How Fossils Are Dated

A Fossil Dating Primer.

The dead speak.  Fossils are the language they use. Fossilization has been a great gift and speaking the language of ‘fossil’ is the only way we know about the history of life on planet Earth. 

At this point in time we have put together a good overview of the history of life on earth but it is far from complete.  Fossilization is a very rare event, there are stringent conditions that have to be met for a living being to become a fossil.  Fossils are precious artifacts. 

At this point you might be thinking, OK, hold on dude, there are fossils all over the place.  But the truth is of all the quintillions of plants and animals that have lived only a very small few have been fossilized for us to find.  No doubt there have been millions if not tens of millions of different species that have lived and died out with no fossil record at all.

This post is about the dating techniques scientists use to estimate a date for fossils.  I have written more on the specific human fossil record here if you are interested.

I am not talking about this kind of dating...

dating a fossil

Paleontology is the science of discovery and study of fossils, Paleoanthropology is the science of discovery and study of the fossils in the human lineage.  So, what exactly is a fossil?

Most fossils are found in sedimentary rocks.  Sedimentary rocks are formed, as the name would imply, from sediments of material at the earth’s surface.  This sediment material is made by erosion and weathering then moved to a place by wind or water and settled into a process called sedimentation.  Sedimentation can also occur from minerals and even shells of creatures that will settle out of suspension from water.

So, a likely scenario for fossil creation is an animal perishes from drowning, or a flood or quickly after death is covered up by sediments and then through time is fossilized as the sediments turn into sedimentary rock. 

Turns out if you ask most people what a fossil is they are likely to say something about bones or dinosaurs.  True enough for sure. However, there are several types of fossils which although not as sexy as old bones are still fossils nonetheless. 

Petrified and permineralized fossils

Petrified bone is the main way we think of fossils.  The process plants and animals go through to become petrified is called ‘Petrifaction’ and petrifaction is accomplished mostly by permineralization. When you see fossils in museums and they are almost certainly form by permineralization.

Here is an example…

T Rex fossil

Trace Fossil

Trace fossils are basically the traces that a living being has left behind to indicate its existence.  These trace elements can be tracks or footprints, nests, eggs, excrement (coprolites) and even bitemarks or claw marks.

Here is an example of footprints from the Triassic…

footprint fossils

Here are some tracks from a slug like animal…

fossilized slug tracks.

Amber Fossils

Amber fossils are really cool and who hasn’t seen the Jurassic Park movie and the essential part amber fossils played in the reconstruction of dinosaur DNA.  Tree resin traps insects, spiders, pollen and other organic material before becoming amber.

Here is an example of amber with an insect entombed…

amber fossil

Chemical fossils

Chemical fossils are substances like fossil fuels.  They are formed by the build up or organic matter under high pressure and temperature coupled with the action of bacteria.

Subfossil

A Subfossil is classified as a fossil not yet completed.  A once living being that may or may not actually complete the fossilization process.

 Living fossils

There is also a category of fossil called a Living Fossil.  These are animals that still live but have changed little in millions of years.  The Nautilus and the Horseshoe Crab and examples.

Dating Fossils

how to date a fossil

All things considered, the accuracy of fossil dating decreases with the age of the fossil, the older the fossil the harder it is to pin down an exact date.  Nevertheless, there are several techniques that can be used to determine age.

The two methods of dating are the relative and absolute.  Fossil sites often employ both methods when dating.

Relative Dating

In the most simple terms, relative dating uses the concept that the deeper layers of the earth will be older than the layers above them. Estimates for the age of fossils can be made from knowing the age of the layer of earth it was found in. 

Absolute Dating

Radiometric or radioactive dating is the main method for dating fossils.  Radiometric dating uses the half-life of isotopes in carbon, uranium and potassium. 

The most commonly known radiometric dating is carbon 14 dating.  Archaeologists use carbon 14 dating quite a bit but for Paleontology and Paleoanthropology the half-life is too short.  The half-life of carbon isotopes is 5,730 years.  What the means is every 5,730 years half of the unstable isotopes of carbon become stable, a measurable effect.  The oldest reliable date you can get with carbon 14 is 50,000 years and that is not nearly old enough for dating the human lineage not to mention other animals like dinosaurs.

Uranium and potassium have a half-life of over a million years allowing for the use in dating of older artifacts.  But how is this accomplished?

Well we have sort of a challenge here. Fossils and the sedimentary rocks they are formed in do not contain uranium or potassium.  However, igneous rock or rock made from cooled magma do contain uranium and or potassium. 

Conveniently volcanic ash contains igneous rock.  Considering the volcanic activity of the earth’s history there are layers of volcanic ash laid down at regular intervals. And the best part is you don’t need a thick layer at all, minuscule particles will do the trick.

This is how it works.  Scientists use a technique called bracketing to determine the age of a sedimentary rock layer.  They age the layer right below the sedimentary rock and right above it.   Using this bracketing method scientists have recorded the age of rock layers worldwide.

Other Dating Methods

There are a few other dating methods used to augment the above.

Luminescence dating which includes optically stimulated luminescence, infrared stimulated luminescence and thermoluminescence dating.  All of which use exposure of mineral grains to sunlight and heat to determine age.

Analyzing amino acids, using tree rings and even using the magnetic shifts of the earth’s poles can be used in some cases to help determine age.

Dating Hominin Fossils

When we are talking about the human legacy comparative analysis is also a very useful tool in fossil age determination.  Presently we have established a really good idea of the timeline for hominins.  New discoveries such as the Rising Star Cave remains of Homo naledi although wonderful in its own right do not change the overall understanding of the human lineage’s timeline.

Today or tomorrow a discovery could be made that will amaze us all, however, when that happens we will be adequately equipped to feel confident enough in the dating of any new discovery using all the tools we have available to us right now

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