QUESTION: What is the minimum amount of time it takes to fossilise something?
ANSWER: Fossils are defined as the remains or traces of organisms that died more than 10,000 years ago, therefore, by definition the minimum time it takes to make a fossil is 10,000 years. But, that is just an arbitrary line in the sand – it means very little in terms of the fossilisation process.
The actual process of fossilisation is highly variable. Really, it is a catch-all term for all processes which yield traces or remains of dead organisms. Assuming we’re talking about preserved remains and not trace fossils, the process of fossilisation that we are usually dealing with is permineralisation, whereby minerals are incorporated into the structure of the preserved tissue (bone, flesh, feathers, etc). During this process, liquids and gases from the living organism are slowly replaced by mineral-rich water. Overtime, the minerals fill the empty spaces and we are left with the familiar structure of a fossil – a living organism that appears to have been turned to stone. Other forms of fossilisation can involve replacement of the original mineral content of the organism with different minerals, or recrystalisation of the same minerals into different forms.
Trace fossils, on the other hand, form when the remains of the organism decay, leaving an empty space or cast of the original organism. Alternatively, they can be formed by compression which can leave a compressed fossil of the original organism, or a trace fossil if the remains are later destroyed.
One thing all types of fossilisation share in common is that they require the dead organism to be trapped underneath layers of sediment. This is what makes the fossilisation such a rare event – if the organism is left exposed after death it will usually be destroyed by decay and scavenging before it has a chance to be fossilised. But, under the right conditions, fossilisation can occur relatively quickly – in the ballpark of hundreds or thousands of years.
Let’s consider just permineralisation – what most of us think of when we think about fossilisation. Permineralisation requires a good availability of minerals such as sulphur, iron, carbon and silica, in the water or sediments around the fossil. The more minerals available, the more rapidly fossilisation can occur, assuming other conditions are optimal.
The speed at which this occurs will also depend strongly on the size of the organism – tiny organisms, eggs or embryos can actually fossilise really quite quickly. Perhaps in a matter of weeks or months. Eggs may be particularly good for rapid natural fossilisation, too, as in many marine species they show adaptations to slow the rate of decay (a good idea if your method of fertilisation involves floating around in the sea for an extended period of time!) which offers a larger window of opportunity for fossilisation. In the laboratory, paleontologists have been able to fossilise lobster and shrimp eggs in just 2 – 8 weeks!
Similarly, researchers in Washington have developed a method that can speed up the process of wood petrification (which begins with permineralisation) in the lab, producing chemically petrified wood in just a matter of days!
Want to Know More?
- PNNL (2005) Instant petrified wood yields super ceramics Press Release
- Martin, Briggs and Parkes (2004) Decay and Mineralization of Invertebrate Eggs Palaeos
- Martin, Briggs and Parkes (2002) Experimental mineralization of invertebrate eggs and the preservation of Neoproterozoic embryos Geology
- Briggs and Kear (1993) Fossilization of Soft Tissue in the Laboratory Science
- Grimes et al (2000) Understanding fossilization: Experimental pyritization of plants Geology
Featured image used under a creative commons license from Wikimedia commons. Original image by Vassil.