MASS EXTINCTIONS OF THE PAST

SUMMARY OF THE FIVE GREAT EXTINCTION EVENTS PROVIDED BY SAMNOBLE MUSEUM: LINK: https://samnoblemuseum.ou.edu/understanding-extinction/mass-extinctions/end-permian-extinction/

(#1: END PERMIAN) When did it happen?: There were two significant extinction events in the Permian Period. The smaller, at the end of a time interval called the Capitanian, occurred about 260 million years ago. The event at the end of the Permian Period (at the end of a time interval called the Changshanian) was much larger and may have eliminated more than three-quarters of species of marine animals. It happened about 252 million years ago and geological evidence shows that it may have taken no more than 200,000 years. In terms of geological time the extinction occurred quickly. Who became extinct? Important groups of marine animals disappeared at the end-Permian extinctions. Trilobites, which had lived in the oceans for more than 250 million years, were lost, along with tabulate and rugose corals. Reef building in shallow seas stopped for about 14 million years until the middle of the following Triassic Period. At that time, an entirely new group of corals, the stony or scleractinian corals, appeared in the oceans. Although they did not become entirely extinct, rhynchonelliform brachiopods, crinoids, shelled cephalopods and snails also suffered significant losses. On land, primitive synapsids (relatives of mammals) disappeared. Some estimates suggest that up to 70 percent of vertebrate genera were lost. Below are some groups of marine animals that became extinct at the end-Permian event. trilobites, Tabulate corals, Rugose corals, goniatitic cephalopods, Productid brachiopods, cladid crinoids. What caused the extinction? Warming of the Earth’s climate and associated changes to oceans were the most likely causes of the extinctions. At the end of the Permian Period volcanic activity on a massive scale in what is now Siberia led to a huge outpouring of lava. The eruptions also produced carbon dioxide, a greenhouse gas that helps warm the planet. The lava flows erupted onto carbon rich rocks like coal and as they were heated by the hot lava, greenhouse gases, including methane, were also produced. The global warming that followed may have increased average ocean water temperatures by as much as 14.5°F (8°C). Much of the carbon dioxide released by the eruptions would have been absorbed by the oceans. High levels of dissolved carbon dioxide in seawater are toxic to many marine invertebrates. Also, the dissolved carbon dioxide would have produced changes in seawater chemistry that may have made it difficult for some marine invertebrates, such as corals, to grow shells or skeletons. If that wasn’t bad enough, there is also geological evidence that the amount of oxygen dissolved in sea water (which invertebrates and fishes breath with their gills) was reduced, probably as a result of changes in ocean circulation.

(#2: END TRIASSIC): When did it happen?: The extinction occurred near the end of the Triassic Period, about 201 million years ago. Who became extinct?: All major groups of marine invertebrates survived the extinction, although most suffered losses. Brachiopods, shelled cephalopods, sponges and corals were particularly hard hit. On land, casualties included the phytosaurs, a group of crocodile-like animals. What caused the extinction?: At the end of the Triassic, the supercontinent of Pangea, which combined all of the modern continents into a single landmass, began to break (rift) apart. As North America separated from Africa and the Atlantic Ocean began to form, volcanic activity on a massive scale introduced carbon dioxide into the atmosphere. This led to global warming and changes to the oceans that were similar to (although not as large) those that occurred at the end-Permian extinction. Reconstruction of Late Triassic global geography: All of today’s continents were combined into the supercontinent of Pangea. Pangea was beginning to break apart. As North America and Africa began to separate there was a vast outpouring of lava. The area of volcanic rocks that formed at this time is shown in yellow. Gases, including carbon dioxide, produced during the eruptions led to global climate change.

(#3: LATE DEVONIAN): When did they happen?: The end-Frasnian extinction happened about 375 million years ago. The oldest of the three extinctions, towards the end of a time interval called the Givetian, occurred about 10 million years before the Frasnian event. The youngest extinction happened near the end of the Devonian period, about 365 million years ago, during a time interval called the Famennian. Who became extinct?: The end-Frasnian extinction was most pronounced in tropical environments, particularly in the reefs of the shallow seas. Reef building sponges called stromatoporoids and corals suffered losses and stromatoporoids finally disappeared in the third extinction near the end of the Devonian. Brachiopods associated with reefs also became extinct. Groups of trilobites disappeared at each of the three extinctions and very few survived into the following Carboniferous Period. Examples of groups of brachiopods and trilobites that became extinct are: Odontopleurid trilobites, Dalmanitid trilobites, Phacopid trilobites, Atrypid brachiopods, Pentamerid brachiopods. How did they happen?: As extinctions were mostly of tropical groups climate change may have been involved, and there is geological evidence for cooling of the global climate at the end-Frasnian event and near the end of the Devonian Period. Cooling may have been caused by a drop in the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas that helps warm the planet, so if levels fall, cooling will follow. In the Late Devonian, large trees evolved and formed the first forests. As plant life expanded, they used up more carbon dioxide in photosynthesis. When dead plant material decays, carbon dioxide is returned to the atmosphere, but some plant material (e.g., leaves) will be buried in swamps, lakes and rivers. This buried plant material removes carbon permanently from the atmosphere and often forms coal. When we mine coal and burn it we return carbon dioxide to the atmosphere and warm the planet.

(#4: END-ORDOVICIAN): When did it happen?: There were two distinct extinctions roughly a million years apart. The first of these began about 443 million years ago. Together, these extinctions may have removed about 85 percent of species of marine animals. Who became extinct?: All of the major animal groups of the Ordovician oceans survived, including trilobites, brachiopods, corals, crinoids and graptolites, but each lost important members. Widespread families of trilobites disappeared and graptolites came close to total extinction. Examples of fossil groups that became extinct at the end-Ordovician extinction: Trilobite family Trinucleidae, Trilobite family Bathyuridae, Brachiopod genus Thaerodonta, Brachiopod genus Plaesiomys, Graptolite family Climacograptidae, Graptolite family Diplograptidae. What caused the extinction? The evidence indicates that climate change caused the extinctions. A major ice age is known to have occurred in the southern hemisphere and climates cooled world-wide. The first wave of extinctions happened as the climate became colder and a second pulse occurred as climates warmed at the end of the ice age. Reconstruction of Late Ordovician global geography (southern hemisphere), showing the south polar icecap (white). The Ordovician continent of Laurentia corresponds to most of present day North America; Baltica included part of modern western Europe. Gondwana was a super-continent composed of most of the major modern continents.

(#5: END CRETACEOUS): When did it happen?: The extinction occurred at the end of the Cretaceous Period, about 65.5 million years ago. Who became extinct?: In addition to the non-avian dinosaurs, vertebrates that were lost at the end of the Cretaceous include the flying pterosaurs, and the mosasaurs, plesiosaurs and ichthyosaurs of the oceans. Important marine invertebrates also disappeared, including ammonites, groups of cephalopods and some bivalves, such as the reef-building rudists and some relatives of modern oysters. Examples of invertebrate groups that became extinct at the end_Cretaceous event. tes that disappeared at the end-Cretaceous extinction. Ammonite (Cephalopod), gryphaeid oyster (Bivalv), Inoceramid (Bivalve). What caused the extinction?: Several lines of geological evidence indicate that an asteroid that was as much as 10 kilometers (6 miles) in diameter hit the Earth at the end of the Cretaceous Period. This evidence includes an ancient impact crater in the Yucatan Peninsula of Mexico (now filled in by younger rocks) that dates to the time of the extinction. The impact would have produced an enormous dust cloud that would have risen up into the atmosphere and encircled the planet. The dust cloud greatly reduced the amount of sunlight reaching the Earth’s surface and prevented photosynthesis by plants on land and plankton in the oceans. As plants and plankton died, extinctions expanded up the food chain, eliminating herbivores and carnivores. If that was not bad enough, dust and debris falling back to Earth was hot and may have triggered widespread wildfires. There is some debate over whether the asteroid was the sole cause of the extinction or whether other factors were also involved. Towards the end of the Cretaceous, volcanic activity in India produced lava flows over a vast area. Some paleontologists and geologists have suggested that gases (e.g., sulfur dioxide; carbon dioxide) released by the volcanoes might have altered the climate. Others point to geological evidence for a fall in sea level that would have reduced the area of shallow seas and, possibly, coastal plains.

SUMMARY AND CONCLUSIONS:

The characteristics of the five mass extinctions described above show that the two essential things that they all have in common are signficant global geological upheaval involving enormous amounts of energy and time scales of millions of years. Humans appeared on earth 66 million years ago and we could have been a creation of the End Cretaceous Extinction event but we surely could not have caused it. Human civilization did not appear until this Holocene interglacial about 8,000 years ago and the industrialization of human civilization did not occur until 100 years ago. The time scale needed for mass extinction events excludes any role for humans and the geological nature of these events also excludes any significant role for humans. That humans are causing a sixth mass extinction in the sequence of the five described above is a fanciful idea of extreme environmentalism but not credible.