Absolute dating fossils
Index fossils are fossils that only occurred during a specific time frame.Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites make the best index fossils.Very rare, and typically found in very recent fossils.Some examples include freezing (woolly mammoths in the permafrost), mummification, fossilization in amber.The atoms in some chemical elements have different forms, called isotopes.These isotopes break down at a very consistent rate over time through radioactive decay.Potassium-40 on the other hand breaks down much slower and is common in rocks and minerals.
Using relative dating the fossil is compared to something for which the age is already known.
While there is no defined date, typically something must be older than 10,000 years to be considered a fossil.
The oldest fossils in the fossil record date from 3.5 billion years ago, however it wasn't until around 600 million years ago that complex, multi-cellular life began was first preserved in the fossil record.
If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer much be younger in age.
By combining these principals, relative dating techniques can usually be used to determine the approximate age of a fossil.