When a plant or animal is buried quickly, it gets enclosed in sediment before it decomposes. As pressure transforms this sediment into rock, a hollow mould of the organism is formed. Gradually minerals seep into this hollow and harden over time to form a detailed, three-dimensional cast. Soft tissue organisms are preserved as impressions between layers of sediment. Perfectly preserved fossils of insects and other small forms of life have also been found trapped inside hardened tree sap.

The most common way an animal such as a dinosaur fossilises is called petrification. These are the key steps:

1. The animal dies.

2. Soft parts of the animal’s body, including skin and muscles, start to rot away. Scavengers may come and eat some of the remains.

3. Before the body disappears completely, it is buried by sediment – usually mud, sand or silt. Often at this point only the bones and teeth remain.

4. Many more layers of sediment build up on top. This puts a lot of weight and pressure onto the layers below, squashing them. Eventually, they turn into sedimentary rock.

5. While this is happening, water seeps into the bones and teeth, turning them to stone as it leave behind minerals.

This process can take thousands or even millions of years.

Credit: Natural History Museum

Picture Credit: Learning Geology

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