Poster Presentation

I created a poster summarising the project and the main findings for a five minute poster presentation. This was created halfway through my blog and therefore does not contain all of the information from the latter posts. If you click on the image then you can zoom to view all of the literature.

Thankyou very much for following this blog.

Conclusions, and Looking to the Future.

Sixth Mass Extinction?

This blog has investigated the claim that the Earth is undergoing a 'sixth mass extinction', and has attempted to determine whether this is a natural fluctuation or a human induced tragedy. This has been done by comparing the present rates of extinction with those from the five previous 'mass extinction events' and discussing what the future could hold, as well as considering the perspective of skeptics. I have also tried to include a number of interesting articles to highlight the importance of this topic, as well as the wide-ranging effects that this loss of biodiversity is having. Throughout this blog I have come to the personal conclusion that despite the inaccuracies known in absolute numbers, the Earth is experiencing an extremely elevated level of species loss, primarily due to the impact of humans. Whether this can be described as the 'sixth mass extinction event' is a matter of opinion. All previous events have resulted in more than 75% of species being lost from Earth (Jablonski 1994). The timescale of species loss is currently not long enough for this to have occurred, however predictions believe that we could exceed this figure by 2100 if we continue as we have, hopefully we can prevent this from occurring.

Evidence

Actual species rates are very difficult to quantify with different scientists expressing different opinions. Most conservative estimates place the current rate at 1000 times the background rate, whereas some place it as high as 10,000 times greater (Call of Life 2009). Wilson estimates that 27,000 species are lost per year, whereas, Ehrlich believes that this may be as high as 130,000 species per year. It must be noted that these estimates rely on a number of assumptions decreasing their accuracy.

Clear links have been identified between human dispersal and the extinction of a number of species, including examples such as the loss of 135 mammal species (representing 70% of North Americas large mammals), when humans migrated from Asia to North America 12,500-10.000 ya, and the loss of 17 of 50 species of lemur in Madagascar after the arrival of humans 2000 ya (CBD 2010). This correlation undoubtedly shows that humans have had a drastic influence in the global loss of species. Human activities that are believed to impact extinctions include: over-hunting, introduction of infectious diseases, increased inter specific competition, habitat destruction, and the introduction of exotic species.

I looked at the previous five 'mass extinction' events to try and determine any similarities between these and the current postulated event. All of these five events had natural causes, unlike the 'sixth mass extinction' and also occurred over a much longer timescale (in the order of millions of years), highlighting how severe the problem is; that the high current extinction rates could be severe enough to carry extinction magnitudes to the 'big five' benchmark in as little as 300 years (Barnosky et al. 2011).

The future appears bleak for global biodiversity. The IUCN considers 11,046 species of plants and animals 'threatened' considering them vulnerable, endangered or critically endangered. There is a significant chance that these species may go extinct in the near future, almost all resulting from human activities. To put this into perspective this accounts for 24% of mammals and 12% of birds. (IUCN 2000). The figures are likely to be much higher due to the large amount of unidentified species, and every update of the IUCN Red List has resulted in an increase in the number of species considered 'threatened'.

Prominent scientist Georgina Mace believes that if nothing changes 14-22% of all species and sub-species could be lost over the next 100 years (www.whole-systems.org/extinctions.html). However, nature film Call of Life (directed by Monte Thompson, 2010) believes that this will be much higher, and if current trends continue within a few decades at least 50% of plant and animals species will disappear forever.

Prominent advocates of the mass extinction include Niles Eldredge, Edward Wilson, and Richard Leakey. The two quotes that really drew my attention to this cause were those of Eldredge and Wilson. 

• Niles Eldredge (1999): "It is well established that the Earth is undergoing yet another mass extinction event, and is clear that the major agent for this current event is homo sapiens".
• Edward Wilson (2002): "At current rates of human destruction of the biosphere, half of all species will be extinct in 100 years"

These two quotes, in a few simple words identify the significance and extent of this problem as well as identifying what they believe is the cause. However, as discussed above this topic is far from simple, and what is even more complex is how we should go about attempting to prevent this disaster.

What should be done?

The natural world could be devastated beyond recognition, with the loss of human life in the billions if we do not act not to prevent further biodiversity loss. However, scientists believe that we still have time to avert the worst of the crisis and save much of the biosphere, but only if we act now. 

Firstly we need to create broad public awareness of this issue, including magnitude and implications. Only then can the whole of society begin to recognise the systematic changes that will be required. The solution is clear, in order to save Earth and secure a future for the human race we need a new worldview. We must accept that technology alone cannot solve this issue, instead we need to create fundamental change, in culture, our minds, and most importantly our hearts. 

References


Barnosky, A. D., et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature. 471: 51-57.
Call of Life: Facing the mass extinction. Monte Thompson (2009)
Centre for Biodiversity and Conservation Conference. Humans and other catastrophes: perspectives on extinction. New York. American Museum of Natural History. Available at:http://www.amnh.org/science/biodiversity/extinction/IntroSymposiumFS.html
Eldedge, N. (1999). Cretaceous meteor showers, the human ecological niche and the sixth extinction. Centre for Biodiversity and Conservation Conference. Humans and other catastrophes: perspectives on extinction. New York. American Museum of Natural History. 
IUCN. (2000). Compiled by C. Hilton-Taylor. 2000 IUCN Red List of threatened species. Thanet Press Ltd.: Margate.
IUCN. (2009). The IUCN Red List of threatened species 2009 update.
Jablonski, D., (1994). Extinctions in the fossil record. Philosophical Transactions of the Royal Society London. B. 344: 11-17.
Wilson, E. O. (2002). The future of life. Vintage.
www.whole-systems.org/extinctions.html

Sixth Mass Extinction Skeptics

As with any controversial topic there are of course skeptics. This skepticism to the idea of the sixth mass extinction have developed over the last 25 years, accusing 'doom sayers' of over-stating their case, or even worse fabricating it.

Two examples of this skepticism are the 1991 article in Science "Extinction: Are ecologists crying wolf?" and the 13/12/1993 issue of US News and World report which ran a cover story titled "The doomsday myths".

Advocates of this skepticism essentially suggest that although ecologists believe that many species are becoming extinct, or are about to become so, they do not actually know for sure.

Julian Simon from the University of Maryland is one of the most prominent anti-alarmists said in 1986 "The available facts...are not consistent with the level of concern" (Simon 1986) and in 1993 in the New York Times described claims by various ecologists that current extinction rates were equivalent to those of a mass extinction as "utterly without scientific under pinning" and "pure guesswork" (Simon 1993).

To understand this skepticism more I will focus on the 1991 Charles Mann article "Extinction: Are ecologists crying wolf?". The article opens with a quote from biologists Paul Ehrlich and E. O. Wilson who warned that biodiversity is in such danger that the US must "cease developing any more relatively undisturbed land" as a "first step to a solution". Charles Mann describes these two prominent scientists as representatives of an "exaggerated and distorted biodogma that runs the risk of impeding solutions to tropical forest deforestation" which Mann describes as a factor that all sides agree is a severe problem.

The paper states that no credible effort has been made to pin down the scientific assumptions behind the mega-extinction scenario, and that by mis-stating the problem both the credibility of science and the effort to preserve biodiversity are in danger. The paper refers to Julian Simon in the New Scientist in 1986 stating that the widely touted estimates of future extinction rates have no empirical basis whatsoever. That some scientists such as Myers envision the disappearance of 25% of the worlds species by 2100, whereas Ehrlich and Wilson conservatively figure the loss at 2-3% by 2100. Mann argues that this discrepancy calls into question the credibility of all such estimates. Wilson agrees that more data is needed, but that the imminence of the extinction problem, particularly in tropical forests is absolutely undeniable.

This skepticism has gained a number of supporters due to the fact that it is startling to imagine that humans may have caused such a large-scale tragedy. The disparity of predictions from different authorities of the scale of the imminent extinction (from 17,000 species per year to 100,000 per year) has also reduced legitimacy.

I personally feel that we need to protect as many species as we can, by acting now, and this skepticism does little to help the cause.

References

Budiansky, S. (1993). The doomsday myths. Us News and World report. 13/12/1993.
Mann, C. C. (1991). Extinction: Are ecologists crying wolf? Science. 253: 736-8.
Simon, J. (1986). Tree Data. New Scientist. 19/06/1986.
Simon, J. (1993). Before skies become entirely barren of birds; what data? New York Times. 25/05/1993.

Cretaceous Extinction Event

Introduction

The Cretaceous extinction event occurred ~65 million years ago and was a large-scale mass extinction of animal and plant species with 46% of genera and 70% of species lost (Barnosky et al. 2011). The event marks the end of the Mesozoic. Widely known as the K-T extinction, it is associated with a geological signature usually present as a thin band of sedimentation found in various parts of the world, known as the K-T boundary (Fortey 1999).

Scientists theorise that the extinctions of the end Cretaceous period were caused by one or more catastrophic events, such as massive asteroid impacts or increased volcanic activity. A number of impact craters, and evidence of massive volcanic activity (such as the Deccan traps) have been dated to the approximate time of the Cretaceous extinction event. These geological events would have likely reduced sunlight, hindering photosynthesis. resulting in a massive disruption in the Earth's ecology. Other more gradual cases have also been put forward including changes in sea level or climate (Macleod et al. 1997).

After the Cretaceous extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.


What was lost?

There was significant variability in the rate of extinction between and within different clades. Those species that were dependent on photosynthesis were more severely affected as atmospheric particles blocked sunlight reducing the amount of solar energy that reached the Earth's surface. This significant plant extinction led to a major rearrangement of the dominant plant groups, severely affected herbivorous animals. Consequently top predators such as the Tyrannosaurus rex also perished.

(Image showing how a Tyrannosaurus rex may have appeared. From www.planet-inspire.blogspot.com)

• Microbiota - The Cretaceous extinction event was one of the most dramatic turnovers in the fossil record for various calcareous nanoplankton that formed the calcium deposits which gave the Cretaceous its name. Statistical analysis of marine losses during this extinction event suggest that the decrease in diversity was caused more by a rapid increase in extinctions as opposed to a decrease in speciation. ~ 55% of diatom species were lost during the extinction along with numerous species of planktonic and benthic foraminifera, suggesting a significant turnover but not a catastrophic extinction of these organisms (Macleod et al. 1997). 
• Marine invertebrates - ~98% of colonial coral species (those that inhibit warm, shallow tropical waters) became extinct during the Cretaceous extinction event. However, solitary corals which generally do not form reefs and tend to inhabit cooler and deeper areas of the ocean were not as significantly impacted. ~35% of echinoderm genera became extinct during this period as well as all but two species of the molluscan class Cephalopoda (those that did survive diverged into modern octopodes, squids, and cuttlefish) (Ward et al. 1991). Other invertebrates also became extinct during this period including rudists (reef-building clams) and inoceramids (relatives of modern scallops). 
• Fish - ~80% of shark, ray, and skate families survived the extinction, along with more than 90% of bony fish families. The marine and freshwater environments that these fish inhabited are believed to have mitigated the environmental effects of this extinction event. 
• Terrestrial plants - There are large amounts of evidence that the Cretaceous extinction severely disrupted plant communities. However, there were important regional differences, with 57% of North American plant species going extinction, yet New Zealand and Antarctica experienced no significant turnover of species. 
• Amphibians - Strong evidence that the majority of amphibians survived the extinction event relatively unscathed (Macleod et al. 1997). Archibald and Bryant (1990) conducted an indepth study of salamander genera in fossil beds in Montana, and found that six of seven genera were unchanged after the Cretaceous extinction event. 
• Non-archosaur reptiles - More than 80% of Cretaceous turtle species survived the extinction event, and all of those species which survived are represented in current species (Novacek 1999). The order Squamata (which is today represented by lizards and snakes were successful throughout the Cretaceous and survived the extinction event and are currently the most successful and diverse group of living reptiles. Their small size, adaptable metabolism, and ability to move to more favourable habitats were key factors in their survival. The mosasaurs and plesiosaurs (giant aquatic reptiles) were not so successful and became extinct by the end of the Cretaceous. 
• Crocodyliforms - Five of the ten families of crocodilians died out during the late-Cretaceous period, with the only apparent trend being that no large crocodile survived (Macleod et al. 1997). 
• Avian dinosaurs - All non-neornithean birds became extinct during the Cretaceous period including a number of groups which had previously flourished. Neornthine birds survived the extinction event due to their abilities to dive, swim,and seek shelter. 
• Non-avian dinosaurs - The majority of scientists agree that all non-avian dinosaurs became extinct during the Cretaceous extinction event. There is no evidence that these creatures could burrow, swim, or dive, and were therefore unable to shelter themselves from the environmental stress that occurred during this period. It is possible that small dinosaurs did survive, however, they would have been deprived of food as both herbivorous and carnivorous dinosaurs would have quickly found that their food was in short supply. Whether the extinction occurred gradually or very suddenly is a contested topic, as both views have support in the fossil record. 
• Mammals - All major cretaceous mammalian lineages survived the Cretaceous extinction event, although they suffered significant losses. In particular the marsupials, which largely disappeared from North America. The mammalian species of this period were generally small, comparable in size to rats. This small size would have helped them to find shelter in protected environments. 

Postulated Causes

There have been several theories on the cause of the K-T boundary which led to the mass extinction, these have typically centred on either impact events, increased volcanism, or a combination of both.

• Impact event - In 1980 Luis Alvarez, Frank Asaro, and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous-Teritary boundary contained a concentration of iridium 30-130 times background levels (Alvarez et al. 1980). Iridium is very rare in the Earth's crust as it is a siderophile element, and the majority of it travelled with the iron and sank into the Earth's core during planetary differentiation. Iridium remains abundant in most asteroids and comets, and the Alvarez team suggested that an asteroid hit the Earth at the time of the K-T boundary (Alvarez et al. 1980). Subsequent research identified the Chicxulub Crater buried under Chicxulub on the coast of Yucatan, Mexico as the impact crater which matched the Alvarez hypothesis size (180 km in diameter) and dating (Pope et al. 1996). The asteroid landed in the ocean and would have caused megatsunamis, evidence of which has been found in several locations in the Caribbean and Eastern USA. The asteroid landed in a bed of gypsum which would have produced vast amounts of Sulfur Dioxide, further reducing the sunlight reaching the planet, and then precipitated as acid rain. There is an ongoing dispute as to whether the impact was the sole cause of extinctions. However, in March 2010 an international panel of scientists endorsed the asteroid hypothesis as being the cause of the extinction, ruling out other theories such as volcanism. They determined that a 10-15 km space rock hurtled to Earth and hit at Chicxulub in the Yucatan peninsula.
• Deccan Traps - Recent evidence shows that the Deccan traps erupted over a period of 800,000 years, spanning the K-T boundary, and may potentially be responsible for the extinction and delayed biotic recovery experienced during the late-Cretaceous (Macleod et al. 1997). It is likely that large volumes of dust and sulfuric aerosols would have been released into the air, which may have blocked sunlight and reduced photosynthesis in plants. Carbon Dioxide may have also been emitted, increasing the greenhouse effect when the dust and aerosols had cleared from the atmosphere. 
• Multiple impact event - A number of other craters appear to have been formed around the time of the K-T boundary, suggesting the possibility of multiple near simultaneous impacts (perhaps from a fragmented asteroidal object). These include the 24 km wide Boltysh crater in Ukraine (65.17 mya +/- 0.64 ma) and the controversial much large 600 km wide Shiva crater beneath the Indian Ocean (~65 mya) (Macleod et al. 1997). 
• Maastrichtian sea-level regression - There is numerous evidence that sea levels fell in the late-Cretaceous by more than any other time in the Mesozoic. This would have greatly reduced the continental shelf area, which is the most species-rich part of the sea, which could have caused a mass extinction. Climate change would have also resulted partly by disrupting winds and ocean currents, and also by reducing the albedo of the Earth resulting in increased temperatures.

As well as these individual causes of the mass extinction a number of scientists feel that it was a combination of multiple causes that led to this, arguably the most famous mass extinction, resulting in the demise of the dinosaurs. 

References

Alvarez, L. W., W. Alvarez and H. V. Michel (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108.
Archibald, J. D. and L. J. Bryant. (1990). Differential Cretaceous-Tertiary extinction of nonmarine vertebrates; evidence from Northeastern Montana. In Sharpton, V. L. and P. D. Ward (Eds.): Global catastrophes in Earth history: an interdisciplinary conference on impacts, volcanism, and mass mortality. Geological Society of America. Special Paper. 247: 549-562.
Barnosky, A. D. et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature 471: 51-57.
Fortey, R. (1999). Life: A natural history of the first four billion years of life on Earth. Vintage.
Macleod, N. et al. (1997). The cretaceous-Tertiary biotic transition. Journal of the Geological Society. 154: 265-292.
Novacek, M. J. (1999). 100 million years of land vertebrate evolution: the Creaceous-Early Tertiary transition. Annals of the Missouri botanical garden. 86: 230-258.
Pope, K. O. et al. (1996). Surface expression of the Chicxulub crater. The Geological Society of America. 24: 527-530.
Ward, P. D. et al. (1991). Ammonite and inoceramid bivalve extinction patterns in Cretaceous/Tertiary boundary sections of the Biscay region. Geology. 19: 1181-1184.
www.planet-inspire.blogspot.com

Triassic Extinction Event

Introduction

The Triassic extinction event occurred ~200 million years ago and formed the boundary between the Triassic and the Jurassic periods. During this event ~47% of genera were lost and ~80% of species (Barnosky et al. 2011). The number of events within this extinction event is disputed, with some studies suggesting that there were at least two periods towards the end of the Triassic period, ~12-17 million years apart. However, recent faunal analysis in the Petrified Forest of northeast Arizona suggests no significant change in the environment during this time (Hunt et al. 2002).


What was lost?

The Triassic extinction event was particularly severe in the oceans, with the conodonts disappearing, along with the majority of marine reptiles. Invertebrates such as brachiopods, molluscs, and gastropods were also severely affected (Raup and Sepkoski 1982).

(Triassic gastropod fossils. From: www.nationalgeographic.com)

The Triassic extinction event was not as equally devastating in terrestrial ecosystems, however a number of important clades of crurotarsans (large archosaurian reptiles) disappeared, as well as the majority of large labyrinthodont amphibians, groups of small reptiles, and a number of synapsids. Some primitive dinosaurs also went extinct, where other more adaptive dinosaurs survived to evolve into the Jurassic (Jacobs 1997).

Some of the surviving plants from the Triassic that went on to dominate the Mesozoic world include modern conifers and cycadeoids (McElwain and Punyasena 2007).

Postulated causes

The causes of the Triassic extinction event are not known with any certainty. The period was accompanied by huge volcanic eruptions that occurred as the supercontinent of Pangea began to break apart ~202-191 million years ago (Marzoli et al. 1999). This led to the formation of the Central Atlantic Magmatic Province (CAMP), which was believed to be one of the largest inland volcanic events since the planet stabilized (Barnosky et al. 2011).

Other possible causes for the extinction event includes global cooling or a bolide impact, however there is little evidence for these causes.

Conclusion

The Triassic extinction event left many empty niches, allowing the dinosaurs to expand and fill them. Over the next 150 million years dinosaurs became increasingly abundant, dominant, and diverse throughout the Jurassic and the Cretaceous, until the final event of the "big five" occurred, which will be discussed in the next entry.

References

Barnosky, A. D., et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature. 471: 51-57.
Hunt, A. P., et al. (2002). No significant nonmarine Carnian-Norian (late Triassic) extinction event: Evidence from the Petrified Forest National Park. Denver Annual Meeting October 27-30. Paper no. 235-6.
Jacobs, L. L., (1997). African Dinosaurs. In Encyclopedia of Dinosaurs. Currie, P. J., K. Padian (Eds.). Academic Press. pp. 2-4.
Marzoli, A., et al. (1999). Extensive 200 million year old continental flood basalts of the Central Atlantic Magmatic Province. Science. 284: 618-620.
McElwain, J. C., and S. W. Punasna, (2007). Mass extinction events and the plant fossil record. Trends in ecology and evolution. 22: 548-557.
Raup, D. M. and J. J. Sepkoski, (1982). Mass extinctions in the marine fossil record. Science. 215. 1501-1503.
www.nationalgeographic.com

Permian Extinction Event

Introduction

The Permian extinction event occurred ~250 million years ago forming the boundary between the geologic periods of the Permian and the Triassic (Barry 2002). This extinction event was the most severe of all five mass extinction events, with up to 96% of all marine species and 70% of terrestrial vertebrate becoming extinct, and is the only known mass extinction of insects (Benton 2005). As such a gargantuan amount of biodiversity was lost, recovery of life after the Permian extinction event took much longer than any other event (Benton 2005).

Many scientists believe that there were upto three distinct pulses of extinction, with several proposed mechanisms for the extinction. It is believed that the earlier phase(s) were due to gradual environmental change, while the latter phase(s) was likely due to a catastrophic event.

What was lost?

Marine organisms experienced the greatest losses during this extinction event. Among the losses were:

• 100% of Blastoids
• 100% of Trilobites (which were in decline since the Devonian, with only 2 genera living before the extinction).
• 100% of Eurypterids
• 99% of Radiolaria
• 98% of Gastropods
• 97% of Foraminifera 
• 59% of Ostracods

(Diagram of a Permian blastoid. From www.accessscience.com)

Statistical analyses of the marine losses during the Permian suggest that the decrease in diversity was due to a sharp increase in extinctions as opposed to a decrease in speciation (Bambach et al. 2004). Those organisms with calcium carbonate skeletons were primarily affected, especially those reliant on ambient Carbon Dioxide levels to produce their skeletons (Benton 2005). The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification (Payne et al. 2004). 

The Permian had a great biodiversity of invertebrate species, including the largest insects to have ever existed (Barry 2002). During the Permian extinction event up to nine insect orders became extinct, and up to ten more greatly reduced in diversity. This is the only known extinction of insects.

A massive rearrangement of plant ecosystems occurred during the Permian extinction event, with many land plants entering an abrupt decline. Large gymnosperm woodlands were replaced by an increase in herbaceous plants such as lycopodiophyta (Looy et al. 2001). Gymnosperms subsequently recovered 4-5 million years later.

Of the terrestrial vertebrates even the groups that survived suffered extremely heavy losses of species, with some of the surviving groups not persisting long past this period. It is known that over 70% of terrestrial labyrinthodont amphibians, sauropsid (reptile) and therapsid (mammal-like reptile) families became extinct (Benton 2005). Large herbivores suffered the greatest losses, with all Permian anapsid reptiles dying out.

(Possibly what a Millerosaurus - an extinct genus of anapsid from the Permian period may have looked like. http://guidetoreptiles.blogspot.com)

Recovery

One disaster taxa such as the hardy Lystrosaurus recovered quickly after the Permian extinction event. Specialised animals that formed complex ecosystems with high biodiversity, complex food webs and a variety of niches took much longer to recover. This is likely due to the successive waves of extinction inhibiting recovery and prolonging the application of environmental stress to organisms, continuing into the early Triassic (Lehrmann et al. 2006). Most scientists believe that recovery did not begin for ~5 million years, and took ~30 million years to complete (Benton 2005).

Prior to the extinction ~67% of marine organisms were sessile and attached to the seafloor, but during the Mesozoid only ~50% were sessile, and the rest free-living. Also prior to the extinction ~50% of marine ecosystems were simple, but post-recovery complex communities outnumbered these simple communities by three to one (Wagner et al. 2006). Bivalves, and motile species also became much more prevalent.

Land vertebrates took an unusually long time to recover from the Permian extinction event, and is believed to have not been complete until the late Triassic. Lystrosaurus, a pig-sized herbivore constituted as much as 90% of the earliest Triassic land vertebrate fauna, smaller carnivorous therapsids also survived. By the late-Triassic dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians, and ammaliforms were also abundant and diverse (Benton 2005).

(What a lystrosaurus was believed to look like. From www.planetdinosaur.com)

Postulated Causes

There are a number of proposed mechanisms for the extinction event, including catastrophic and gradualistic processes. Any hypothesis about the cause must explain: the selectivity of the event, which primarily affected organisms with calcium carbonate skeletons; the large period before recovery started; and the minimal extent of biological mineralization once the recovery began (Knoll 2004).

• Impact event - If an impact event was a major cause of the Permian extinction, it is likely that the crater would no longer exist. 70% of the Earth's surface is ocean, and the Earth has no ocean floor crust older than 200 million years due to sea-floor spreading and subduction. It has also been speculated that craters created by very large impacts on land may be masked by extensive lava flooding from below after the crust is punctured or weakened.
• Volcanism - The final stages of the Permian saw two flood basalt events, the Emeishan and the Siberian Traps (Wignall et al. 2009). These eruptions may have caused dust clouds and acid aerosols, which would have blocked out sunlight and thus disrupted photosynthesis both on land and in the photic zone of the ocean, leading to a collapse of food chains. This may have killed land plants, and organisms with calcium carbonate shells as was experienced in the Permian extinction. Carbon Dioxide would have also been released, leading to global warming. 
• Anoxia - There is evidence that the oceans became deficient in Oxygen towards the end of the Permian, with a notable and rapid onset of anoxic deposition near East Greenland (Wignall and Twitchett 2002). Uranium/Thorium ratios also confirm this. This would have resulted in widespread extinctions except for anaerobic bacteria living in the sea-bottom mud. This anoxia may have been due to global warming slowing down or stopping the thermohaline circulation, reducing the mixing of oxygen in the ocean. 
• Pangaea - Half-way through the Permian all the continents joined to form the supercontinent of Pangaea. This configurations severely decreased the extent of shallow aquatic environments, the most productive part of the seas, exposing formerly isolated organisms of the rich continental shelves to competition from invaders. Ocean circulation and atmospheric weather patterns were also both affected, creating seasonal monsoons near the coasts and an arid climate in the vast continental interior. The formation of Pangaea did not affect terrestrial organisms.

Conclusion

It is likely that a combination of causes, most likely a sequence of catastrophes, each one worst than the previous, led to the Permian extinction event. This led to the greatest loss of biodiversity the Earth has seen during the Phanerozoic, which took millions of years to recover.

References

Barry, P. L. (2002). The great dying. Science and Technology Directorate, NASA.
Benton, M. J. (2005). When life nearly died: The greatest mass extinction of all time. Thames and Hudson.
http://guidetoreptiles.blogspot.com
Knoll, A. H. (2004). Biomineralization and evolutionary history. In P. M. Dove, J. J. DeYoreo and S. Weiner (Eds.) Reviews in mineralogy and geochemistry.
Lehrmann, D. J. et al. (2006). Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from South China. Geology. 12: 1053-1056.
Looy, C. V., et al. (1999). The delayed resurgence of equatorial forests after the Permian-Triassic ecologic crisis. Proceedings of the national academy of sciences. 96: 13857-13862.
Payne, J. L., et al. (2004). Large perturbations of the carbon cycle during recovery from the end-Permian extinction. Science. 305:506-9.
Wagner P. J., et al. (2006). Abundance distributions imply elevated complexity of post-palaeozoic marine ecosystems. Science. 314: 1289-1292.
Wignall, P. B. and R. J. Twitchett, (2002). Permian-Triassic sedimentology of Jameson Land, East Greenland: incised submarine channels in an anoxic basin. Journal of the geological society. 159: 691-703.
Wignall, P. B. et al. (2009). Volcanism, mass extinction, and carbon isotope fluctuations in the middle Permian of China. Science. 324: 1179-1182.
www.accessscience.com
www.planetdinosaur.com

Devonian Extinction Event

Introduction

The Devonian extinction event was the second of two major extinctions that affected the evolutionary fauna of the Palaeozoic (Brenchley 2001). This extinction event occurred in two parts: Firstly the Kellwasser event which occurred at the beginning of the Devonian period ~374 million years ago (Racki 2005). The second part was the Hangenberg Event, which occurred at the end of the Devonian period ~359 million years ago (Caplan and Bustin 1999). During this mass extinction event 35% of all genera and 75% of all species became extinct, the lowest figures of all five mass extinction events (Barnosky et al. 2011).

It is evident that there was a significant loss of biodiversity during the Devonian period, however the extent of time during which these events took place is less certain, with estimates ranging from 500,000 to 25 million years (Stigall 2011). It is also not clear whether two periods of mass extinction occurred during the event, or a series of smaller ones. Some scientists believe that the Devonian extinction event may consist of as many as seven distinct events, over the period of ~25 million years.

The late-Devonian world was very different from that of today. The continents were arranged very differently with a supercontinent Gondwana covering the majority of the Southern continent. The continent of Siberia occupied the Northern hemisphere, while Laurussian existed on the equator and was drifting towards Gondwana. By the end of the Devonian the continents of Euramerica and Gondwana were beginning to converge to form Pangaea.

The biota of the Devonian was very different from that of the Ordovician. The plants which had been on land in forms similar to mosses, lichens and liverworts since the Ordovician had evolved to develop roots, seeds, and water transport systems, allowing them to live in areas that were not constantly wet, consequently forming wide-ranging forests on the highlands. The oceans had also undergone significant changes, and were now home to massive coral reefs, and the first tetrapods were beginning to evolve leg-like structures.

(How the seas during the Devonian period may have appeared. From www.geology.wisc.edu.)

What was lost?

The Devonian mass extinction event primarily affected the marine community, most significantly affecting shallow warm-water organisms. The most important group affected by the Kellwasser event were the reef builders, including stromatoporoids and the rugose and tabulate corals. The collapse of the reef system was so severe that major reef-building did not recover until the Mesozoic era. Further taxa that were severely affected during the extinction event include the brachiopods, ammonites, acritarchs, trilobites, and conodonts. As with most extinction events specialist taxa occupying small niches were greater affected than those with wider tolerances (McGhee 1996). 

The Hangberg event impacted both marine and freshwater communities, impacting ammonites and trilobites, as well as jawed vertebrates including our tetrapod ancestors (Sallan and Coates 2010). The Hangenberg is linked to the extinction of 44% of high-level vertebrate clades and the complete turnover of the vertebrate biota (Sallan and Coates 2010). 

Postulated causes

The sedimentological record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction. During the middle and late Devonian there is evidence of widespread anoxia in oceanic bottom waters, the rate of Carbon burial rapidly increased, and benthic organisms were decimated especially in the tropics and reef communities (Algeo 1998). There is also strong evidence for high-frequency sea-level changes throughout the kellwasser event, the Hangenberg event has also been associated with sea-level rise followed rapidly by glaciation-related sea-level fall (Brezinksi et al. 2009).However, the cause of these changes is open to debate.

Possible triggers include:
1/ Bolide impact. However, there is no secure evidence of a specific impact during this event. Craters which are believed to be of this age often cannot be dated with sufficient precision to link them to the extinction event, and those which have been dated precisely have been found to be not contemporaneous with the extinction (Racki 2005).
2/ Plant evolution. During the Devonian land plants underwent an extremely significant phase of evolution, increasing their maximum height from 30 cm to 30 m. This increase in height was made possible due to the evolution of advance vascular systems allowing the growth of complex branching and rooting systems. Seeds also developed during this time allowing dispersal in areas previously inhospitable to plants such as upland and inland areas.
This affected weathering due to the plant root systems breaking up the upper layers of bedrock and stabilising a deep layer of soil. Soil promotes the chemical breakdown of rocks, releasing ions acting as nutrients to plants and algae. If these nutrients were input into a river eutrophication and subsequent anoxia may occur, which may have caused an extinction.
Increased weathering of silicate rocks would have likely drawn down Carbon Dioxide from the atmosphere, decreasing levels from ~15 times present levels to ~3 times. This reduction in concentrations would have likely led to global cooling.
The increase of plants on the continents during the Devonian would have also had a significant effect on Carbon Dioxide levels. An increase in photosynthesizing land plants is likely to have reduced Carbon Dioxide levels and produced a cooler climate. There is evidence of glacial deposits in northern Brazil (located in the South Pole during the Devonian) suggesting widespread glaciation at this time. This switch from a warm climate to a much cooler one may have led to extinctions of many species.
If these two reductions in Carbon Dioxide had acted alongside each other it is likely that significant environmental change would be experienced, with the Earth being pulled out of the 'greenhouse' state and into the 'icehouse' state that continued through the Carboniferous and Permian.


References

Major changes in the biota of the Devonian appear to have been caused by rapid environmental changes, yet the causes of these changes is still debated. Environmental changes may have occurred due to an extra-terrestrial impact, or due to the significant vegetation changes that occurred during the Devonian. This extinction event had the smallest magnitude of all five events, however ~75% of all species were still lost. These gaps in biota were subsequently filled by other species adapted to the new 'icehouse' environment of the Carboniferous.

References

Algeo, T. J. (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: 113-130.
Barnosky, A. D., et al. (2011). Has the Earth's sixth mass extinction already arrived? Nature. 471: 51-57.
Brenchley, P. J. (2001). Extinction: Late Ordovician mass extinction. Encyclopedia of Life Sciences.
Brezinski, D. K. et al. (2009). Evidence for long-term climate change in upper Devonian strata of the central Appalachians. Palaeogeography, Palaeoclimatology, Palaeoecology. 284: 315-325.
Caplan, M. L. and R. M. Bustin. (1999). High-resolution isotope stratigraphy of the Devonian-Carboniferous boundary in the Namur-Dinant Basin, Belgium.
McGhee, G. R. (1996) The late Devonian mass extinction: the Frasnian/Famennian crisis. Columbia University Press.
Racki, G. (2005). Toward understanding late Devonian global events: few answers, many questions. Understanding late Devonian and Permian-Triassic biotic and climatic events: Towards an integrated approach. 5-36. Elsevier.
Sallan, L. and  M. Coates (2010). End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates. Proceedings of the National Academy of Sciences. 107: 10131-10135.

Stigall, A. L (2011). Speciation decline during the late Devonian biodiversity crisis related to species invasions. Public Library of Science.
www.geology.wisc.edu