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Lecture 11

Lecture 11 BIOC50 Mass Extinctions and Cenozoic Diversification

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Biological Sciences
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Jason Weir

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Lecture 11 Mass Extinctions and Cenozoic Diversification 1) Mass Extinctions: • It’s estimated that perhaps 98% of species that have ever evolved have gone extinct. Currently, there are estimated to be between 7 and 100 million species on our planet. If no species had ever gone extinct, we would currently have 343 million to 4.9 billion species on the planet! • Extinction is a natural and important component of macro-evolution. Populations or taxa which fail to adapt to a set of ecological circumstances may go extinct while other populations or taxa which do adapt survive. • In order for biodiversity levels to remain the same through time, rates of extinction must equal rates of speciation. • When speciation rates exceed extinction rates we get an increase in biodiversity • When extinction rates exceed speciation rates we get a decline in biodiversity a) Species naturally arise and go extinct. • Plotted here is the number of marine genera through time. The green represents genera with detailed fossil records, the grey includes genera with less detailed fossil records. • We see a general increase in species diversity from about 200 Mya to the present. • When extinction rates greatly exceed speciation rates we get a mass decline in biodiversity. This usually indicates a mass extinction event. Mass Extinctions: a) The big 5: Yellow triangles 1) late Ordovician 2) late Devonian 3) late Permian (P / T) 4) late Triassic 5) late Cretaceous (K / T) Blips and fluctuations: represent periods where extinction rate > speciation or speciation > extinction Whole genera extinctions and fluctuations prove that they are indeed mass extinction rates: Major increase in extinction Permian and Triassic. Possible causes of mass extinction events: • Mass extinctions generally happened “suddenly” in geologic time, usually over the course of thousands to a few millions of years • Mass extinctions were global phenomena, thus we need to look for global causes. • Plausible clauses include: 1. Disruption of ocean circulation, resulting in changes to ocean chemistry. The end result: global change in climate ( stimulate glaciations in Pleistocene) 2. A rapid change in sea level: again change in global climate 3. Enormous volcanic eruptions: release of volcanic gasses and ash results in global cooling and makes photosynthesis difficult. 4. Extra-terrestrial impact by asteroids: fire storms and shock waves result in massive die offs initially; dust ejected into atmosphere hinders photosynthesis and result in global cooling. i) Late-Ordovician • Major die-offs of marine species • The mass extinction event near the end of the Ordovician seems to be closely linked with a major climate change. • A first pulse of extinction happened as a big ice age began, the second as it ended ii) Late-Devonian • Major die-off of coral reefs and their associated faunas • The most gradual decline of mass extinction • Climate change, major changes in sea levels and ocean chemistry as well as several (not terribly large) meteor impacts at this time. It is debated whether the meteor impacts played a major or minor role or any role at all. iii) Permo-Triassic (P / T) (250 Mya) • The largest mass extinction event of all time. • This extinction event was recognized over 150 years ago to define the end of the Paleozoic Era and the beginning of the Mesozoic Era.  57% of all marine families  96% of all marine species  84% of all marine genera • Was a rapid event, probably < 1 million years • It also affected terrestrial ecosystems  In a well preserved terrestrial fossil record from the Southern Hemisphere, coal beds which formed the great swampy forests of the Ordovician and Permian stop abruptly at the P / T boundary in the fossil record and don’t show up again until about 6 million years after the extinction event.  This suggests a significant drop in global photosynthesis no trees that formed these coal beds • CAUSE: Siberian Volcanism appears to be the best proposed explanation  The largest known volcanic episode in Earth history occurred in Siberia at 251.2 Mya, immediately preceding the mass extinction event.  Eruptions caused a lot of ash that blocked off the sunlight and reduced photosynthesis and we were left off with fungi and we see that there was a lot of fungi during the 250 mya. o In addition to plate tectonics, Earth has plume tectonics.  Occasionally, an event at the boundary between Earth’s core and mantle sets a giant pulse of heat rising toward the surface as a plume.  As it approaches the surface, the plume melts the crust to develop a flat head of magma that can be 1000km across and 100 km thick. o Penetrating the crust, the magma plume generates enormous volcanic eruptions pouring out hundreds of thousands of cubic kilometers of lava and releasing volcanic ash. o One such massive plumage erupted in Siberia exactly at the P / T boundary forming what are known as the “Siberian Traps”, gigantic lava fields that cover 4 million square km. o “Siberian Traps” lasted at full intensity for only ~ a million years. o Release of ash and acidic gasses would have blocked sunlight, killing off both terrestrial plants and aquatic photosynthetic based ecosystems resulting in food chain collapse. Would have resulted in immediate cooling. o CO2 emitted could have resulted in global warming in the aftermath o There are extreme abundances of fossil fungal cells in land sediments at the P / T boundary. o This fungi enriched layer may represent a single world-wide crisis with fungi breaking down massive amounts of plant vegetation that had been catastrophically killed. o Siberian Volcanism appears to be the best proposed explanation o Such a fungal layer is rare in fossil record. o Not everyone is convinced by the Siberian Traps explanation for the P / T extinction event and other theories have been proposed. For example, some have proposed meteor impacts (lack of solid evidence) at this time may have caused the extinctions in combination with volcanism and other factors. o Some even suggest that intense volcanism may have been triggered by meteor impacts. o ( he pretty much skipped the last 8 dots) iv) Late Triassic (200 Mya) • CAUSES: unknown. Possibly a combination of metior impact (none found) and large volcanic eruptions combined with climate change. • Many of the earlier diapsid reptile and even earlier amphibian-like tetrapod groups go extinct, allowing dinosaurs to take over. Still a minor extinction compared to the P/T  20% of all marine families  55% of all marine genera v) K / T mass extinction (65.5 Mya) • Dinosaurs and Pterosaurs right up to, but not after the K / T boundary allowing mammals and birds to take over  17% of all families  50% of all genera  Extinctions  75% of all species • CAUSES: o 30 years ago it was suggested that a large asteroid, ~10km in diameter had caused the K / T extinction event. o PROOF: The evidence was an abnormally high concentration of Iridium immediately at the K / T boundary layer in the geological column.  Iridium concentration (parts per billion): went up to 3.7 ppb at 65.5 mya  Iridium is rare on Earth but plentiful in asteroids. o In 1990 the Chixulub crater was identified on the Yucatan coast of Mexico.  The crater has a diameter of about 180 km which would have been formed by an asteroid ~ 10 km in size Right under the Iridium layer  The age of the crater is believed to correlated with the 65.5 Mya iridium layer. theres Ejecta layer (dustoandLook at slides 11-15 dirt) As we go further away from  Asteroid caused a shock wave that forms ejecta and forms world wide blocking crater photosynthesis Ejecta hit at the red and purple dots area and slowly dissipates as you go farther from
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