Earth's Major Extinctions

Geolor's Earth Issues
Earth's Major Extinctions

Extinction

Extinction is the result of animals or plants being unable to adapt to changing conditions. In some cases, scientists have identified the problem which resulted in the loss of an entire species. More often, extinction is created by a complex interaction between many problems, as in the case of the dinosaurs.

Extinction is the process in which groups of organisms (species) die out. If the birth rate is less than the death rate over time, extinction results. Extinction is a natural result of evolution. Species go extinct when they are unable to adapt to changes in the environment or compete effectively with other organisms. Well over 99 per cent of the species that have ever lived have gone extinct. Credits

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). Credits

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:
1. Catastrophic Agents- such as meteorite impacts and comet showers,
2. Earth Agents- such as volcanism, glaciation, variations in sea level, global climatic changes, and changes in ocean levels of oxygen or salinity Although these agents can explain mass extinction, the causes of mass extinction events remains relatively unknown. Credits

Geologic Time Charts

Here is a A Concise, but short Geologic Time Chart of Earth's History.
Here is a more detailed Geologic Time Chart.
A Time Scale With Graphics - Long Version.
A Shorter Version of the Time Scale with graphics.

The Nemesis Hypothesis

Nemesis hypothesis of Raup and Sepkoski, theorizes that there is a periodicity of 26 million years to mass extinctions which is caused by collisions with comets from the Oort cloud as they are perturbed in their orbits by a dark star (a companion star to the sun). The search for the "Nemesis Oort cloud" continues.

Major Extinction Events Overview

The five largest mass extinctions in Earth's history occurred during:

The late Ordovician period (about 438 million years ago) - 100 families extinct - more than half of the bryozoan and brachiopod species extinct.

The late Devonian (about 360 mya) - 30% of animal families extinct.

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.

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).

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 urchins and many others. Credits

On the Destiny of Species

Marine Fossil Record Shows Extinctions Happened at Regular Times

A Mathematical Model for Mass Extinction

Minor Extinctions of Earth History Triassic, Jurassic, Oligocene and Neogene.

Patterns of Extinction The concept of periodicity.

BBC's The Extinction Files: Mass Extinctions: Introduction This website covers Late Cambrian to the End Of the Cretaceous; discusses the Fossil Record and Theories of Extinction for each Period.

Changing Paleoclimates and Mass Extinctions: A New Model for Climatic Change

Phanerozoic Extinctions

Mass Extinctions Of The Phanerozoic Menu
This website describes extinctions and their causes from Precambrian all the way into the Holocene. It covers, in addition to Precambrian, the Cambrian, Ordovician, Devonian, Permian and Late-Cretaceous Era extinctions, then ends with the Holocene epoch.

Speculated Causes of the Permian Extinction

The Permian Mass Extinction Geological setting, species affected and causes.

Extinctions Tied to Impact from Space
Update on the cause of the Permian Extinction; week of Feb. 24, 2001.

The Great Dying
"250 million years ago something unknown wiped out most life on our planet. Now scientists are finding buried clues to the mystery inside tiny capsules of cosmic gas."

The Permo-Triassic extinction Overview, species affected and causes; Easy to navigate menu.

Earth's Climate During the Permian Extinction Universe Today.

The Great Dinosaur Extinction

The Extinction of the Dinosaurs: Most dinosaur species perished in background extinctions that occurred throughout their Mesozoic Era. The other dinosaur species died out during the several mass extinctions that occurred in the Mesozoic: at the end of the Triassic (213 million years ago), during and at the end of the Jurassic (at 190, 160, 144 mya), and during and at the end of the Cretaceous (at 120, 82, and 65 mya). Credits

-- THEORIES --

Asteroid Impact Extinction Theory

Catastrophism and Mass Extinctions

Meteorite Impacts and Mass Extinctions

The Alvarez Asteroid Impact Theory

Cretaceous-Tertiary Extinction

Dinosaur Extinction Page

Extinctions Due to Impacts, Past and Future

Dinosaur Extinction: The Extinction of the Dinosaurs: A Research Program at the Milwaukee Public Museum

The Cretaceous-Tertiary Asteroid Theory

Sea Clue to Death of Dinosaurs BBC News

The Asteroid Hypothesis Dinosaur Extinction: Smithsonian Dept. of Paleobiology.

NewScientist.Com Reports on new discoveries relating to dinosaurs; both current and archived reports.

The KT Event This series of web pages has been developed by the Year IV MSci Geology students at the University of Bristol.

Links are:

101 Crazy Theories about Dinosaur Extinctions
The pattern and timing of extinction
Evidence for Late Cretaceous Climatic Change: Isotopic and Palaeobotanical
Gradual extinction and climate change
Timing and Rate of Other Extinctions; What else died out?
Evidence for Impact, 1980-1990
The Chicxulub Impact Site - Discovery and Effects
Impact Geology, Chemistry and Physics

Each page links to even more information. Good job!