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

Ordovician Extinction Event

Introduction

The Ordovician extinction event was the first of two major extinctions that seriously affected the Palaeozoic evolutionary fauna (Brenchley 2001). The early-mid Ordovician was a time when fauna established the complex suspension-feeding communities, characterized by brachiopods, bryozoans, crinoids, and corals, which reached an equilibrium generic diversity by the start of the Late Ordovician.  (Sepkoski 1995). This equilibrium was disturbed after ~20 million years by the Ordovician extinction at ~439 million years ago. The extinction occurred in 2 main phases an estimated 0.5 to 1 million years apart.

(How the seas during the Ordovician period may have appeared from www.geology.wisc.edu)

The mass extinction seriously affected both benthic and planktonic faunas across all latitudes in both clastic and carbonate environments, and in deep and shallow marine regions (Brenchley 2001).

What was lost?

The Ordovician mass extinction event eliminated an estimated 85% of marine species, 55% of genera, 22% of families, however only a few orders, and no classes or phyla (Brenchley 2001). Both benthos and plankton were affected with high levels of generic extinction among the pelagic conodonts (76%), graptolites (85%), sessile benthic brachiopods (54%), rugose corals (69%), tabulate corals (72%) and benthic trilobites (58%) (Sepkoski 1995).

(Image showing changes in diversity across the 2 phases of the Ordovician mass extinction. Numbers in bold are the estimated generic diversity before and after the event, and the percentages are the amount of generic extinction at each phase of extinction. From Brenchley 2001.)

The first phase of extinction amongst the majority of groups was sharp. Graptolites suffered very significant losses losing ~75% of their genera, and trilobites and brachiopods lost ~50% of their genera, however, conodont losses were relatively small (Sheehan 1988). During this interval a new brachipod fauna, the Hirnantia fauna and the Mucrohaspis triblobite association appeared (Brenchley 2001). These eurytopic (can tolerate a wide range of environmental conditions), cool-adapted, opportunist species established themselves across a wide range of environments from high-latitudes to the marginal tropics. In the tropics the Edgewood brachiopod fauna became established alongside new low-diversity associations of eurytopic corals.

                                          (Trilobite fossil. From National Geographic 2011.)

The second stage of extinctions was also sharp, with the majority of groups excluding the graptolites further depleted, with the conodonts experiencing the greatest losses. Many long-established clades of trilobites that had survived the first phase of extinction disappeared, alongside the newly established Hirnantia and Edgewood brachiopod faunas and the Edgewood coral faunas (Brenchley 2001).

Immediately after the late-Ordovician mass extinction event most groups remained depleted, characterized by the residual members of the pre-extinction Ordovician faunas. There was also a rapid appearance of new genera among the conodonts.

Postulated Causes

There is a close correlation between the two phases of extinction and the growth and decay of the Gondwanan ice caps, suggesting that the rapid climatic changes that disrupted a long-established greenhouse climate may have played a major role in causing the Ordovician extinction event (Brenchley 2001). The first phase of extinction coincided with the initiation of major glaciation and the second phase with the decay of the ice caps. The growth of continental ice is reflected in an estimated global sea level fall of 50-100m and the waning of the ice caps by a similar magnitude. Oxygen isotope stratigraphy reflect the presence of a major ice cap, and a fall in shallow marine temperatures of up to 8°C, even in tropical regions (Brenchley et al. 1994). A synchronous positive shift of ~7‰ in ¹³C implies a major change in Carbon cycling such as may result from an increase in productivity and increased organic Carbon storage in sediments or deep-water (Brenchley et al. 1994).

(Environmental and biotic changes associated with the Ordovician extinction event, PDB is the international carbonate standard.. From Brenchley 2001).

Causes of the first phase: The decrease in marine temperatures which started abruptly during the first phase is likely to have eliminated those species living within a specific temperature range, particularly in tropical regions where the faunas were most likely adapted to greenhouse conditions. Sea level change was modest during this period, and is unlikely to have been a factor in the first phase, but may have subsequently played a role in the extinction of shallow marine faunas of tropical carbonate shelves (Brenchley 2001). Faunas inhabiting on or above slope areas may have been affected by the changes in ocean circulation associated with the development of thermohaline circulation in response to the cooling of high-latitude waters. This new circulation likely resulted in vigorous upwelling, which may have raised the thermocline and oxygen minimum zone and resulted in an overabundance of nutrients and substances toxic to the plankton inhabiting the near-surface mixed layer (Wilde et al. 1990). This may account  for a number of extinctions including gratolite assemblages and the planktonic and benthic trilobites. The conodonts mainly inhibited shelf waters which escaped the effects of upwelling, which explains why populations did not deplete significantly.

Causes of the second phase: The second phase was coincidental with a rise in sea level, temperature, a change in Carbon cycling (suggesting a return to warm stratified oceans), and widespread anoxia (Brenchley 2001). This rise in temperature is likely to have favoured the warm-adapted survivors of the first extinction, and played a key role in the elimination of the cool-adapted forms among the Hirnantiabenthic and nektobenthic faunas. Species sensitive to Oxygen deficiency would have been eliminated and high sea level may have restricted many shallow marine habitats.

Conclusion

Major biotic changes in the Late Ordovician appear to have been caused by a rapid change from a long history of stable greenhouse conditions preceding the extinction into and then out of an ice house climate with a number of associated changes. A mass extinction of species was experienced without eliminating any major groups, and without radically changing the ecological structure of communities. The holes in the structure of communities were progressively filled during the early Silurian with new taxa, but with little ecological innovation.

References

Brenchley, P. J. (2001). Extinction: Late Ordovician mass extinction. Encyclopedia of Life Sciences.
Brenchley, P. J, et al. (1994). Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period. Geology. 22: 295-298.
National Geographic (2011) at www.nationalgeographic.com
Sepkoski, J. J. Jr (1995). The Ordovician radiations: diversification and extinction shown by global genus-level taxonomic data. In Cooper, J. C. et al. (eds.) Ordovician Odyssey: Short papers of the Seventh International Symposium on the Ordovician system. pp. 393-396. Fullerton: Pacific Selection, Society of Sedimentary Geology.
Sheehan, P. M. (1988). Late Ordovician events and the terminal Ordovician extinction. New Mexico Bureau of Mines and Mineral Resources Memoir. 44: 405-415.
Wilde, P. et al. (1990). Vertical advections from oxic or anoxic waters from the main pycnocline as a cause of rapid extinctions or rapid radiations. In Kauffman, E. G. and O. H. Walliser (eds.) Extinction Events in Earth History, Lecture Notes in Earth Sciences 30. pp. 85-98. Springer-Verlag: Berlin.
www.geology.wisc.edu

Summary of the Sixth Mass Extinction

Prior to discussing in further depth the previous five 'mass extinction' events and comparing them to the proposed current event, I thought that it would be useful to summarise the 'Sixth Mass Extinction'. Large amounts of information will be drawn from the Nature article "Has the Earth's sixth mass extinction already arrived?" (Barnosky et al. 2011) which provides a fantastic overview of the sixth mass extinction, introducing elements such as comparing past extinctions and looking to the future, which are to be discussed later in this blog. I will also draw upon other sources of information that I came across during my blogging hiatus.

Introduction

Over the last 3.5 billion years it is estimated that 4 billion species have evolved on Earth, however, over 99% of these species have since become extinct (Novacek 2001). This shows that extinction is a very common occurrence, however, it is normally balanced by speciation.

This balance wavers, and several times over the Earth's history extinction rates have appeared elevated. Only five times have these rates qualified for 'mass extinction' status. Different causes are believed to have led to these periods, and the extent above background level has varied substantially. The common factor between them all is that extinction rates during these events have been higher than any other geological interval of the last 540 million years, exhibiting a loss of over 75% of estimates species on Earth (Jablonski 1994). 

Five 'Mass Extinction' Events (from Barnosky et al. 2011)

Ordovician - Ended ~443 mya. 57% of genera lost and 86% of species. Cause: Onset of alternating glacial and interglacial episodes, repeated marine transgressions and regressions, uplift and weathering of the Appalachians affecting atmospheric and ocean chemistry, sequestration of Carbon Dioxide. 

Devonian- Ended ~359 mya. 35% of genera lost and 75% of species. Cause: Global cooling followed by global warming tied to the diversification of land plants with associated weathering, paedogenesis, and the draw-down of global Carbon Dioxide. There is also evidence for widespread deep-water anoxia and the spread of anoxic waters by transgressions. The timing and importance of bolide impacts is still debated. 

Permian - Ended ~251 mya. 56% of genera lost and 96% of species. Cause: Siberian volcanism, global warming, spread of deep marine anoxic waters, elevated Hydrogen Sulfide and Carbon Dioxide concentrations in both marine and terrestrial realms, ocean acidification. Evidence for bolide impact is still debated.

Triassic - Ended ~200 mya. 47% of genera lost and 80% of species. Cause: Activity in the Central Atlantic Magmatic Province (CAMP) thought to have elevated atmospheric Carbon Dioxide levels which increased global temperatures and led to a calcification crisis in the oceans. 

Cretaceous - Ended ~65 mya. 40% of genera lost and 76% of species. Cause: Bolide impact in the Yucatan is thought to have led to global cataclysm and caused rapid cooling. Preceding the impact biota is believed to have already been declining, this may be for a variety of reasons including: Deccan volcanism leading to global warming, and tectonic uplift altering biogeography and accelerating erosion potentially leading to ocean eutrophication and anoxic episodes. 

Sixth Mass Extinction

Increasingly scientists are recognising modern extinctions of species and populations (e.g Barnosky et al. 2011, Ceballos and Ehrlich 2002, Hughes et al. 1997, IUCN 2010). Documented numbers are likely to be serious underestimates as the majority of species have not yet been formally described (Dirzo and Raven 2003). Such observations suggest that humans are now causing the sixth mass extinction through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate (e.g Barnosky et al. 2011, Myers 1990,  Pimm et al. 1995). If this is the case recovery will not occur on a timescale relevant to human beings, as evolution of a new species typically takes hundreds of thousands of years, and recovery from a mass extinction event probably occurs over millions of years (Barnosky et al. 2011). 

Data Disparities

Only certain kinds of taxa (most notably those with fossilizable hard parts) and a restricted subset of the Earth's biomes (generally temperate latitudes) have adequate data for direct fossil-to-modern day comparisons. 

Fossils are widely acknowledged to be a biased and incomplete sample of past species, but modern data sets also have important biases, with less than 2.7% of the approximately 1.9 million named extant species have been formally evaluated for extinction status by the IUCN (IUCN 2010). 

Despite limitations of both the fossil and modern records, scientists are working around the diverse data biases to attempt to avoid error in extrapolating from what they do know to inferring global patterns.

Defining 'Mass Extinctions' Relevant to History

Extinction involves rate and magnitude which are distinct, but interlinked. Rate is the number of extinctions, divided by the time over which this occurred. Magnitude is the percentage of species that have gone extinct. 

Mass extinctions were originally declared by rate, when the pace of extinction appeared to become significantly faster than background extinction (Novacek 2001). However, recent studies suggest that both the Devonian and Triassic events occurred due to a decrease in origination rates rather than an increase in extinction rates (Barnosky et al. 2011). 

Thus a 'mass extinction' is when extinction rates accelerate relative to origination rates such that >75% of species disappear within a geologically short interval (typically <2 million years). Therefore we need to determine current extinction rates and identify how closely historic and projected biodiversity losses approach 75% of the Earth's species. 

Background Rate

Numerous studies (eg. Myers 1990, Pimm et alMSY (Extinctions/Million species years) where background rates are estimated from fossil extinctions that took place in 1 million year time-slots (Wake and Vredenburg 2008). For current rates the proportion of species extinct in a comparatively very short (one to a few centuries) timescale is extrapolated to predict the rate over 1 million years. This relies on the assumption that extinction rate relies constant over 1 million years, which is untrue according to empirical data (Barnosky et al. 2011). This results in rates much faster or slower than what the average rate would be over the one million year period. 

Current extinction rates determined by Barnosky et al. (2011) using this approach varied from 24-693 species extinctions per million species year depending on the approach. These figures are greatly above the background rate of 1.8 species extinctions per million species years. 

Combined Rate-Magnitude Comparisons: Looking to the Future

As rate and magnitude are so intimately linked, a question of critical importance is whether current rates would produce 'big five' magnitude 'mass extinctions' in the same amount of geologic time that we think most 'big five' extinctions spanned. Barnosky et al. (2011) believe so, stating that current extinction rates for mammals, amphibians, birds and reptiles if calculated over the last 500 years are as fast or faster than all rates that would have produced the 'big five' extinctions over hundreds of thousands or millions of years. 

The high current extinction rates could be severe enough to carry extinction magnitudes to the 'big five' benchmark in as little as 300 years (as determined by Barnosky et al. 2011), however, future research is greatly needed, and will be further discussed in a subsequent blog entry. 

References

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Wake, D. B., and V. T. Vredenburg. (2008). Are we in the midst of a sixth mass extinction? A view from the world of amphibians. Proceedings of the National Academy of Science. USA. 105: 11466-11473.