Fossils come in three forms: replacement fossils, cast fossils and mould fossils. A replacement fossil occurs when the actual cells of the organism are replaced with minerals. Mold fossils occur when the organism decays, leaving only an impression in the stone behind. Cast fossils occur when new minerals fill a mould fossil, creating an exact duplicate of the organism.
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The Fossilization Process
Fossilisation occurs over a span of millions of years. The process takes a very long time because it relies on minerals, carried by water, to turn organic material to stone. In the case of replacement fossils, certain chemicals are washed away over time and replaced by other minerals. Mold fossils slowly turn to stone because they are under great pressure, usually from a large amount of water, that hardens them. The water also leaches away the organic material inside, leaving a cavity. In the case of cast fossils, it takes a long time to build up layers of mineral inside the cavity of a mould fossil.
Some organisms are more prone to fossilisation by specific methods. In general, hard materials such as bone and shell are much more likely to become fossilised than soft materials that can easily decay, such as plant matter. However, plant fossils do exist, primarily in the form of mould fossils, because although they decay quickly they can leave an impression behind in the earth which later hardens into stone.
Fossils form in sedimentary rock, which is rock deposited over a period of time, often by rivers and lakes. Examples of sedimentary rock include limestone, shale and sandstone. The exact kind of stone a fossil is preserved in gives clues as to what the environment was like during the preservation process. For example, sandstone fossils indicates either desert conditions or shallow, silty water.
Two for One
Depending on modern environmental conditions, it is entirely possible for a fossil hunter to find both the cast and mould fossils of the same organism, since the cast fossil may actually still be embedded in its mould. In fact, in some areas hunters can pry up chunks of sediment riddled with embedded fossils.
Cast and Mold Fossils vs. Trace Fossils
There is another, less common kind of fossil that can be easily confused with cast and mould fossils. These are trace fossils, and they show signs of animal behaviour. For example, a preserved dinosaur footprint may look like a cast fossil. In fact, it underwent many of the same procedures. However, the distinction lies in that it is the result of the behaviour of the organism rather than the deteriorating body of the organism.
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