Fossil Guide - what are fossils?
This new page is intended to provide information to schools on fossils - what they are, how they form and how they change over time. If you have a question about fossilization you feel we ought to add to this page then please contact us and we will see if we can provide an answer here. The content will gradually grow as we add more answers. Pictures and diagrams will also be added where possible.
For general inquiries about geology and fossils you can also visit our Frequently Asked Questions (FAQ) webpage on the dinosaurs page of our website.
What is a fossil?
A common dictionary definition of fossil is: the remains or impression of a plant or animal hardened in rock.
They are the remains of a once-living organism, generally taken to be one that lived prior to the end of the last glacial period, i.e. fossils are older than 10,000 years. Most dictionary definitions refer only to animals or plants but now we would say that it was any once-living organism because we have to include those microscopic creatures from other kingdoms. The term includes skeletons, tracks, impressions, trails, borings, casts and coprolites.
Fossils are usually found in consolidated rock, but can also be found in softer mudstones and shales. They are most commonly found in sediments, but rare specimens are found in some igneous rocks (compressed ash) and low-grade metamorphic rocks (slate and marble).
There are grey areas where there may be some debate about whether the object has become a fossil (for example wooly mammoths living 20,000 years ago were recovered from the frozen tundra of Siberia - are these true fossils or just ancient deep-frozen carcases?). An object that is on its way to becoming a true fossil is sometimes called a 'sub-fossil'.
In its original sense, fossil meant anything dug up from the earth, including ores, precious stones etc. The modern use of the word dates from the late 17th Century. The word 'Fossil' comes from the French fossile, from the Latin fossilis, from fodere foss, meaning dig).
Fossils as a key to the past
Excavated fossils, and the rocks they came from, can provide a glimpse into environments of the distant past. Long before the Earth was discovered to be billions of years old philosophers debated the strange objects that were appearing in old quarries and crumbling cliffs.
Today we use fossils to help produce charts of temperature change in the sea and atmosphere, and to establish the shape and global position of pieces of the Earth's crust before they formed today's continents.
How do fossils start to form?
First of all an organism has to die. If it is accidentally buried in sediments (muds, clays and sometimes grits and sands) it stands a chance of becoming a fossil. If it isn't buried then scavenging and decay will reduce the body (of the plant or animal) and it will never become a fossil. The sediments that cover the body are the result of natural weathering of other rocks and minerals and it is these that will determine how hard the fossil becomes, which parts preserve and what colour it may become. If the organism is buried on the sea-floor then a steady rain of fine material (dead plankton, shell fragments and mud) wil eventually cover it and anything else that doesn't move.
Why are fossils from land animals and plants rarer than those from the sea?
Organisms that lived on the land are rarely buried quickly enough. Other animals eat them and the weather destroys the remains if they are left on the surface. Although there are many land animals and plants available to turn into fossils very few end up in the fossil record. Those land organisms that do end up fossilized have usually been rapidly covered in soft muds as the result of flooding, or have fallen into ponds or rivers.
Can fossils tell us what happened before the organism became a fossil?
Yes, sometimes we find teeth marks preserved on bones; or marks where creatures like snails tracked their way across the bones; or piles of crunched bone within a skeleton where something was munching its way through. We have also found thin lines of black carbon around dinosaur bones - evidence for the former roots of plants growing down into the buried dinosaur. This is an area of special study called taphonomy - which means 'the laws of burial'.
After initial burial, what happens next?
Once the creature, or plant, is fully covered the risk of being disturbed goes down. The soft tissues that are left may rot away to leave just the hard parts like bones, teeth, stems and seed cases. More muds and other sediments like sands can build up and the organism gets buried under increasing depths of sediment.
The ground may become saturated with water and heavy minerals like iron compounds and barites and the temperature and pressure gradually increases. Sometimes the pattern of any soft parts may be preserved as imprints. Soft parts get crushed.
The individual particles of the sediment get squashed closer together and the rock can become 'lithified' - turned to stone. Some sediments like limestones, sandstones and mudstones can become very hard, but other like shale and clay may remain soft and friable. Fossils preserved in softer rocks may themselves be very fragile. Over time the fossil may continue to change depending on what subsequently happens to the rock.
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