Ecology And Environment

Major Earth Extinctions five



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Extinction takes place when a species of animals or plants can no longer adapt to the changes in the Earth. Rather than just one problem there are many combinations and interactions of different situations that cause extinction to come about.

If the birth rate is lower than the death rate, this will, over time, cause extinction. If a species cannot compete for food, shelter etc.. with other species, it will go into extinction. This the natural result of evolution. It is believed that ninety-nine percent or more of all species that ever roamed the earth is now extinct. Extinction has no preference to land or sea. Animals suffer at the hands of extinction more than plants. Plants are resistant to mass extinction. Extinction happens roughly every 26 million years or so.

Knowing the timetable helps scientists determine what caused the disappearance of species in that event.

There were five major extinction events on earth:

* The late Ordovician period
* The late Devonian period
* The Permian Period
* The late Triassic period
* The Cretaceous-Tertiary

The late Ordovician period which occurred about 438 million years ago wiped out one hundred families of plant and animal species.

Life was plentiful and flourished in the Ordovician period but by the end of the period significant mass extinction occurred. Mollusks and arthropods where the dominant species of the oceans. Diversity of life on land was yet to take place.

Mass extinction toward the end of the Ordovician period affected planktonic forms and some group of trilobites which died out completely and the Asaphida were greatly reduced. It is speculated the late Ordovician-Silurian events were caused by an ice age due to it being one of the coldest periods in Earths history in the last 600 million years.

The ecological system of that time reached a complexity that persists until the present day. This was a time of rich sea life and trilobites and brachiopods in particular. Reef forming corals began to appear along with an abundance of calcifying animals.

The late Devonian period, about 360 million years ago, left roughly thirty percent of all animal families extinct. The Kellwasser event took place in this period and the Hangenberg Event, as a second mass extinction, closed this period.

Although there was a massive loss of biodiversity during the Devonian period, it is not clear as to the time period the extinction took place. It is also uncertain whether it was one final blow or if the extinction took place over several million years.

Plants and insects had colonized the planet during the Devonian period and corals built massive reefs in the oceans. The extinction seemed to target creatures of the oceans with brachiopods, trilobites and corals being the hardest hit. Corals were nearly totally extinct and only returned during the Mesozoic. It is uncertain as to what caused these extinctions.

Sea level changes and ocean anoxia, triggered possibly by global cooling or/and oceanic volcano are among the theories which caused this extinction. An extraterrestrial comet or some other object impacting the earth could also be the cause. Rather than a single event, this could have been caused by some new invading species. Jawed vertebrates were apparently unaffected by the Kellwasser event.

The world looked completely different during the late Devonian period. The continents were joined together at that time making a super continent, Gondwana covered much of the southern hemisphere, Siberia occupied the northern hemisphere and the equatorial continent, Laurussia was drifting towards Gondwana. Laurussia was formed by the collision of Laurentia and Baltica.

Plants developed roots which allowed them to live on dryer land away from areas that were constantly wet. This led to huge forest on the highlands. Tetrapods were evolving leg like appendages.

The Permian period was roughly 245 million years ago. The world was hot and dry during the Permian period. A single giant continent dominated Earth called Pangaea which contained vast deserts. This period ended most extinctions in Earths history, 90% of marine and 70% of terrestrial species were eliminated.

Sea Levels were low which could have caused the mass extinction of sea life that depended on shallow coastal areas for their survival. The Tethys ocean dominated the Mesozoic era. The climate varied between extreme hot and cold. The first modern trees came into existence during the Permian period.

By the end of the Permian period, trilobites and many other marine life were extinct. Fungi, arthropods and different types of tetrapods were included in the very diverse terrestrial life.

Relatives to the cockroach were the most successful in the Permian period, as it had a number of formidable advantages over other animals. Roughly 90% of the insects of this period were cockroach like, with the dominant aerial predators being dragonflies. The Permian period is the only known period of Mass extinction of insects.

The late Triassic period was about 208 million years ago,  and during this period 35% of animal families went extinct. The time span of the Late Triassic was between 228 million years ago and 199 million years ago.

Plateosaurus, Coelophysis, Eoraptor and many other dinosaurs evolved during this period. Fossils of this area are found in Africa but they are more common in the south than the north. Africa's strata from this period has not been thoroughly researched but the boundary separating the Triassic and Jurassic pinpoints an extinction of global impact.

In the area known as Tubingen (Germany) characteristic for the boundary of Triassic and Jurassic, a bone bed can be found.

The Cretaceous-Tertiary extinction occurred about 65 million years ago. It is widely known as the K-T event, associated with a geological signature known as the K-T boundary, a thin band of sedimentation which can be seen in certain parts of the earth.

This mass extinction event is the marking of the end of the Mesozoic era and the starting of the Cenozoic era. Many researchers call the event by a different name, the Cretaceous-Paleogene extinction event.

Finding non-avian fossils below the K-T boundary demonstrates that non-avian dinosaurs went extinct during the boundary event. Some dinosaur fossils were found above the K-T boundary but they were fossils that were eroded from their original resting place and preserved by later sediments.

Theory has it the K-T extinctions were caused by massive asteroid impact or volcanic eruption. These events could have blocked out the sunlight upsetting the ecology of the earth. Many believe the extinction was due to a combination of events over time.

Species who were depending on photosynthesis became fewer in number or went extinct due to the blockage of the sun and it caused a major reassignment of dominant plant groups. Herbivorous creatures faded away as the plant life they depended on vanished. This lead to the extinction of animals like the Tyrannosaurus Rex.

Probably, the omnivores, insectivores and carrion eaters didn't become extinct because there was an increased amount of food source for them. Few animal groups in stream communities suffered extinction because they fed on detritus ( non-living food) that washed into the streams from land.

There is some debate regarding mass extinction and expansions through the K-T boundary. This has been studied since the 1930's.

There is good evidence of extinction of jawed fish across the K-T boundary with a large fossil record. About 80% of sharks, rays and skates families survived the extinction along with 90% of boney fish families.

All scientist agree that non-avian dinosaurs died out at the K-T boundary. Since they weren't capable of burrowing beneath the earth or swimming, they had no way of protecting themselves from the worst part of the environmental disaster.

Some dinosaurs did survive the K-T event, however, and live to this very day, and today they are known as birds.

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ARTICLE SOURCES AND CITATIONS
  • InfoBoxCallToAction ActionArrowhttp://www.geolor.com/EarthIssues/Earths_Major_Extinctions_geolor.htm
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Ordovician
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Late_Devonian_extinction
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Permian
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Late_Triassic
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Cretaceous%E2%80%93Tertiary_extinction_event