Fossilisation stages. Illustration. showing the process by which organic matter fossilises. At far left, a Stegosaurus dinosaur (Late Jurassic, 150 million years ago) dies on the bank of a river. The body becomes covered in sediments and the gradual replacement of the organic matter with minerals (red arrows) forms a fossil, preserving the shape and structure of the original organism. The exposure of the fossil is shown at far right following erosion of the overlying rock layers.