How Fossils are made
After their death, some animals were quickly buried by sandstorms, sinking in mud etc.
More and more sediment covered the remains over time.
The harder parts of the animals that didn't rot like bones and teeth were sheathed in the newly-formed sediment.
With no scavengers, quick burial and not much weathering, over time different parts of the animal turned into fossils.
After a long time, the chemicals in the animals' buried bodies underwent a series of changes. Water and minerals seeped into the slowly decaying bone and replaced the chemicals with rock-like minerals. The fossilization process involves the original minerals in the animals' carcass dissolving and being replaced with other minerals.
The result of this process is a heavy, rock-like copy of the original object - a fossil. The fossil has the same shape as the original object but chemically is more like a rock. Some of the Hydroxyapatite (a major bone part) does remain but is saturated with silica.
