EXTINCTION
[Fossils]
A species of animal becomes extinct when every one of its kind has died. Extinction is the ultimate fate of all species, becuase it has been happening since life appeared on earth.
There are many reasons for extinction. A species can be competitively excluded by a closely related species, where the species lives can disappear and/or the organisms that the species exploits could come up with an unbeatable defense. Some species enjoy a long tenure on the planet while others are short-lived.
Most species of animals living today can be traced back as fossils only a few million years. Fossils show that extinction rates have varied through time, with four or five episodes of mass extinction interrupting periods when the diversity of living things increased.
The history of life on this earth includes many episodes of mass extinction in which many groups of organisms were wiped off the face of the planet. Mass extinctions are followed by periods of radiation where new species evolve to fill the empty niches left behind. It is probable that surviving a mass extinction is largely a function of luck.
In the greatest known extinction episode - at the end of the Permian period, 250 million years ago - 96 percent of ocean species and more than 50 percent of all species disappeared. This coincides with the formation of Pangaea II, when all the world's continents were brought together by plate tectonics. A worldwide drop in sea level also occurred at this time.
The most well-known extinction occurred at the boundary between the Cretaceous and Tertiary Periods, 65 million years ago. This extinction eradicated the dinosaurs. The event was probably caused by environmental disruption brought on by a large impact of an asteroid with the earth. Scientists have discovered in Mexico a buried impact crater (thought to be from a large comet striking the earth) that is 200 km (124 mi) in diameter. They have found that the impact and the dinosaur extinction occurred nearly simultaneously.
When the dinosaurs died out, it was the best thing that ever happened to the mammals, because the mammalian radiation occurred. Mammals coexisted for a long time with the dinosaurs but were confined mostly to nocturnal insectivore niches. With the eradication of the dinosaurs, mammals radiated to fill the vacant niches. After millions of years, almost all of the large animals on earth were mammals. The Age of the Dinosaurs was over. The Age of the Mammals was beginning.
[Triceratops] [Rhino]
To find food and survive in these new niches, mammals evolved bodies that were often similar to the reptiles they replaced. For example, bulky dinosurs with horns were replaced by bulky mammals with horns.
[Pterosaur] [Bat]
Pterosaurs died out and the bats replaced them. Like the pterosaurs, bats had wings of skin.
[Plateosaurus] [Mammoth]
Large plant-eating dinosaurs with long necks were replaced by large plant-eating mammals with trunks. The trunks worked like long necks to reach food.
[Velociraptor] [Tiger]
Meat-eating dinosaurs with claws and sharp teeth were replaced by meat-eating mammals with claws and sharp teeth.
[Ichthyosaurus] [Dolphin]
In the ocean, marine reptiles with sleek bodies and sharp teeth died out. They were replaced by mammals with sleek bodies and sharp teeh.
Currently, human alteration of the ecosphere is causing a global mass extinction.
According to one theory, mass extinctions result from drastic environmental changes (e.g. fire, acid rain, global cooling) that follow events such as meteorite or comet impacts or massive volcanic eruptions. Supporters of this theory believe that the extinction of dinosaurs took place over several million years.
An opposing theory holds that an asteroid or comet crashed into the earth and caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs.

[T-Rex] [Stegasaurus] How Do We Know Dinosaurs Didn't
Live Beyond 63 Million Years Ago?

In layers of rock that are between 63 and 90 million years old, there has been many dinosaur fossils. In the newer rocks immediately above the 63-million-year-old marker, there are only skeletons of reptiles and many mammals, but no dinosaur skeletons.
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