The end of the dinosaur age

By the end of the Cretaceous period, there were many species of dinosaurs, all very specialised and successful. Then, 65 million years ago, at the point in time between the Cretaceous period and the next period, called the Tertiary period, dinosaur life ceased to exist. Dinosaurs were not the only ones affected - 70 percent of the Earth's species were killed in this mass extinction, which is referred to by scientists as the K-T extinction. In this term, the 'K' refers to the Greek work kreta, which means chalk (a mineral found in many Cretaceous deposits), while the 'T' refers to the Tertiary period.

The K-T extinction is not unique, and it is not even the largest mass extinction. There have been five major and at least twelve minor extinctions in the last 650 million years. The largest of these was the extinction at the end of the Permian period, which was the last period before the Mesozoic era began. From 90 to 95 percent of all of the Earth's species died out, allowing the dinosaurs to appear.

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Organisms affected by the K-T extinction

During the K-T extinction, all the dinosaurs died out. They were not the only organisms, or living things, to become extinct, however. Pterosaurs, ichthyosaurs, plesiosaurs, some fish species, shelled molluscs such as ammonites and belemnites, and a number of plant species also disappeared during the K-T extinction.

Other organisms did survive the K-T extinction, however. Mammals, some birds, crocodiles, turtles, lizards, snakes, amphibians, insects, some marine animals and flowering plant species survived. They were the ancestors of similar plants and animals that are alive today.

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Many of the animals that survived were smaller animals. Some scientists believe they were able to survive because they ate a variety of foods, while the diets of some of the larger dinosaurs were more specialised. After the K-T extinction, organisms were able to evolve without competition from the dominant dinosaurs.

Evidence for the K-T extinction

The main evidence for the K-T extinction is that around 65 million years ago the fossil record of dinosaurs and other organisms simply stopped. This suggests that something killed many organisms.

Another piece of evidence for the K-T extinction is a layer of clay at the K-T border, or the border between rocks made during the Cretaceous period and rocks made during the Tertiary period. This border is thought to have been deposited between 65 and 64.5 million years ago. This clay has high levels of iridium, which is an element that is not common on the surface of the Earth. Many scientists believe that the iridium layer at the K-T border is somehow connected with whatever killed the dinosaurs. There is also evidence that the climate in the Tertiary period was cooler than it had been in the Cretaceous period.

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What caused the K-T extinction?

There are a number of theories that attempt to explain the great extinction at the end of the Cretaceous period. Some of them are given little credit by scientists. Some people believe there is a twin to our sun, for example, called Nemesis, that comes close to the Earth periodically. Others believe that the dinosaurs became sick with a mysterious disease, or that cosmic rays from some extraterrestrial event irradiated the Earth. These theories are not very widely believed because they cannot be proven or disproven. In addition, many of them do not account for the extinction of other animals, especially marine animals, and most of them do not entirely fit in with the evidence, such as the iridium layer at the K-T border.

There are two main groups of theories that are widely accepted among scientists today: Earth-based theories and extraterrestrial theories. Earth-based theories suggest that something that happened on Earth was responsible for the K-T extinction. These theories state that plate tectonics, combined with volcanic activity, caused organisms to become extinct by changing the Earth's climate and making it cooler. Extreme volcanic activity could block out the sun's rays, making the temperature drop rapidly. Plate tectonics could also disrupt the Earth's climate by changing worldwide ocean and wind currents. Earth-based theories state that the extinction took place gradually, over a long period of time. These theories are supported by the iridium at the K-T border because iridium is found deep within the Earth's core, and volcanic eruptions could bring it to the surface.

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Extraterrestrial theories suggest that the K-T extinction was caused by something outside Earth. This commonly takes the form of an asteroid collision, or many asteroid collisions, with the Earth. An asteroid collision would throw tonnes of dust into the air, blocking out the sun's rays and cooling the climate. There is evidence that an asteroid large enough to cause worldwide disturbances did hit the Earth around 65 million years ago near the Yucatan Peninsula in Mexico, Central America. The asteroid theory is also supported by the iridium layer, as iridium is found in some types of asteroids.

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Both of these groups of theories are widely accepted by many members of the scientific community. It is very difficult to explain what happened to the dinosaurs because no human beings were present, and the only data we have is from fossil and geological records.

Birds - a living legacy?

Some scientists believe that in one sense, there are still dinosaurs alive today. These 'dinosaurs' would have evolved significantly, probably from smaller dinosaurs during earlier parts of the Mesozoic era. They evolved to grow feathered wings instead of front legs or arms, allowing them to fly, and their back legs developed specialised feet used to grasp branches. Today, we call these 'dinosaurs', birds. While not all scientists are in agreement as to whether or not birds are directly descended from dinosaurs, it is interesting to think that in some way, dinosaurs possibly did survive the mass extinction 65 million years ago.

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