1. The struggle for life on earth
Mass Extinctions
The struggle for life on earth

2. Mass Extinction - Definition
A significant proportion of species become extinct (between 30% and 95%)
The extinctions operate across a wide range of environments and lifestyles
The extinctions occurred rapidly (over a few million years) and were due to one or more physical factors

3. The “Big Five” Mass Extinctions identified since the Cambrian
Extinction Event
Date of Extinction
End of Ordovician
443 Million years ago
Late Devonian
375 Million years ago
End of Permian
251 Million years ago
End of Triassic
200 Million years ago
End of Cretaceous
65 Million years ago

4. The 5 Major Mass Extinctions
Gaps between extinctions vary from 51 Ma to 135 Ma with an average gap of 94.5 Ma

5. Percentage of Species Wiped Out
Ordovician-Silurian - 85%
Late Devonian - 82%
Permian-Triassic - 96%
End Triassic - 76%
Cretaceous-Tertiary - 76%
Causes: Ordovician- sea level falls; KT, Dev, PT, T : Asteroid; KT, T, PT - Flood Basalt

6. Extinctions During the Phanerozoic
The grey section of the graph shows all genera, or groups of species, extant at a given time. The green shows the portion of those genera that are well-resolved, meaning scientists have a good understanding of the group, with few gaps. The yellow and red triangles show the “Big Five” and present-day mass extinctions, and the blue triangles show other large extinction events. Biodiversity does change through time. By that, we mean that the number of species present on Earth changes over time, and the composition of species changes as well. For example, there are no dinosaurs left on the planet today, but we know that at one point in the past they thrived. That is a change in biodiversity. The most striking changes we see in biodiversity have come at mass extinctions. There have been five major mass extinctions through Earth’s history, which many smaller extinction events in between. In addition, there is background extinction; this is the term given to species extinctions that happen at a fairly regular rate at all times, and are not the result of a sudden natural disaster or impact event. But overall biodiversity has increased!
The Phanerozoic is from 542 Ma (Cambrian) to the present

7. End of Ordovician Mass Extinction – 443 Ma
70% of marine species became extinct
Tropical faunas badly affected especially coral reefs
Main groups affected Trilobites, Graptolites, Echinoids, Brachiopods
The third largest extinction in Earth's history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number. At the end of the Ordovician Period, the world entered an intense ice age, possibly brought about by the location of the supercontinent Gondwana over the southern pole. The formation of large ice sheets meant sea levels fell dramatically, perhaps by as much as m. This particularly affected the corals and bryozoans that were living in shallow inland seas, which drained of water. Global cooling spelt disaster for warm-adapted species that had nowhere to migrate to. Then, after about 1 million years, the glacial conditions ended rapidly. Sea levels rose, water low in oxygen blanketed the shallow marine habitats, and deep ocean waters stagnated, delivering a second blow to the marine life that had managed to survive.

8. Late Devonian Mass Extinction – 375 Ma
A series of events that lasted around 10 Ma
Three quarters of all species on Earth died out in the Late Devonian mass extinction, though it may have been a series of extinctions over several million years, rather than a single event. Life in the shallow seas were the worst affected, and reefs took a hammering, not returning to their former glory until new types of coral evolved over 100 million years later. The causes of this drawn-out mass extinction are far from certain, but it was a time of major environmental change. The first phase has been linked to a lack of oxygen in the ocean (anoxia), combined with rising sea levels and global cooling. The appearance of forests may have played a role in the mass extinction million years ago. Some scientists think that the increased diversity and amount of plant cover on land triggered the anoxia and cooling. New plant root systems boosted soil formation and promoted the weathering of rocks, releasing nutrients into the oceans. This may have stimulated algal blooms that used up oxygen in the water when they decayed. Meanwhile, photosynthesising plants and the weathering of silicate rocks would have reduced the amount of the greenhouse gas carbon dioxide in the atmosphere, causing temperatures to fall. Large volcanic eruptions in the Vilnuy Province in eastern Siberia may have also induced climate change. Large meteor impacts have been blamed too. But, although glassy globules of melted rock tell us that impacts did occur in the Late Devonian, so far none match the timing of the extinctions. Many experts believe that an intense ice age, brought about by the supercontinent Gondwana drifting back over the South Pole, is responsible for the later round of extinctions.
Cephalopods, Fish and Corals most affected

9. End Permian Mass Extinction – 251 Ma
The largest mass extinction event removing 95% of marine species and 50% of marine families
Trilobites, Cephalopods, Bryozoans, Corals, Crinoids badly affected
Around 251 million years a devastating event threatened to wipe out life on Earth entirely. No one knows what happened exactly but more than 95% of all species in the sea disappeared and life on land was also brought to brink of collapse. Such events are known as mass extinctions. There have been five “big” mass extinctions since the invasion of land, 500 million years ago, but this extinction was by far the biggest of all time. The perplexing thing is that no one really knows what caused it. Some scientists argue that there was a massive volcanic eruption at that time that went on for thousands of years but others disagree that this was the cause. The Permian mass extinction has been nicknamed The Great Dying, since a staggering 96% of species died out. All life on Earth today is descended from the 4% of species that survived.
Major faunal and floral overturn on land
Marks the boundary between dominance by the Palaeozoic and Modern Fauna

10. Permian Mass Extinction
100% trilobites
98% crinoids
97% foraminifera
99% radiolaria
100% blastoids
Most severe of all mass extinctions. Life in the sea almost extinguished – 95% of species died out. Occurred over about 10 Ma. Effects on land floras and faunas seem to have been less severe. Pangaea was assembled. Leads to reduction in areal extent of shallow water environments. Brings into competition faunas from similar environments in different continents. Sea level low. Overall reduction in shallow water environments, but not likely to be so severe as to cause extinction of 95% of life. Reduced oxygen in the ocean-Oceanic anoxic events or anoxic events (Anoxia conditions) refer to intervals in the Earth's past where portions of oceans become depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia develops - euxinia refers to anoxic waters that contain H2S hydrogen sulfide. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincide with several mass extinctions and may contribute to these events. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. It is believed oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming and elevated levels of greenhouse gases. Enhanced volcanism (through the release of CO2 and other greenhouse gases) is the proposed central external trigger for the development of these events.[
97% ammonites
100% acanthodians
96% brachiopods
100% eurypterids

11. End Triassic Mass Extinction – 200 Ma
Multiple event mostly affected the land where over 95% floral species eliminated
During the final 18 million years of the Triassic period, there were two or three phases of extinction whose combined effects created the Triassic-Jurassic mass extinction event. Climate change, flood basalt eruptions and an asteroid impact have all been blamed for this loss of life. This extinction is probably the least understood of the big five, because of the lack of accessible sediments for scientists to study. Most of the evidence suggests falling sea levels were probably responsible for the longer-term extinction patterns. As the warm shallow seas decreased, reefs died and other marine organisms faced increased competition in less space. On land the lack of water would have led to more extreme temperatures and seasons. When deep water spread back over the continents it was low in oxygen, resulting in further marine extinctions. While the ultimate cause of this sea fall and rise isn’t certain, it appears to be associated with the start of a volcanic rift forming between the Americas and Africa and Europe. This would eventually produce the Atlantic Ocean. At the end of the Triassic, very large eruptions occurred along the rift zone (known as the Central Atlantic Magmatic Province) for about 500,000 years. The environmental changes these caused put further stress on life on Earth.
Around 30% marine species became extinct – mainly reef dwellers, Ceratites, Brachiopods and Bivalves

12. End Cretaceous Mass Extinction – 65 Ma
Around 70% of all species wiped out
Dinosaurs, Reptiles, Ammonites, Belemnites, Brachiopods, Bivalves, Foraminifera

13. End-Cretaceous Mass Extinction
100% belemnites
100% ammonites
few radiolaria
Average species life is 2 million years, so you have to assess the extinction rate above this baseline. It was more rapid, but not instantaneous. Although effect on different groups varied, in the sea it seems a less severe extinction than that of Permian. Most major groups in the sea continued across the boundary into the Tertiary. On land, the effects were more dramatic, with disappearance of large land animals and low latitude floras.
100% dinosaurs
100% pterosaurs
few plants
100% mosasaurs
90% foraminifera
100% ichthyosaurs

14. Dinosaur extinction theories

15. Causes of Mass Extinctions
Supercontinent Formation
Extra-Terrestrial Impacts
Flood Basalt Eruptions
Methane Hydrates and Global Warming
Rapid and Major Glaciation Events

16. Supercontinent Formation
Pangea formed at the end of the Permian
Resulted in fewer continental shelves and lack of habitat for shallow marine organisms
Coincided with huge decline in the numbers of shallow marine species
Caused rapid fluctuations in climate, unstable weather patterns and extreme aridity in the interior of the land mass

17. Supercontinent Formation
A single continent reduces the input of nutrients to oceans from rivers and estuaries
This decreases the amount of nutrients available for shallow water marine life and may have also altered the salinity

18. Supercontinent Formation
A supercontinent positioned close to one of the poles can initiate major glaciations
Glaciation causes sea levels to fall and there is a significant reduction in shallow water marine environments
A supercontinent located over a pole can also lead to the coverage of the Earth in ice, a condition known as ‘Snowball Earth’

19. Extra-Terrestrial Impacts
Asteroids 1 km diameter strike the Earth every 500,000 years
Father and son team Luis & Walter Alvarez advanced the idea that impact of an asteroid about 10 km across could explain the mass extinction. Caused clouds of dust, blocking solar radiation. With no photosynthesis, most plants die out. Herbivores had nothing to eat, and carnivores soon ran out of herbivores to eat.
Large collisions with 5 km diameter asteroids occur approximately about once every 10 million years
The last known impact of an object of 10km diameter or larger was 65Ma

20. Extra-Terrestrial Impacts
Local destruction of habitats – impact blast, shockwave, ignition of wildfires
Billions of tonnes of debris injected into the atmosphere resulting in rapid global cooling

21. Extra-Terrestrial Impacts
Impact in the sea – billions of tonnes of water vapour injected into the atmosphere resulting in a greenhouse effect and rapid global warming

22. Flood Basalt Eruptions
Eruptions last between 0.5 and 2.0 million years and can erupt enough basalt to cover the whole of the USA to a depth of a kilometre
Local destruction of habitats and initiation of wildfires

23. Flood Basalt Eruptions
Billions of tonnes of carbon dioxide and sulphur dioxide released into the atmosphere during eruptions
Both are powerful greenhouse gases and will contribute to very rapid global warming

24. Flood Basalt Eruptions
When it rains the sulphur dioxide will come back down to Earth dissolved in rainwater and the acid rain will kill vegetation on a large scale
With vegetation dying, all food chains will be affected and also the oceans may become acidified with disastrous effects on marine life

25. Siberian volcanism in the Permian period
An animated slide. The first picture gives the accepted area of the Siberian Traps. Our recent research at the University of Leicester has demonstrated that basalts recovered from oil-exploration boreholes into the West Siberian Basin are the same as the Siberian Traps in terms of age and geochemistry. Thus the proven area of the Siberian Traps is double what was previously thought. Reichow, M., Saunders, A. D., White, R. V., Pringle, M. A., Al'Mukhamedov, A., Medvedev, A. & Kirda, N. P Ar/39Ar dates from the West Siberian Basin: Siberian Flood Basalt Province doubled. Science 296, The Siberian Traps are the remnants of widespread volcanic activity that occurred in northern Pangaea, about 250 m.y. ago. The timing of the extinction is, with the resolution of current dating methods, identical with the main eruptions of the Siberian Traps. The reduction in animal and plant species was not the only feature of the late Permian and early Triassic. There was a pronounced shallow-ocean anoxic event, that appears to have been global in scale, affecting the continental shelves. There was an increase in the amount of fungal (or algal) remains at the P-Tr boundary, and indirect evidence shows significant global warming and climate change.

26. Methane Hydrates in Ocean Floor Sediments
Large volumes currently locked into ocean floor sediments
Stable under low temperatures of deep ocean
Methane clathrate (CH4·5.75H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.

27. What is methane hydrate?
Methane molecule (CH4)
in a 'cage' of H2O molecules
Structure held together
by hydrogen bonds
This is methane (formed by decay of organic matter) that gets trapped in water molecule ‘cages’, at relatively low temperatures and high pressures within sediment on the ocean floor. It can be released by either increasing the temperature of the sediment (by increasing the temperature of the bottom-water), or by decreasing the pressure (dropping sea level). In the Permo-Triassic scenario, release of methane hydrate could have been achieved either by warming the whole oceans, or by changing circulation patterns so that warmer water penetrated to deeper depths. Acknowledgement: USGS, via
"The ice that burns"

28. Methane Hydrates in Ocean Floor Sediments
Global warming may result in deep ocean temperatures rising and the release of large volumes of methane from ocean sediments
The rapid release of large amounts of methane into the atmosphere will result in highly accelerated global warming

29. Methane Hydrates in Permafrost
Immense amounts of methane from organic decay are trapped in marine sediments in a frozen form. Perhaps twice as much carbon stored in methane hydrates as in all fossil fuels Methane Hydrates are a potential energy source, but may have a role in extinctions. Flood basalt eruptions result in global warming which warms the oceans. Methane hydrates become unstable and methane begins to bubble up out of sediments and permafrost. Methane is released into atmosphere (greenhouse gas) and the result is further global warming.

30. Glaciations-Possible Causes
Supercontinents positioned in high latitudes/close to the poles
Milankovitch cycles: Precession, Obliquity and Eccentricity

31. Glaciations-Effects on Ecosystems
Loss of habitat as ice masses grow
Could eventually lead to ‘Snowball Earth’ scenario
Contraction of climatic belts towards the equator
Global cooling, decreased productivity of primary producers

32. End Cretaceous Mass Extinction – 65 Ma
K-T extinction
K-T confusingly describes the change-over period between the Cretaceous and the Tertiary. The 'K' is Greek, representing the word Kreta, or chalk. The 'Cret' of Cretaceous also comes from the same word, indicating that there was a lot of chalk laid down during this period1. The term was coined to delineate between two periods, the delineation point being the extinction event. This was established well before the bolide impaction theory was aired, but mentioning the K-T event is now more or less synonymous with the bolide impact. It has, in fact, proved such an unshakable image that it will be quite a while before (in the public eye at least) any other theory will get a fair hearing.
A large bolide (asteroid or meteorite 10km in diameter) collided with the Earth 65 million years ago

33. And then what? Fireball Tsunami Wildfires Dust and darkness Acid rain
Increased CO2 and global warming
The bolide theory is very neat, and there is a large amount of evidence to back it up. There is a fine layer of clay that delineates the Cretaceous from the Tertiary - this sedimentary layer has been found all over the globe. Tests performed on this layer reveal the presence of an unusually high concentration of iridium. Iridium is not uncommon on Earth, but the concentrations found at the K-T boundary clay layer indicate an extra-terrestrial source. Put bluntly, the only bodies in which such large concentrations of iridium are found are bolides. This layer points to a single, global event during which this material was distributed widely. One explanation of this distribution is a massive impact. Other evidence comes in the form of distressed quartz. Distressed quartz develops unusual properties when subjected to extreme impaction. Many examples of distressed quartz have been found in the same layer as the iridium, as have microtectites (tiny balls of glass found near impact craters, another symptom of a bolide collision). The question of cause and effect remains, though. Was the bolide the cause of the extinction event, or did it arrive, coincidentally, during an established extinction process? Simply because the two events occurred at the same time, it cannot be stated with any confidence that the bolide collision definitely caused the K-T extinction.
Acknowledgement:

34. Evidence - Asteroid Impact Location
Under the seas surrounding the Yucatan platform of Mexico lay a secret that was undiscovered for 65,000,000 years. Named the 'Chicxulub Crater' after the nearby village of Puerto Chixulub, the crater is all that survives of a massive bolide collision with the Earth. The impact of an item approximately 12.5 miles wide left a crater of between 90 and 125 miles across. It is posited that one of the initially devastating effects of such an oceanic impact would be a wall of water up to five miles high. The wall obviously wouldn't stay there for long - it would come crashing down and become an unstoppable tsunami, flooding the land for miles and miles around, and covering the previously fertile soil with salt - ruining the chances of anything growing back. While this was going on, portions of the bolide and fragments of the Earth's crust would have been hurled into and out through the atmosphere - some so violently that they would have achieved a temporary orbit before raining down to Earth. Dust and soot would have filled the air and darkened the skies - smaller meteors and burning debris from the main site would also rain down. The heat generated by the impact would have started raging fire storms which would sweep across continents. An estimated 77,220 cubic miles of the Earth's crust was vapourised and ejected from the Chicxulub site. What follows this event in the usual narratives is the tale of a slow death for the dinosaurs. There was no food, for in the ensuing darkness no photosynthesis could occur. Cold dinosaurs plodded around blindly, choking on dust and dying in their droves across this shattered and ruined planet. An estimated 70% of all species perished. The face of the Earth would never be the same again. But was it really so bad?
Location of possible impact site discovered on the Yucatan Peninsula in Mexico at Chicxulub
The impact structure is a circular depression about 180 km in diameter

35. Evidence for Asteroid Impact LocationB
A. Gravity survey onshore and offshore of the Yucatan Peninsula
B. 3D map of gravity and magnetic field variations reveals the Chicxulub crater, now buried beneath tons of sediment. This view is looking down at the surface, from an angle of about 60°.

36. Evidence – The K-T Boundary Layer
The presence of a thin 2cm layer of iridium-rich clay found all over the world within sedimentary rocks
The K-T Boundary Layer dates the same everywhere at 65.5 Ma +/- 0.3 Ma
Iridium is a transition element, rare on Earth but found in meteorites.
First proposed by Luis Alvarez in 1980
The clay layer at Cretaceous–Tertiary (K–T) boundary contains unusually high levels of the element iridium. Iridium is normally extremely rare in sediments, but is more abundant in extra-terrestrial materials. Hypothesis is that iridium has come from the fragmented asteroid. Clay also contains grains of quartz that show damage consistent with impact. Abundance of fern spores above K–T layer, indicates widespread devastation of plants and recolonization of landscape.

37. Evidence – Shocked Quartz
Shocked quartz is found worldwide, in a thin layer at the boundary between Cretaceous and Tertiary rocks.
It was first discovered at nuclear testing sites and later in craters caused by meteorite impacts as at the Barringer Crater near Flagstaff, Arizona in the USA

38. Evidence – Shocked Quartz
Shocked quartz has a microscopic structure different from normal quartz.
Under intense pressure, but relatively low temperature, the crystalline structure of quartz is deformed along planes inside the crystal.
These planes, which show up as lines under a microscope, are called planar deformation features (PDFs), or shock lamellae.

39. Evidence – Tektites (Glass Spherules)
Tektites (from Greek tektos, molten) are natural glass rocks up to a few centimetres in size.
Most scientists agree they are formed by the impact of large meteorites on Earth’s surface.
Tektites are black or olive-green in colour and their shape varies from rounded to quite irregular.
Tektites (Glass Spherules) from 1 to 8 mm in diameter are found within a radius of 600 to 1,000km of the Chixulub Crater in Mexico

40. Evidence – Soot from Wildfires
The K-T Layer has high concentrations of carbon in many locations,
suggesting that the asteroid impact may have generated wildfires.
Large areas of vegetation would have been destroyed in a short time,
soot fell to Earth and was incorporated into sedimentary rocks

41. Evidence – Tsunami Deposits
It is thought that the asteroid impact occurred in the sea and initially produced a crater km wide and 30 km deep
This would have displaced vast volumes of seawater and generated a series of very large tsunamis possibly over metres in height
The tsunamis would have travelled great distances inland, and in Texas at Waco there are large-scale sedimentary deposits thought to be of tsunami origin.

42. Evidence – Tsunami Deposits
The tsunami deposits in Waco, Texas are thought to be from the waves generated by the K-T asteroid impact.
The sediments are estimated to have been deposited at least 300km inland by the tsunamis!

43. Animation to show Impact of a large asteroid with the Earth
Impact of a body 10 km across with the Earth would make a big hole. For many years after the idea of an impact was suggested, no appropriate crater was known. Then a crater (now filled with sediment) was discovered at Chicxulub (CHICK-sa-Lube) in Mexico and seems to be about the right age. Many people saw this as final proof that an asteroid impact caused the death of the dinosaurs.
The impactor's estimated size was about 10 km in diameter and is estimated to have released 4×1023 joules of energy, equivalent to 100,000,000 megatons of TNT on impact.

44. Never mind the bolides……
Flood Basalt Eruptions 66 Ma – The Deccan Traps
Erupted mainly over a period of 30,000 years
Over 2000 metres thick and cover 500,000 km²
May have originally covered 1,500,000 km²
Caused a global drop in temperature of 2°C
The Deccan Traps are often cited as a rival to the bolide theory for the cause of dinosaur extinction. These were a range of active volcanoes covering approximately 200,000 square miles of India. They had been discharging poisonous gasses into the atmosphere for years, causing acid rains, generally depleting the biosphere and adding lethal cocktails of gasses to the atmosphere (not to mention large volumes of soot and dust). Interestingly, most of the lava from the Deccan Traps was released between 60,000,000 and 65,000,000 years ago - almost dead on the K-T boundary. But demonstrating that Deccan volcanism was the principal cause faced daunting hurdles. For over two decades, the main Deccan eruptions were shown to have occurred over less than 800,000 years (within magnetic polarity C29r, spanning the KTB). Determining where, within this major eruptive phase, the KT mass extinction occurred, remained problematic. Models of the Deccan Traps’ biotic and environmental consequences generally underestimated the duration, eruption-rate and gas emissions by orders of magnitude. This led workers to think that volcanism could not have been one of the major causes of the KT mass extinction. This view is rapidly and radically changing, principally due to three recent studies. Chenet et al. have estimated that 80% of the 3500m-thick Deccan Traps were erupted over a very short time - possibly less than 10,000 years - with most of that time taken up by the quiet periods between eruptions. The entire Deccan lava pile was erupted in three phases with the first and smallest phase at 67.4Ma, the main phase at or near the KTB and the last smaller phase in the early Danian. Self et al., measuring sulphur and chlorine gas concentrations in glass inclusions, determined that one cubic kilometre of erupted lava would have released between 3.5 to 5.4 teragrams of SO2, and one teragram of HCL. These huge amounts of S and Cl gases, released over a very short period at the end of the Cretaceous, would have had severe environmental consequences. Then Keller et al. revealed in 2008 that the mass extinction coincided with the end of the main phase of Deccan volcanism. These results are based on sedimentological, microfacies and biostratigraphic data of 4-9m-thick inter-basalt-flow sediments in four quarry outcrops in SE India. Here, the so-called Rajahmundry traps mark the end of the main phase of Deccan volcanism, and include the world’s longest lava flows, extending 1300km into the Bay of Bengal. Sediments immediately below mark the mass extinction in planktic foraminifera. Sediments directly overlying the lower trap basalts contain early Danian planktic foram assemblages of zone P1a, which mark the subsequent evolutionary recovery following the KT mass extinction. These results strongly suggest that Deccan volcanism played a critical role in the KT mass extinction, which took place after the last mega-pulse of the main phase of Deccan volcanism. Deccan volcanism has emerged as a credible cause for the KT mass extinction and the most serious challenge to the impact hypothesis. Moreover, the discovery of rapid and voluminous Deccan eruptions at KT time suggests that Ir and other PGE contributions may have been far greater than originally assumed and could even account for at least some of the Ir anomalies. Whether the impact killed the dinosaurs outright, or tipped them over the edge to extinction is open for question. One thing is certain: as long as there were no witnesses, we'll never really know what happened.
The term Trap is derived from the Dutch for stairs and refers to the step-like landscape of the area

45. The Sixth Mass Extinction?
Following their evolution humans have caused huge species loss, and this has accelerated in since the 19th century
Species are becoming extinct at a rate of about 4000/year, 100/day, 1 species every 15 minutes.
NASA
Shortly after the evolution of Homo sapiens in Africa, about 130,000 years ago, modern humans spread worldwide. Around 15,000 years ago many large mammals like the woolly mammoth started to go extinct and it is possible (though difficult to prove) that human hunting was responsible. As human society continued to develop, forests were cleared for agriculture, and the industrial revolution began, so the rate of extinction related to human activity increased. It is difficult to estimate the rate at which species are going extinct today but famous ecologist E.O. Wilson believes that half of all species will go extinct before We have already noted that there have been five mass extinction events over the past 500 million years. Given the facts, it seems unavoidable to believe that we are now in the midst of a sixth mass extinction.
Are we now in the midst
of a sixth mass extinction?
Today

46. Meanwhile….the K-T debate continues…
31 August 2002 “Cold was killing dinosaurs long before the asteroid commonly thought to have been their downfall hit, according to scientists.”
11 June 2002 “Dino heatwave recorded in leaves” … “analysis of fossil leaves from 65 million years ago shows there was a sudden and dramatic rise in carbon dioxide in the Earth's atmosphere. “
From BBC News – for full stories see:

47. … and the P-Tr debate rumbles on…
6 June 2002 “A huge outpouring of molten rock 250 million years ago may have been the decisive factor in the deaths of nearly all lifeforms on the Earth at that time.”
23 February 2001 “Earth's biggest mass extinction 251 million years ago was triggered by a collision with a comet or asteroid, US scientists say.”
One of the difficulties that teachers face is evaluating media reports that sometimes say the exact opposite of one another. Sadly, this arises from the fact that science (and particularly, Geology) doesn’t make headlines unless it’s presented as absolute truth, preferably with some sort of dire implications. The real truth is that in many case, we just don’t know, but it doesn’t make a “story” to say “X could have happened, but we’re not quite sure yet, and even if we were sure, we can’t prove what the implications are”.
From BBC News – for full stories see:

48. Most mass extinctions that have afflicted life on Earth during the past 500 million years have occurred during times of major volcanic eruption and all were accompanied by major changes in climate, sea level and oxygenation levels in the ocean. Among the five major mass extinctions, only the end-Cretaceous (KT) displays a close coincidence of four factors - an iridium anomaly (commonly assumed to represent an impact), an impact crater (Chicxulub), a large igneous province (the Deccan Traps) and major climate and sea level changes The KT mass extinction also differs in that it follows the longest period ( Ma) of low background extinction. Throughout the Cretaceous, generic diversity had increased, accelerating during the Campanian and peaking during the late Maastrichtian, prior to the mass extinction.

49. Conclusions
The largest extinctions of the last 300 m.y. correlate with massive volcanism.
At least one also correlates with a significant meteorite impact.
There are far more impact and volcanic events than there are mass extinctions.
This rules out simple causal relationships between volcanism and extinctions, or impact and extinctions.
The “Murder on the Orient Express” hypothesis shows that combinations of driving factors are probably more important.
These complications lead to conflicting reports in the media
The Moth in the Ointment There are several reasons to believe that the Chicxulub impact crater was not quite as devastating as popular science programmes would have you believe. The humble moth survived quite nicely. Moths breathe through 'spiracles', an apparatus which requires clean air. Most fly sprays work by clogging these spiracles, and suffocating the insect to death. If the air was choked with dust and soot, you would expect moths to be quite high on the extinction list. Second to this, moths survive by eating pollen. No pollen, no moths. So a single season without flowering plants would spell the end for the moth. So due to its presence, we must concede that the rhythms of the flowering plants were not disturbed by this terrible event. It was commonly accepted that the dinosaurs died out because they were cold-blooded, and couldn't regulate their core temperatures effectively in the new darkness, but frogs and newts (both cold blooded) survived admirably, as did crocodiles and alligators. This may have something to do with their semi-aquatic nature. But the ammonites, which were wholly aquatic, died out with the ichthyosaurs, the mososaurs, and the plesiosaurs, to name but a few. Correlation is not the same as causation!

50. That’s All Folks!