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What is a fossil?
A fossil is physical evidence of former life, from before recorded human history.

How do fossils form? (adapted from http://www.discoveringfossils.co.uk/whatisafossil.htm )

‘Fossilisation’ refers to a variety of often complex processes that preserve organic remains.  It happens under these conditions:

* rapid and permanent burial/entombment – protecting the specimen from environmental or biological disturbance

* oxygen deprivation – limiting the extent of decay and also biological activity/scavenging

* continued sediment accumulation (as opposed to an eroding surface) – ensuring the organism remains buried in the long-term

* The absence of excessive heating or compression, which might otherwise destroy it.

Fossil evidence is typically preserved within sediments deposited beneath water, partly because the conditions outlined above occur more frequently in these environments, and also because the majority of the Earth’s surface is covered by water (70%+). Even fossils derived from land, including dinosaur bones and organisms preserved within amber (fossilised tree resin) were ultimately preserved in sediments deposited beneath water i.e. in wetlands, lakes, rivers, estuaries or swept out to sea.

In the following example a fish is used to illustrate the stages associated with fossilisation… This is just one example, in reality there are countless scenarios that create the conditions necessary for fossilisation


Having reached adulthood and returned to its birth place to spawn, this particular fish reaches the end of its life and dies. Soon after death the body of the fish becomes water-logged and sinks to the seafloor. More often than not the carcass would be pulled apart and scattered by scavenging crustaceans and other fish, however on this occasion the absence of any large scavengers leaves the fish relatively undisturbed.
A fish returns to its place of birth to spawnThe dead fish settles to the seafloor

After several weeks the fish is partially decomposed. Despite the calm conditions on the seafloor, several thousand meters into the bedrock pressure is building along an active geological fault.

Suddenly the stressed rock slips, sending shockwaves to the rock above and causing the sediment nearby to mobilise. The mobile sediment travels across the seafloor burying the fish in the process, in what is often termed a rapid burial event.

Once entombed beneath the sediment the remaining flesh and soft tissue are broken down by bacteria, leaving just the skeleton in the position of burial.
Fish body tissues have mostly decayedMobilised sediment buries the fish

Rapid burial is a common component for optimal fossilisation, as prolonged exposure would otherwise increase the likelihood of disturbance from scavengers and/or currents. Burial may also occur quickly if a high volume of sediment is deposited in the area following a period of heavy rain that delivers sediment from major rivers (for example).

Sediment accumulation and permineralisation

Over time the skeleton is gradually buried deeper by accumulating sediment. Slowly the weight of the sediment compacts the underlying areas, pressing the grains together, driving excess water out, and depositing minerals in the pores, and ultimately turning the soft sediment to hard rock – a process known as lithification.

As this process takes place, minerals contained within the waters-saturated sediment replace the original minerals in the skeleton and fill any voids formed as parts of the skeleton dissolve. The process of mineral replacement is known as per-mineralisation and results in a re-mineralised copy of the original skeleton.

Fish buried beneathe sedimentLayers of sediment build up above the fish

Left: Several months pass and all that remains of the buried fish is its skeleton.

Right: As times passes more sediment accumulates above the fish and the skeleton is gradually compressed and permineralised.

Uplift and exposure

Many millions of years pass and the rock remains buried deep within the bedrock; however tectonic forces associated with the collision between neighbouring continental plates have begun to buckle and uplift the bedrock, raising it above sea level and exposing it to erosion. Gradually, the exposed rock is stripped away, until eventually the top of the fish’s skull is visible at the surface.

Geological forces distort the sedimentThe fossil fish is exposed at the surface

Left: Over time the rock is distorted and uplifted by geological forces associated with continental movement, raising it above sea level.
Right: The uplifted rock is exposed to weathering and gradually erodes away, eventually exposing the tip of the fish’s skull at the surface.

Discovery and extraction

Finally, having lain beneath the ground for millions of years, the partially exposed skull is spotted by a palaeontologist, who undertakes a careful extraction of the skeleton. The process requires patience and precision work to avoid damaging the specimen; a generous amount of rock is retained around the specimen to protect it.

Roy Shepherd extracts the fossil fishChalk fossil fish

What are the types of fossils?
( adapted from http://quizlet.com/1584445/5-different-types-of-fossils-flash-cards/ )

Mold (imprint) fossils – When a leaf, feather, bone or even a body of an organism leaves an imprint on sediment, which hardens and becomes rock

Cast fossils – When minerals fill in the hollows of an animal track, a mollusk shell, or another part of an organism

Fossil fuels – Fuels formed by the remains of dead plants and animals. These include petroleum, coal and natural gas. Although technically a “fossil”, this isn’t usually what people mean by that term.

Actual remains – The body of an organism, with all the parts intact. Usually preserved in ice, amber, or tar. There may be some organic molecules still intact, such as collagens, other proteins, or even short strands of DNA.

Petrified wood – When minerals replace wood or stone to create either petrified wood or a mineralized fossil


What do we learn from fossils?


Fossils are among the most valuable sources of information about the Earth’s history. They tell us about the organisms that lived on Earth from the time of the oldest fossils, about 3.8 billion years ago, to the present.

By studying fossils we can learn not only about the creatures and plants of the distant past, but how they grew, what they ate, how they interacted, and many aspects of their behavior.

Fossils reveal many fascinating facts about the past, but they do a lot more. Do you own anything made out of plastic? Plastic comes from oil, which also provides gasoline, gas heat, and many other necessities of modern life. Fossils are one of the most useful aids to finding oil, because oil tends to accumulate in the pores of particular rock layers.

Rocks of different ages contain different fossils. Study of microscopic fossils brought up in chips of rock during drilling of wells has led to many major oil and gas discoveries. Also, the oil itself is derived from fossil remains of ancient organisms.

Study of fossils has led to important new understanding about how life evolved on earth and about diseases, both ancient and modern. Fossils also help us understand past climates, including ice ages and periods that were warmer than our present climate.

Knowledge from the study of fossils is helping geoscientists understand global warming and its effects. By studying the catastrophic extinction of the dinosaurs and many other life forms at the end of the Cretaceous Period, geoscientists have gained insight into the evolutionary implications of impacts by extraterrestrial objects. Investigating the physical and chemical characteristics of fossil organisms that lived during times of drastic climatic change helps us understand the implications of the changes we are making in our own environment.

Information about Earth history, practical help in finding energy resources, and information that helps us anticipate the effects of possible environmental changes are not the only benefits derived from fossils.

Fossils are beautiful. Many thousands of people collect, buy, sell, and trade fossils all over the world. Some jewelry and furniture are made from fossils, and many stone buildings are made from stone that is composed largely of fossils. Many people collect fossils simply because they are beautiful, but others do so because fossils tell fascinating stories. Neither Barney the Dinosaur nor Jurassic Park would exist if there were no fossils.

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