The five largest mass extinctions in Earth's history occurred during:five largest mass extinctions 1.The late Ordovician period (about 438 million years ago) families extinct - more than half of the bryozoan and brachiopod species extinct. 2.The late Devonian (about 360 mya) - 30% of animal families extinct. 3. At the end of the Permian period (about 245 mya) - Trilobites go extinct. 50% of all animal families, 95% of all marine species, and many trees die out. 4. The late Triassic (208 mya) - 35% of all animal families die out. Most early dinosaur families went extinct, and most synapsids died out (except for the mammals).synapsids 5. At the Cretaceous-Tertiary (K-T) boundary (about 65 mya) - about half of all life forms died out, including the dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites, many families of fishes, clams, snails, sponges, sea urchinspterosaurs plesiosaursmosasaursammonites and many others.

Extinction Processes 1. Extinction strikes in both the land and the sea. 2. On the land, while animals suffer repeatedly, plants tend to be highly resistant to mass extinctions. 3. Preferential disappearance of tropical forms of life during mass extinctions. 4. Tendency of certain groups of animals to experience them repeatedly (for example, trilobites and ammonoids). 5. Alleged equal spacing, or periodicity in geological time (occurring about every 26 million years). These similarities between distinct extinction occurrences aid paleontologists in determining the agents the agents that perpetuated the disappearances of species in each extinction event. Such agents are currently divided into two types: -Catastrophic Agents- such as meteorite impacts and comet showers, -Earth Agents- such as volcanism, glaciation, variations in sea level, global climatic changes, and changes in ocean levels of oxygen or salinity -Combination of the two - could catastrophic agents trigger these earth agents? Although these agents can explain mass extinction, the causes of mass extinction events remains relatively unknown.

Terrestrial Agents: A.Magnetic field switch This theory believes that a switch in the Earth's magnetic field allowed an influx of deadly radiation. The problem with this theory is why would this have affected only the larger animals, and selectively at that? Sea creatures would also be unlikely to be affected, being partly shielded by the water. B. Climate change This theory holds that changes in continent position killed dinosaurs by decreasing temperature. During the Jurassic and Cretaceous periods the continents were separating and oceans opening, leading to cold and changeable conditions that dinosaurs could not adapt to, and that also affected plankton and disrupted the food chain. C. Volcanic activity The idea here is that extreme volcanic activity caused huge dust clouds that blocked off much of the sun's heat and light for many months, even years, leading to a dramatic drop in temperature in both land and sea and a decrease in photosynthetic activity in such organisms as marine plankton and land plants. These organisms are at the base of the food chain, and any changes at their level are magnified repeatedly as you go up the chain, with maximum effects at the top among the larger predators and herbivores. D. Acid rain Shells of microfossils show a sharp increase in strontium isotope ratios up to the Cretaceous/Tertiary (K/T ) boundary and then a slow decrease afterwards. The strontium is derived from weathering of rocks, runs off into rivers and seas and is absorbed by the shells. The rapid increase is interpreted as due to a dramatic increase in such weathering due to acid rain, and is much greater than could reasonably be explained by the impact of a large meteorite. It could, however, be associated with increased volcanic activity. Extraterrestrial Agents: A.Cosmic rays This theory maintains that cosmic rays from an exploding star caused mutations or killed the animals outright. The question must be asked - Why were these mutation effects so selective? Why were all organisms not affected? A further problem with this idea is that no supernova is known from the right time period. B.Meteor impact In this theory a 10Km diameter meteor hit the Earth. Such a meteor has more explosive power than all current nuclear weapons combined, and would punch a hole and lift 10,000 cubic kilometers of dust into the air, darkening the sky for months and causing temperatures to drop from 20 to -10 degrees centigrade. Evidence produced in favour of this theory rests largely on a thin layer of clay deposited world wide at the end of the Cretaceous period that contained unusually high levels of iridium. Iridium is an element once present in the earth's crust. However, even at the time of the dinosaurs most terrestrial iridium had long since sunk into the molten core of the earth, and little remained in the crust. Meteorites are relatively rich in iridium and associated metals, and many also contain minute diamonds fractions of micrometers in diameter that are also found in large numbers in the K/T boundary clay.

Extinctions-flood basalts

Global Climatic Cycles in Geologic Time from L E Waite (2002), modified from Fischer (1984)

Coral/Algal Extinctions from L E Waite (2002)

Temperature change last 2000 years

Temperature different time scales… Glacial and Interglacial Periods