How do we determine the age of a fossil? How many thousands or millions of years ago did the discovered organism live? How do we identify the geological period to which a formation belongs? Geologists and paleontologists use two primary dating methods: relative and absolute. The relative method is quicker but less precise, while the absolute method requires laboratory analysis to examine the find. By understanding the decay rate of certain isotopes and how much they have decayed, we can accurately determine the age of an organism or, more often, the rock in which it was found.

The relative dating method is used when we already know what and where something was found. For example, if we find a Tyrannosaurus tooth in the Hell Creek Formation, we don’t need a laboratory to establish its exact age. We already know when Tyrannosaurus lived and the period covered by the Hell Creek Formation.

But what if we encounter unexplored layers of rock? In this case, index fossils come to our aid. These are typically specific organisms unique to a particular period. Their age has already been determined using absolute methods. For instance, brachiopods are valuable index fossils. These invertebrates not only help us determine the age of the rocks but also provide insight into the physical and geographical environment of the studied area. For example, brachiopods found on the banks of the Shiderty River in 2014 indicated that the rocks were 345-400 million years old and that the area was once a warm sea with an average annual temperature ranging from +5 to +25 degrees Celsius. Consequently, the remains of other organisms found in the same layer as these brachiopods are also dated to the Devonian period.

It’s possible to use multiple indices. Imagine we find a formation containing known brachiopods dating back 345-400 million years. In the same layers, alongside the brachiopods, we find trilobites dated to 410-390 million years ago. With some simple arithmetic, we can estimate the formation’s age to be between 400 and 390 million years.

Additionally, in the relative dating method, it’s essential to remember that layers are deposited sequentially. If we find fossils whose age we know, the layer above them will be younger, and the layer below will be older.

Precise dating of fossils using the absolute method is done through radiometry (radioisotope dating). Radiometry uses various radioactive isotopes, which act like clocks. The consistent radioactive decay of isotopes helps us determine the exact age of rocks from different geological eras, from the tools of our ancestors to the precise age of the Earth itself.

Volcanic rocks, which are deposited in layers, often assist in determining the age of a fossil. By dating the volcanic layers above and below a fossil, we can estimate the age of the discovered remains.

The challenge with the absolute method is that we may not always find the necessary isotope for a particular epoch. For example, when it comes to radioisotope dating, carbon dating (radiocarbon dating) often comes to mind. However, it is rarely used for dating fossils because its accuracy is best for remains younger than 60,000 years. In that time, the C-14 isotope undergoes 10 half-lives, reducing its amount by 1,000 times.

The half-life is the time it takes for half of a given quantity of isotopes to decay. For C-14, this period is 5,730 ± 40 years. In other words, after 5,730 years, half of the isotope decays, and after another 5,730 years, half of the remaining amount decays, and so on.

But what if we need to determine an age of millions or hundreds of millions of years? Other methods using different isotopes are available for this purpose.

Uranium-Lead Dating involves the use of uranium isotopes: uranium-235 or uranium-238. The uranium-lead method is one of the oldest and best-studied methods for dating rocks that are hundreds of millions or billions of years old. The accuracy of this method is very high; for rocks 2 billion years old, the margin of error is ± 2 million years (0.1%). One of the advantages of this method is its broad age range. The half-life of uranium-235 transforming into lead-207 is 700 million years, while uranium-238 transforming into lead-206 takes 4.5 billion years. Sometimes the uranium-thorium-lead method is used with the isotope thorium-232. The transformation of thorium-232 into lead-208 has a period of 14 billion years.

Lead-Lead Dating examines the presence of three isotopes in rocks: lead-206, lead-207, and lead-204. This method is used to determine the age of meteorites and rocks that have lost uranium-235 and uranium-238 isotopes. The lead-lead method was used to determine the age of the Earth. The ratio of lead-207 to lead-206, resulting from the decay of uranium-235 and uranium-238, respectively, in Earth’s rocks and meteorites, provided the date of the planet’s formation. The most accurate figure to date is 4,567,200,000 ± 600,000 years.

Potassium-Argon Dating is useful because potassium is found in many materials, such as mica, clay minerals, volcanic sediments, and evaporites. Due to its long half-life, the potassium-argon method is used to date fossils older than 100,000 years.

These are far from the only ways to determine the age of a find, whether using relative or absolute methods. We face few difficulties when dating a fossil whose type and location are known. It’s more challenging when a specimen lacks or has lost precise information about its discovery site. This often applies to old museum exhibits, finds by amateur paleontologists, or fossils confiscated from illegal diggers.