How Do We Know How Old The Fossils Are?

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Most of us are already familiar with fossils and how they are formed. However, have you ever wondered how the age of these fossils is determined?

For palaeontologists, there are two methods available to determine the age of a fossil. These are:

  • Absolute Dating
  • Relative Dating

Absolute Dating Method

Absolute dating uses radiometric dating methods to determine the age of a fossil (or a rock). Essentially, this method uses the radioactive minerals found in fossils and surrounding rocks. Interestingly, these radioactive minerals almost act like a geological clock, providing a clear-cut age of a fossil.

As we all know, the atoms in certain elements have various forms called isotopes. Over time, these isotopes tend to “break down” at a constant, measurable rate. By measuring the ratio of the parent isotope (Original) to the amount of the daughter isotope that it breaks down into, the age of the specimen can be determined.

This rate of radioactive decay is defined as half-life. For instance, assume a radioactive isotope has a half-life of 10,000 years; so after 10,000 years, exactly half of the radioactive isotope would have decayed from its parent isotope. The same can be said for the next 10,000 years.

In archaeology, carbon dating is a popular method of estimating the age of artefacts and fossil remains. However, since it has a half-life of 5730 years, it is unsuitable for dating fossils older than 75,000 years. Hence, potassium-40 is used as it has a half-life of 1.2 billion years and is quite abundant in fossils and surrounding rocks.

Relative Dating Method

Most of the time, the age of fossils is found through relative dating. As the name suggests, to estimate the age of a fossil, it is compared to other fossils or rocks where the age is already known.

However, if the age of the surrounding rock formation where a fossil was found has not been dated, then palaeontologists resort to using index fossils to estimate the age via correlation. Typically, specimens which are used as index fossils are known to only occur within a very specific age range. Moreover, index fossils have very wide geographic distributions – such as trilobites, brachiopods and ammonites. Hence, if an unknown fossil occurs alongside any of these index fossils, then the unknown fossil’s age range may fall within the age range of the selected index fossils.

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Main Article: Everything You Need to Know About Fossils

Frequently Asked Questions on the age of fossils


What are the methods available to check the age of fossils?

There are two methods available to determine the age of a fossil, these are:

  • Absolute Dating
  • Relative Dating


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