rise of humans.
It had been noticed that extinction events tended to occur at near regular intervals of about 26 million years. This suggestion has been opposed by others who believe the data it is based on is flawed, claiming that the spacing is not always 26 million years. There are a number of events on the earth that have been proposed as causes of mass extinctions that seem to recur in regular cycles. Although not always precise, there does appear to be a tendency for mass extinctions to occur about 26 million years apart.
Meteorite impacts also show some degree of cyclicity, occurring at intervals of about 26-30 million years. There is also some evidence that increased levels of volcanism, and the increased movements of the tectonic plates, 2 processes that are connected when the volcanoes involved are situated on plate margins, also appear to show some degree of cyclical activity. If changes in climate is the main factor in mass extinctions they could be caused or affected to some degree by the previously mentioned cyclical phenomena. It has been suggested that these processes could be controlled to some extent by the influence of the solar system and the even the rotation of the galaxy.
There are 2 main groups of hypotheses of periodic mass extinctions, catastrophism and gradualism. Hypotheses based on catastrophism are mainly based on a regular periodic timing of extinction events. The only known mechanism for a more or less precise timing of events is extraterrestrial, the forces acting on the Earth through its interaction with the solar system.
Gradualism-based hypotheses invoke terrestrial mechanisms such as volcanism, glaciation, global climate changes, sea level changes. Recently hypotheses based on the science of complexity have been proposed. According to these hypotheses species-species interactions sometimes lead to instability that ripple through entire ecosystems. Rats arriving on an island that exterminate a key species could lead to devastation of that entire ecosystem.
There are a number of major extinction patterns recognised.
There are 2 main types of event that palaeontologists associate with the known mass extinction events - catastrophic events such as meteorites and comet impacts, and geological and atmospheric processes such as large-scale volcanism, sea level changes, changing levels of oxygen and carbon dioxide in the atmosphere, and salinity of the oceans. Other processes that have been suggested recently are the acidification of the oceans and the large-scale release of methane from melting permafrost and the vast areas of methane ice on the seafloor at a number of places around the world.
The latter 2 being a worrying possibility in the present bout of global warming. 2 Scientific expeditions have recently observed areas of the Arctic Ocean where methane is being released from the marine permafrost so rapidly that it is bubbling to the surface, not just raising dissolved methane levels that were being measured by the scientists on the research vessels.
Recent studies have been finding that hypoxic zones around the world have been expanding horizontally and vertically. In 2006 the regular late summer peak in hypoxic areas off the coast of the northwest US found that hypoxic water had moved over part of the continental shelf and had actually become anoxic, killing sea stars, sea cucumbers, marine worms as well as fish.
Climate models have been predicting reduced oxygen levels in the oceans as water temperature increased, and combined with other known problems such as acidification of the oceans it is beginning to look suspiciously like a rerun of the events that occurred in the oceans during the Permian mass extinction.
Not all agree with the times allocated to mass extinctions. Prothero (1994) suggested that at least some of the apparent mass extinctions are the result of extinctions that occurred at various times throughout a period being totalled as though they all occurred at the end of the Period. Prothero also suggests that the periodicity of extinctions is more apparent than real because of the this lumping together of extinctions at the end of a period.
Permian-Triassic Crisis - 2 Pulses of Extinction
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|