What is a Fossil?

There are several definitions of “fossil”. The most complete is; “The remains or traces of past life more than ten thousand years old.” This definition is not perfect because the figure of 10,000 is arbitrary: there is nothing fundamentally different between remains 9,000 and remains 11,000 years old, but as it happens we have ten fingers and think of numbers in tens.

Why are Fossils Special?

Fossils tell us virtually everything we know about the history of life on our world and provide many tantalising clues about our own origins.

The vast majority of living things have not been and will not be fossilised. This is because in most cases, a dead organism will simply rot away, be eaten by scavengers, or be torn into dust by wind or waves.

Even when buried to avoid decay, an organism might be crushed by the pressure exerted by new layers of sand or stone being placed on top of it, or dissolved by chemicals in the rock. The soft parts of creatures, such as muscles and fat, are particularly susceptible to all these forms of damage, and virtually all fossils are of hard parts only. Creatures consisting entirely of soft parts, such as worms, are vary rarely fossilised, although their tracks and traces remain.

Having survived all this, fossils themselves are often obliterated by volcanoes, snapped or squashed by the movement of the earth's crust, or simply buried too deeply for us to ever find them. For these reasons, even the millions of fossils in our possession today account for only a tiny portion of life’s great menagerie.

How are Fossils Formed?

Assuming then, that an organism is not disturbed or digested too early, and that conditions are sufficient, what processes might turn it into a fossil? There are several.

The first is called "petrification". Decaying organisms are exposed to environmental minerals, such as the silica from volcanic ash, which creep into their structures and crystallise, forming a mineral replica of the original creature. This accounts for the silicification (petrification by silica) of ancient trees, which are found petrified in spectacular colours: these indicate the presence of elements like iron and silicon.

More commonly, the organism is buried in sediment containing mineral salts, which gradually replace the organic tissue until a perfect facsimile is produced. The fine details preserved by this method are often astonishing, and sometimes faithful even down to the level of individual cells. Suitable minerals include pyrites, phosphates and calcite.

A second type of fossil is formed when the remains of an organism, buried in sediment, dissolve away and leave behind a mould. Should the sediment undergo diagenesis (turn into a rock), this mould itself is a fossil. However, often percolating mineral solutions fill the mould, creating a cast similar to the directly petrified fossils described above. Often both the cast and the original mould are preserved, the former inside the latter, and can be separated into two separate fossils of the same creature, each the inverse of the other.

A third type of fossil, often preserving soft tissue, forms when the creature is caught in a sticky substance such as tree resin (forming amber) or tar (often mixed with sand), which then undergoes diagenesis, ensuring preservation. Amber in particular often preserves such soft and delicate bodies as ancient spiders, insects, lizards and frogs with a high degree of fidelity, although the actual bodies themselves are often replaced with a thin carbon film. This would make DNA extraction much more difficult than Jurassic Park suggests!

Our definition covers not only remains but also traces of past life. Trace fossils are just that – scratches, trails, footprints, droppings (coprolites) and burrows are often preserved by the methods described above. Some extinct organisms, particularly those with only soft parts, are known only by their trace fossils. Trace fossils also provide valuable information about how organisms behaved, how much they weighed, what they ate and how fast they could move. Common examples are worm burrows and trilobite tracks.

There are a number of modes of preservation that fit our definition but are nevertheless not regarded as true fossilisation. One of them is mummification, the drying out of an organism in sterile conditions, which is only a temporary pause in disintegration and does not produce fossils more than a few thousand years old. The same is true of freezing, which has kept Siberian mammoths spectacularly intact for thousands of years.