Fossils: how do they form and what can they tell us?

A 440-480 million year old fossil Trilobite I found in Wales. Trilobites roamed our oceans for over 270 million years before going extinct - in comparison us humans have existed for less than 200,000 years. Finding these incredible impressions makes you wonder what trace will we leave?

Fossils are the remains, trace, or imprint of an organism that has been preserved in the ground since a past geological time. Studying fossils is called palaeontology and helps us understand what the earth was like in the past and how life has evolved over time. They also help identify different layers of rock, which is an important part of exploring for oil and finding water sources called aquifers. 


When an organism dies its body normally decomposes away. The body is often eaten by the microbes that live all around us and within all animal bodies (including our own), recycling the nutrients back into the ecosystem. However, sometimes when the conditions are exactly right the body (or often just its  skeleton) doesn't decay and settles in the mud or at the bottom of the sea or lake bed and becomes buried by sediment. This prevents further decay and decomposition by microbes, who either can’t get enough oxygen or can’t get to the remains to decompose it. Next, a few different things can happen.

One way this can form a fossil is perminerisation (which creates the dinosaur fossils and most ammonites). The sediment builds up over time increasing the pressure on the lower layers and turning the area to rock around the remains. Slowly over thousands of years the remains are dissolved by water seeping through the rock, leaving a hole the same shape as the original remains creating a mould. Over time minerals in the ground water (depending on where) enters the mould, forming a cast of the remains. Over time due to tectonics the rock may rise to the surface and is worn away by erosion to reveal the fossil. 

As previously said, fossils can form in many different ways  and perminerisation is only one way. Fossils can also form by compression, replacement, casting and preservation. All but one of these are similar to perminerisation. The odd one out is preservation, where the actual soft tissue is preserved. Examples of preservation include mammoths preserved in ice or insects captured in amber (like in the original Jurassic Park).


Fossils tell us where they lived and when and can tell us even what they ate and how they died.  They keep a record of long-extinct species we wouldn’t otherwise know about and have shown us that life has existed for over 3.5 billion years, though its difficult to tell the difference between even-earlier lifeforms and patterns naturally occurring non-biologically.  Fossils have given us a record of evolution, showing us where one species ends and others begin. Fossils showed us how the first land creatures shed their shells and grew bones inside instead, find out why they did in my post Changing Fashions: Shells to Skeletons

One example shows us how fossil  have provided information about evolution and also climate change throughout the earths life. In the 1960s fossil colonies of Cyanobacteria showed us that the rise of Cyanobacteria came just before the oxygenation of the atmosphere (making it the likely cause), which led to a mass extinction and the subsequent explosion of aerobic (oxygen breathing) life .

All different kinds of fossils have told us about our own species tale. Pollen micro-fossils found in the teeth of Neanderthals for instance showed that Neanderthals ate a similar diet of plant-stuffs and even cooked and prepared their meals. 

It all makes you wonder what trace we will leave?