All life on earth, from single-celled microbes and simple fungus to dinosaurs and mammals, is compelled to adapt to changes in their environment, which includes efficient competition with individuals of other species, following the process of natural selection. If any species cannot adapt, it will die out, becoming extinct, in other words, totally eliminated.
The fossil record provides strong evidence that of all the species which have ever existed, 99.9 % are now extinct. Millions of species have been extinguished during the earth's history through two fundamentally different processes: background extinctions or mass extinctions.
There has always been, throughout earth's history, a normal background rate of extinction, punctuated by few mass extinctions.
Extinctions caused by moderate environmental changes or normal biological interaction are called background extinctions. Extinctions have been a normal part of the life cycle and are occurring throughout time. The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates per million years. 
These extinctions are caused by small changes in the environment, climate or habitat (a change in average temperature for example, would alter the sex ratio in hatchlings as happens with crocodiles, turtles and tuatara ), depleted resources (e.g. a change in available diet) or by inter- or intra-species competition. An example of species inter-competition is the accidental introduction of a poison snake, Boiga irregularis, into Guam island. This resulted in the extinction of 13 endemic species of birds - a case of background extinction caused by biological (predator-prey) interaction.
Punctuating the process of background rate extinctions are occasional mass extinctions that result in the total elimination of a large number of taxa (groups of life forms collectively categorized taxonomically as kingdom, phylum, class, order, family, genus, or species). Mass extinctions are, relatively speaking, sudden. They result in the global decrease in the number and diversity of life forms. These extinctions have taken place from time to time throughout the existence of life on Earth. Mass extinctions are defined by the four criteria:
- 1. Extinctions occur all over the world.
- 2. A large number of taxa disappear.
- 3. Species belonging to different ecological groups (e.g., terrestrial and marine; benthic and pelagic) disappear.
- 4. The extinctions are clustered in a short amount of geological time (usually, less than one million years).
Major mass extinction events
The Precambrian and Ediacaran Mass Extinctions
- Precambrian time (4.6 billion to 543 million years ago)
- Ediacaran period (523-543 million years ago). 
- Both Precambrian time and Ediacaran period (also known as the Vendian) were host to at least one mass extinction each.
Extinctions are proposed to have affected the earliest organisms on Earth. Approximately 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction. 
The Ordovician-Silurian Extinction'
- Ordovician period (510- 438 million years ago)
- Ordovician extinction (440-450 million years ago)
This extinction, in which an estimated 85% of all species became extinct, is the second only to the Precambrian extinction in devastation. It resulted in the elimination of one third of all brachiopod and bryozoan families, many groups of conodonts, trilobites, and graptolites and ended a large part of the reef-building fauna. In total, more than one hundred families of marine invertebrates ceased to exist in this extinction. 
The Devonian Extinction
- Devonian Period (417 to 354 million years ago)
- 354 ma in which 85% of species became extinct. Approximately 30% of all families were extingushed.
The end-Permian Extinction
About 250 ma at the end of the Permian Period and the beginning of the Triassic in which most notably, trilobites became extinct (an estimated 15,000 species). More than ninety percent of all species became extinct including about fifty percent of all animal families. 
The end-Triassic Extinction
- Triassic Period (251-200 million years ago)
200 ma 76% of species became extinct and the number of families were reduced to 35%. Synapsids  were virtually extinguished and diapsids were able to take advantage of the gaps in the large niche the synapsids left. The only remaining synapsid group is mammalia.
There may have been more than one extinction event, it is not completely clear. The overwhelming event at the end of the Triassic however is clear. Ammonids and bivalves were nearly wiped out, conodonts and basal archosaurs all became extinct.
The end-Cretaceous Extinction
- Cretaceous Period (145 to 65 million years ago)
Theories of mass extinction causes
Approximately 439 million years ago at the termination of the Ordovician and the beginniing of the Silurian a mass extinction took place. One probable cause was brought on by the drop in sea levels as glaciers formed followed by rising sea levels as the glaciers melted. This resulted in the extinction of twenty-five percent of marine families and sixty percent of marine genera (the classification that includes species).
About 250-251 million years ago the Permian Period came to an end. Leading theories of the cause include a comet or asteroid impact, flood volcanism from the Siberian Traps and related loss of oxygen in the seas, and wide-spread volcanic eruptions triggered by the impact of a comet or asteroid. The extinction marking the Permian-Triassic transition destroyed an estimated ninety-five percent of all species: fifty-three percent of marine families, eighty-four percent of marine genera and seventy percent of land species (e.g. plants, insects and vertebrates).
The end-Permian extinction was very sudden in onset. This has been interpreted as an collision with an asteroid or some other type of meteor of very large proportions. However, evidence for the flood volcanism theory indicates that for a period of approximately 600,000 years, volcanic eruptions in the area of what is now Siberia deposited about 2 million km3 of basaltic lava spread over an area about the size of present day Europe to a depth of about 400 to 3,000 meters on the only continent of th time, Pangea. The lava in the Siberian Traps has been dated and its age does coincide with the same period of the Permo-Triassic demarcation period. It took about 100 million years for the levels of post-extinction biological diversity to reach those of the Permian.
At the end of the Triassic, estimated at 199 million to 214 million years ago, there may have been massive floods of lava erupting from the central Atlantic magmatic province and triggering the opening of the Atlantic Ocean. The resultant volcanism may have led to deadly global warming further impacting life on earth. The estimated death toll was twenty-two percent of marine families and fifty-two percent of marine genera.
The transition between the Cretaceous and Tertiary, about 65 million years ago, was marked by an extinction that may have been caused or accelerated by the impact of an asteroid several miles in diameter. The asteroid created the Chicxulub crater on the Yucatan Peninsula and beneath the Gulf of Mexico. There are other possible causes, gradual climate change or massive volcanic eruptions from Indias Deccan Traps. Whatever the cause, the extinction wiped out sixteen percent of marine families, forty-seven percent of marine genera and eighteen percent of land vertebrate families, including the dinosaurs. 
- Raffi S. and Serpagli E., 1993 - Introduzione alla Paleontologia. Utet, Torino (italy), 654 pp. ISBN 88-02-04672-7
- Extinction Thomas J. Herbert, Professor of Biology, College of Arts and Sciences, University of Miami
- Background extinction Kent Holsinger 2007-08-25, University of Connecticut
- Extinction Glossary of terms from Northern Arizona University
- Helen M. Regan, Richard Lupia, Andrew N. Drinnan, and Mark A. Burgman (2001). The Currency and Tempo of Extinction. The American Naturalist, volume 157 (2001), pages 1–10
- What Killed The Dinosaurs? The Great Mystery University of California Museum of Paleontology
- Tuatara Wellington Zoo
- The dates are in dispute. See for example Introduction to the Vendian Period
- Deep Time
- Geological time gets a new period
- Ediacaran Period
- What is Extinction? University of Bristol
- Great extinction came in phases BBC April 1, 2005. Retrieved June 18, 2007
- Boost to CO2 mass extinction idea Helen Briggs, BBC News science reporter Aug. 28, 2005. Retrieved June 18, 2005
- Asteroid 'destroyed life 250m years ago' David Whitehouse, BBC News Online science editor February, 23, 2001. Retrieved June 18, 2007
- Boost to asteroid wipe-out theory Paul Rincon BBC News Online science staff. May 13, 2004. Retrieved June 18, 2007
- Double whammy link to extinctions Paul Rincon, BBC News Online science staff. April 1, 2004. Retrieved June 18, 2007
- often referred to as reptiles with mammal like features
- Archosauria (the "ruling reptiles") were a major group of diapsids. Archosauria were differentiated from other diapsids in that they had antorbital fenestrae (single openings in each side of the skull, in front of the eyes). The only remaining archosauria today are birds and crocodiles.
- Archosaur Lineage University of California Museum of Paleontology
- Synapsida Synapsida Systematics University of California Museum of Paleontology
- What is Mass Extinction? Bristol University
- The Extinctions Bristol University
- When life almost died out European Commission, Magazine of European Research, APril 2005
- The Five Worst Extinctions in Earth's History Lee Siegel Sept. 2000