Addendum to SIO15
The Five Great Mass Extinctions
The San Diego Union Tribunes had an article by Richard A. Lovett in its Quest
section on Wednesday, November 12, 2003. The article focused on a possible
release of enormous amounts of methane hydrates that could have triggered Earth's
greatest mass extinction in the Permian period. There was a nice summary
of the other extinctions as well which is given below. This should be compared with the
(sometimes rather thin) summary of lecture 22. The mass extinctions happened near the end of geological periods (times of periods vary by author and differences to the lecture 22 web page reflect error bars). Phrases in italic are added by me.
Ordovician
- Summary: second greatest extinction of marine organisms
- Time of period: 510 - 438Mio years ago
- Setting:Shallow tidal seas in tropical regions brought about extensive diversification of marine organisms, including new types of cephalopods (the forerunners of squid), corals, the mosslike bryozoans and bivalves. Nautilus may have reached several feet. (more than 600 families of marine life)
- Probable Cause of Extinction: Rapid cooling of surface waters. As seas retreated into ice sheets, marine habitats drastically changed. (decrease in precipitation according to Abbott's figure 10.25. According to atlas significant drop in sea level. It also cites a, contradictory, increase in precipitation.)
- Species affected: More than 100 families of marine invertebrates wiped out.
Devonian
- Summary: as much as 70% of marine invertebrates lost.
"Age of the Fishes". Extinction was not a sudden one but seemed to have taken several tens of Mio years.
- Time of period: 410 - 360Mio years ago
- Setting:Shallow seas flooded great areas of the continent. First appearance of primitive sharks, bony fish, air-breathing lungfishes and the coiled mollusks called ammonoids (the atlas placed ammonites (?) into the Jurassic). World's oceans were dominated by corals. On land, first amphibians, insects and great swamp forests.
- Probable Cause of Extinction: Meteor impact or glaciation in the south polar area that cooled surface water. (no evidence for this in Abbott's Figure 10.25, sea level did drop by 300ft after the extinction, according to atlas).
- Species affected: Corals were so severely affected the they remained relatively uncommon for 150 Mio years. Brachiopods, trilobites, condodonts and jawless fish suffered. Little impact on freshwater and land animals and plants.
Permian
- Summary: greatest mass extinction known
- Time of period: 286 - 250 Mio years ago
- Setting:Formation of supercontinent Pangea and all-time low sea level ( only time sea level was below today's level!) created for the first time more continents than oceans. Extensive development of terrestrial animals, including insects, amphibians, reptiles and mammal-like reptiles.
- Probable Cause of Extinction: Geologic events may have triggered methane hydrate release from sea floor, dramatically reducing atmospheric oxygen levels. Other causes include extensive production of flood basalts in Siberia (largest on Earth). Meteor impacts are also discussed). Extensive global warming associated with the break-up of Pangea, rise in sea level though no particular change in precipitation have also been considered. The release of methane hydrates (or gas hydrates in more general terms) is an interesting idea but I wonder why this would happen in a cold global climate. Methane hydrates are frozen lenses in submarine sediments. Link to Wikipedia
- Species affected: 96% of marine species (250,000 down to 10,000), including trilobites, and 75% of terrestrial vertebrates decimated. (63% tetrapods, 30% amphibians)
Triassic
- Summary: three extinction events spanned the period
- Time of period: 250 - 206 Mio years ago
- Setting:Warm and hot, with vast areas of desert. huge marine reptiles probably swam in small schools, feeding on mollusks. Dinosaurs became abundant and diversified. Appearance of first true mammals.
- Probable Cause of Extinction: Climate change (cooling, drop in sea level) or meteor impact. One large impact crater exists in Canada, although new dates place it out of time with extinction events. (increased volcanism seems out of phase with extinctions though Iridium finds point toward increased volcanism near the extinction)
- Species affected: Great devastation to sponges, marine snails, bivalves, cephalopods, brachiopods and marine reptiles. On land, 35 families of insects and 8 families of reptiles died out.
Cretaceous
- Summary: known as the dinosaur extinction
- Time of period: 144 - 65 Mio years ago
- Setting:Continents drifted apart to approach their current position. Emergence of flowering plants. Major diversity in marine reptiles, bivalves, ammonoids and corals.
- Probable Cause of Extinction: metero impact created gigantic dust cloud that shrouded Earth, halting photosynthesis and leading to massive animal die-offs. Alternatives include massive productions of flood basalt (Deccan Traps in India). End of cretaceous also became drier and cooler. Dramatic drop in sea level (could imply increase in oceanic spreading rates??).
- Species affected: 75% to 85% of species, including 50% of vertebrates. Non-avian dinosaurs, pterosaurs, marime reptiles perished. The ammonites disappeared. Mammals, birds, reptiles and amphibians were largely unaffected.