Mass Extinction

Late Ordovician

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The Late Ordovician occurred approximately 443.8 million years ago. The extinction took place during the Ordovician period, which spanned 485.4 mya and 443.8 mya.
Before the Late Ordovician extinction the planet was inhabited by fusulinids which shared the oceans with stromatolites, foraminiferans, and radiolarians, all which were tiny, simple, single-celled​ life forms and on land simple cryptospores lived.

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85% of all life on Earth was wiped out and it ranks as the 2nd deadliest because over 100 marine families died out and species living in shallow waters were more negatively affected than species living in deep waters. Terrestrial species affected include conodonts, trilobites, bryozoans, and brachiopods.
After the Late Ordovician extinction, life began to become more complex, as nonvascular plants appeared, and complex, multi-celled aquatic and marine life in the form of fish came to be.

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The continents did not have the same location as they were in today. The supercontinent known as Gondwana had formed from the colliding of the land masses known today as Antarctica, Africa, South America, India, and the Sahara Desert.
The supercontinent of Laurentia was just beginning, and at this time only included North America. Other landmasses​ existed separately from each other, and thus formed the Iapetus ocean, and the Paleo-Tethys ocean.

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A variety of evidence obtained from geologic and fossil records prove the Late Ordovician extinction took place. Moreover, scientists turn to ancient blocks of ice containing gas, proving how atmospheric concentrations have changed.
Burst of Gamma Rays - 6,000 light years away, a hypernova might have occurred, having the potential to remove half of Earth’s ozone​ and expose Earth’s surface creatures to UV radiation.

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Glaciation-A combination of a cooling change in climate and the hiding away of water in the ice caps brought an end to many habitats shared by several communities. The supercontinent known as Gondwana ceased its movement directly above the South Pole, the formation of glaciers began.
Volcanism-Volcanic eruptions may have blasted silicate rocks into the atmosphere. These rocks would have removed CO2 from the atmosphere as they eroded, resulting in a major decline in atmospheric carbon dioxide.

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Laurentian Brachiopods
The Late Ordovician extinction brought an end to the dominance of single-celled​ organisms over the Earth, the seas were no longer ruled by microbes.
The Late Ordovician rise to the dominance of multicelled organisms, as fish came to rule the seas.

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Bryozoan

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Conodont

Late Devonian

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Occurred about 375 Million Years Ago
The majority of organisms were aquatic; a large amount of marine animals and coral reefs. Life had started to colonize on land, with simple terrestrial life. 19% of Marine, and 50% of Terrestrial life were wiped out. After, large animals started to take shape, growing limbs.
The sedimentological record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction.

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What caused these changes is somewhat more open to debate. Many reef-builders and jawed vertebrates went extinct.
Oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter, is the leading cause.
Most if not all scientist agree.There are many other accompanying theories that some have suggested; namely effects from ‘the greening of the earth’ (Plants using CO2 for photosynthesis, leading to a decrease in a greenhouse gas) and Magmatism.

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This was the start of terrestrial life. Right before this mass extinction, small insects, and plants started to develop. These plants developed seed transportation systems in this time, allowing them to thrive away from water
This was also the start of tetrapods; four limbed organisms.

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Tatracotolla

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Cystoidea

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Graptolites

End-Permian (sometimes called “Permian-Triassic”)

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The Permian-Triassic extinction took place during the Permian period, an era that approximately started 299 and came to an end 252 million years ago.
The Permian extinction is known as the most devastating mass extinction, as 95% of marine life was wiped out, along with 70% of terrestrial life.Over half of all taxonomic families went extinct. Despite these loses, the Permian period contained a great amount of genetic diversity, and diverse marine and terrestrial life.

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Several types of organisms lived in the water such as acanthodians, fusulinids, rugose corals, crinoids, etc. There were several types of insects, plants, and terrestrial organisms. New organisms came to be after the extinction, when the Triassic period was well underway such as large mammals and reptiles.
During the Permian period Earth looked very different from its appearance today. During this era, the smaller continents of the planet had been forced together to form Pangea.

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A great ocean called Panthalassa spanned the majority of the planet. During this era, the continents of North America, South America and Africa were one large landmass. Europe and Asia were much more separate than they are today, and were more as the continents of North and South America are now: partially connected by land, yet separated and individual. Also, the land of Asia and Europe were also smaller than now.

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Scientists have several articles of evidence to support their ideas on this extinction. They have used the ratio between stable carbon isotopes to back up their theory that an alteration in the carbon cycle caused the extinction.

Although marine organisms suffered the most loss, a variety of terrestrial & aquatic organisms went extinct in this mass extinction. Marine organisms that went extinct include blastoids, a group of echinoderms, trilobites, acanthodians, the first form of jawed fish.

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Terrestrial organisms like the massive moschops, pelycosaurs, & several insects became extinct. 
Possible causes for this extinction include temperature crises, or the dramatic changes of ocean temperatures, making like habitats uninhabitable for many organisms. Another possible cause is an alteration in the carbon cycle. A 3rd theory to the extinction is a population boom of CH4 producing microbes, increasing the amount of CH4 in the ocean and the atmosphere, significantly warming the oceans.

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Another possible cause is that pangea caused the extinction of many species, as some microbes died as the tectonic plates were slammed together. Another explanation is that a meteorite struck the Earth, releasing shockwaves, flames, and toxic particles into the atmosphere.
Scientists don't agree on a single explanation for the extinction. Some agree an alternation of the carbon cycle caused the extinctions, while others believe that the explosion of CH4 emitting microbes caused the extinction.

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The End Permian extinction was the most devastating extinction to have ever occurred on the face of the Earth. Half of all species families were lost, significantly affecting food chains, food webs, and ecosystems across the planet. The End Permian extinction also gave rise to larger, more complex marine reptiles to appear. Some scientists regard these very marine reptiles to be the ancestors of some of today’s marine species.

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Maschop

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Trilobite

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Blastoid

End-Triassic

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Occurred about 201 Million Years Ago
Approximately half of life on earth at the time was rendered extinct; this includes 34% of marine life. Most life was marine and reptilian, or small and bacterial. Many small ecological niches were wiped out in this extinction, allowing room for the dinosaurs to expand, and become the dominant species.
It is thought at the time that the reason for a lowed biodiversity is the result of a decrease in speciation, rather than an increase in extinctions.

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The exact cause of the end-Triassic extinction is up for debate. Many think it was caused by climate change & rising sea levels resulting from the sudden release of large amounts of CO2. The volcanism of the first 40,000 yrs of this time was particularly intense and coincided with the beginning of the mass extinction. Other authorities suggest that the heating caused by rising CO2 concentrations in the atmosphere could have melted massive amounts of CH4 trapped in permafrost and undersea ice.

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It allowed dinosaurs to take reign of the earth.
Plants were almost not affected; this allowed these new organisms such as dinosaurs to thrive right off the bat, not having to wait for new evolutions as a source of food.

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Conodont

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Ammonoidea

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Archosauromorpha

End-Cretaceous (also known as “Cretaceous-Tertiary” or “K-T”)

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The End-Cretaceous extinction occurred about 66 million years ago.
This extinction killed off 80 percent of all the species on earth at the time. Dinosaurs and many marine invertebrates were eliminated in this extinction. Mammals seemed to not be affected in this extinction and thrived after this extinction took place.
The continents on the earth were just beginning to form into the continents that we have today.

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Since the early 1980s, much attention has been focused on the asteroid theory formulated by Walter Alvarez & Luis Alvarez. This theory states that a bolide (meteorite/comet) impact may have triggered the extinction event by ejecting a huge quantity of rock debris into the atmosphere, enshrouding Earth in darkness for several months or longer. With no sunlight able to penetrate this global dust cloud, photosynthesis ceased, resulting in the death of plants and the disruption of the food chain.

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There is much evidence in the rock record that supports this hypothesis. A huge crater 180 km (112 miles) in diameter dating to the latest Cretaceous was discovered buried beneath sediments of the Yucatán Peninsula near Chicxulub, Mexico.
Most notably this extinction brought down the reign of the dinosaurs and many marine and terrestrial invertebrates.
Scientists theorize that a giant meteor struck the earth which accelerated global climate change.

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Most notably this extinction saw the end of the dinosaurs.
After this extinction mammals started to be the dominant species on the face of the earth.

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T-Rex

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Velociraptor

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Pterodactyl