What is Mass Extinction? A mass extinction occurs when a large fraction of all living species becomes rapidly extinct. The fossil record shows that at least five major mass extinctions have occurred in the past 500 million years. Impacts of asteroids on Earth are suspected as a primary cause of mass extinction.

Percentage of families that went extinct Mass Extinction 60 40 Percentage of families that went extinct 20 500 400 300 200 100 Time (millions of years ago)

1.The Ordovician Extinction Since more water was in ice form, the sea level lowered all over the world, causing a reduction of space for life on continental shelves. The most affected animal group was the marine invertebrates, in which more than 100 families were wiped out. This extinction occurred at end of the period, about 440-450 mya. It is thought to be caused by a global cooling, which caused the continent Gondwana to glaciate. Geologists have found glacial deposits in the Saharan desert, which provided the evidence for this theory.

The species that survived the Ordovician extinction rediversified during the Silurian and Devonian periods. Sharks made their first appearance during this time, and reef-builders such as coral were flourishing in the oceans. The first land plants were forming, which allowed forests to arise. Other new species included amphibians and insects.

2. Devonian Extinction (~350mya) This event is theorized to be caused by an episode of global cooling similar to that of the Ordovician extinction. This time, the glacial deposits have been found in northern Brazil. However, meteorite impacts have also been thought a possible cause of this mass extinction, although the evidence remains inconclusive. This extinction had little effect on land animals, mostly affecting (again) the marine life, in particular, the reef-builders.

The Permian Extinction At the end of the Permian period (about 245 mya), the most severe mass extinction occurred, wiping out 90-95% of marine and 70% of land species. Many theories of what caused this extinction have been considered: The glaciation of Gondwanaland, the primary theory for the two previous mass extinctions, the increased fluctuation of global temperatures caused by north and south pole glacier events, volcanic eruptions occurring in Siberia, and The formation of super-continent Pangaea, which possibly caused a reduction of space on the continental shelf.

The Permian Extinction’s Effects The main effect of this mass extinction was the increased number and variety of reptiles, both on land and in the ocean. This made way for what has been called the “age of the dinosaurs,” because it helped to wipe out competing species.

Triassic-Jurassic Extinction (~200mya) During the Triassic period, there was a super continent call Pangaea. In the mid-Jurassic period, Pangaea broke apart into Laurasia and Gondwanaland Volcanic eruptions and the split of Pangaea caused another mass extinction The continental drift was accompanied by volcanic activity and continued during the Cretaceous period at a fast pace.

K-T Extinction The K-T impact caused the extinction of many species including the dinosaurs and 60-80% of life at that time. Sudden extinction thought to be caused by an asteroid “Impact Hypothesis” In 1980, Walter Alvarez of U of California at Berkeley found a layer of iridium in sediments that dated from the time of the K-T extinction.

Impact site

Greenhouse Earth, Icehouse Earth, and sea level change Greenhouse Earth: when there is little to no ice on Earth including on the Polar Ice Caps. The temperatures are very high Icehouse Earth is when worldwide temperatures are very low. Scientists are scared that there will be no ice on Earth because sea level will rise.

Sea Level Changes During the beginning of the Cretaceous period, global sea levels were really high because there was no polar ice. Later in this period, the sea levels dropped. Because the climate cooled off tremendously, many animals were driven to extinction but unique dinosaurs were able to evolve on different continents. During the Tertiary period of the Cenozoic era, extreme cooling occurred. But during the Quaternary period, cooling continued resulting in a series of ice ages. This cooling led to the high rates of evolution and extinction among mammals.

Diversification and Speciation after Mass Extinction Surviving Species take over vacant habitats and resources Speciation takes every 100 to 100,000 year 10 to 10,000 new species per year Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.

PERIOD ONSET MAJOR EVENTS ME Paleozoic Cambrian 540 Ma most animal phyla present, diverse algae Ordovician 490 Ma first jawless fishes, animal diversification 1st Silurian 445 Ma first bony fishes, colonization of land Devonian 420 Ma first insects and amphibians, fish diversify 2nd Carboniferous 355 Ma extensive forests, first reptiles, insects radiate Permian 290 Ma reptiles radiate, insects are diverse 3rd Mesozoic Triassic 250 Ma early dinosaurs, first mammals, marine inverts. diversify 4th Jurassic 200 Ma first birds, diverse dinosaurs Cretaceous 145 Ma flowering plants and mammals diversity, dinosaurs continue diversification 5th Cenozoic Tertiary 65 Ma radiation of mammals, birds, flowering plants, pollinating insects Quaternary 2 Ma humans evolve, extinctions of large mammals

Is there a potential Sixth Major Extinction Is there a potential Sixth Major Extinction? Species are becoming extinct at a rate of about 4000/year 100/day 1species every 15 minutes Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.

2050 50% of all species on the planet will be either endangered or extinct Habitat destruction Global Warming 25% mammalian species 15% bird species Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.