Understanding Mass Extinctions

• Mass extinctions are significant biotic transitions that have greatly affected the diversity of life on Earth.
• Extinction is vital as without it, there may not be sufficient ecological space for new species to emerge.
• The average species lifespan is a few million years before extinction.
• It's estimated that 99.9% of all species that have existed are now extinct, with most due to normal background extinction.
• Mass extinction happens when multiple lines of descent terminate simultaneously.
• Mass extinctions are studied due to their dramatic nature and implications for our future, showing the biosphere can be wrecked and healed with a new set of organisms.
• A mass extinction episode is followed by diversification and adaptation of the survivors.
• Mass extinctions lead to declines in both diversity and abundance of organisms, are typically global events, and cut across taxonomic groups affecting organisms in a wide array of environmental habitats.
• Each species has a geological range defined by its first and last appearance in the stratigraphic record.

The Big Five

• There are five major mass extinction episodes recognized from the fossil record.
• These are the end-Ordovician, late Devonian, end-Permian, end-Triassic, and end-Cretaceous extinctions.

End-Ordovician Extinction

• Occurred around 443 Ma, causing the extinction of 52% of documented species.
• Uniquely among the Big Five, was limited to marine organisms, as land was not yet colonized.
• The cause is probably global cooling, with evidence of glaciation in Gondwana.
• Global seawater temperatures dropped by an estimated 5°C during this period.
• Cooling climate could trigger a mass extinction due to changes in ocean temperature and a drop in sea level.
• Glaciers are made of water, and that water has to come from somewhere.
• Dropping sea level by more than 330 feet reduces the amount of habitat they have in which to dwell, and increases competition between individuals.
• Contributing factors to the cooling include Gondwana's position over the South Pole and decreased greenhouse gas levels.
• The Taconian Orogeny may have reduced CO2 levels through silicate weathering.
• The Ordovician Meteor Event, with an influx of L-chondrite meteorites, might have caused a reduction in sunlight due to stratospheric dust.

Late Devonian Extinction

• Took place over 20 million years, with major die-offs at 374 Ma and 359 Ma.
• Resulted in the destruction of 99% of reefs and severe losses in the fish world and marine groups.
• Likely cause is the proliferation of black shales due to increased organic carbon.
• Tree roots accelerated the release of nutrients, leading to eutrophication and dead zones.
• Black shale is rich in organic carbon.
• The removal of organic carbon from the biosphere lowered CO2 levels, possibly resulting in global cooling.
• Glaciogenic sedimentary layers during the late Devonian support evidence of a new build-up of glacial ice

End-Permian Extinction

• The most extreme extinction event in Earth's history, with 62% of marine genera and severe impacts on terrestrial biota becoming extinct.
• Roughly 17% of species on Earth survived.
• Referred to as "The Great Dying".
• Global warming appears to be the general cause, with two pulses: the Capitanian at 260 Ma and the end-Permian at 252 Ma.
• Oxygen isotope studies indicate a 6°C warming of the global ocean.
• Extra CO2 in the late Permian atmosphere trapped heat energy.
• CO2 likely came from The Emeishan Traps and the Siberian Traps large igneous provinces which erupted during the late Permian.
• The Siberian Traps represents a very big eruptive episodes in Earth history.
• The level of CO2 in the atmosphere reached to at least 3000 ppm, and maybe as high as 30,000 ppm.
• As the polar oceans warmed up to roughly the same temperatures as the tropics, the ocean became stagnant.
• Stagnant oceans led to anoxia, recorded by black sedimentary deposits.
• The oceans probably became euxinic, infused with deadly hydrogen sulfide gas (H2S), with purple sulfur bacteria blooming in profusion.
• During the Siberian Traps eruption, a layer of rock salt was pierced, releasing ozone-destroying chemicals that increased UV light exposure.
• The early Triassic seas were overtaken by the clam Claraia.
• The survivors went on to diversify into the Modern Fauna; mollusks would now reign supreme, scleractinian corals would build reefs, and echinoids, sea stars and sea cucumbers would now represent the echinoderms (replacing crinoids and blastoids).

End-Triassic Extinction

• About half of Triassic genera didn't make it into the Jurassic, with ammonoids and bivalves decimated.
• Numerous marine reptiles were snuffed out and conodonts disappeared.
• The cause was likely a spike in CO2 levels and subsequent global warming due to the eruption of a large volcanic province.
• The breakup of Pangaea and the opening of the Atlantic Ocean led to the Central Atlantic Magmatic Province (CAMP).
• The Central Atlantic Magmatic Province (CAMP) may have erupted even more basalt than the Siberian Traps.
• Plant fossils indicate a significant decrease in stomata density, indicating high CO2 levels.

End-Cretaceous Extinction

• Dinosaurs, ammonitic ammonoids, and other giant reptiles went extinct.
• The Cretaceous was generally hot, with no evidence of glacial ice.
• The Western Interior Seaway developed in North America due to high sea levels.
• The Deccan Traps, a large igneous province in western India, erupted.
• In 1980, the Alvarez team published a paper that showed evidence for an extraterrestrial impact as the cause.
• The team found high levels of iridium in a clay layer between limestone strata.
• Other evidence of an impact included spherules and shocked quartz.
• The Chicxulub crater in the Yucatán Peninsula was identified as the impact site.
• The impactor was estimated to be about ten kilometers across.
• Recently, Robert DePalma and colleagues published a new site they call “Tanis” in North Dakota which shows evidence for a major extraterrestrial impact.
• Fish fossils have impact spherules caught in their gills.
• Kill mechanisms included radiative heat, blunt force trauma, forest fires, tsunamis, ocean acidification, and nuclear winter.

Other Mass Extinctions

The Great Oxidation "Event"

• Earth transitioned from an oxygen-free atmosphere to one with free oxygen between 2.4 Ga to 2.0 Ga.
• Prior to this, the atmosphere lacked significant free oxygen because of unusual chemical sedimentary rocks (banded iron formations/BIFs).
• Oxygen is a waste product of photosynthesis, so any photosynthetic organism produces it in proportion to the amount of glucose it generates.
• Of particular note are cyanobacteria, which are preserved from Archean and younger times as stromatolites.
• Free oxygen was deadly to sulfur-reducing bacteria and other microbes.

The Cambrian Explosion

• Was preceded by soft-bodied, sessile Ediacaran biota.
• The Cambrian animals destroyed the Ediacaran ecosystem.
• The Ediacaran period a time of microbial mat-dominated ecospace.
• The Cambrian critters could swim high above, such as the fearsome predator Anomalocaris, while many others burrowed into the sediment below, which disturbed the continuity of the bedding.
• The very oldest beds of the Cambrian are defined by the first appearance of the trace fossil Treptichnus pedum.

Eocene-Oligocene Transition

• Earth's climate shifted into a cooler state.
• Antarctica separated from Australia and South America, creating the circum-polar current.
• Deep sea ocean temperatures dropped globally by 4°-5°C, and glaciers grew on Antarctica.
• Three substantial bolide impacts in the northern hemisphere: Popigai in Siberia (35.7 ± 0.2 Ma), Toms Canyon offshore from New Jersey (~35 Ma), and Chesapeake Bay, Virginia (35.5 ± 0.3 Ma).
• Climate shift caused turnover of organisms, with grasslands increasing and mammals adapting to cooler conditions.

Pleistocene Megafauna

• Targeted large mammals and birds.
• Causes for megafauna extinction probably relate to humans.
• Timing varied by continent, coinciding with the arrival of humans.
• The extinction of megafauna was least in Africa, where animals evolved alongside humans.

The Modern Biodiversity Crisis

• May be another mass extinction caused by human impact.
• Factors contributing to the crisis include habitat destruction, invasive species, disease, pollution, and climate change.
• Rates of extinction may match or exceed those of the Big Five mass extinctions.

The Last Life on Earth

• In about 2.5 billion years, life on Earth will cease.
• Plate tectonics will slow down and stop, causing the magnetic field to weaken and be lost.
• The Sun will grow in volume and consume the Earth.