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Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.

Unfortunately, these elements don't exist in dinosaur fossils themselves.

Each of them typically exists in igneous rock, or rock made from cooled magma.

Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.

Types of Fossils There are two main types of fossils: body fossils and trace fossils.

A body fossil is the physical remains of an animal. Some very small organisms, called microbacteria, leave behind microscopic particles called filaments.

The theoretical limit for C-14 dating is 100,000 years using AMS, but for practical purposes it is 45,000 to 55,000 years.

If dinosaur bones are 65 million years old, there should not be one atom of C-14 left in them.

Uranium is present in many different rocks and minerals, usually in the form of uranium-238.

Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree.

Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct. These are: Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.

It's accuracy has been verified by using C-14 to date artifacts whose age is known historically.

The fluctuation of the amount of C-14 in the atmosphere over time adds a small uncertainty, but contamination by "modern carbon" such as decayed organic matter from soils poses a greater possibility for error. Thomas Seiler, a physicist from Germany, gave the presentation in Singapore.

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