Fish fossils (Knightia sp.) preserved in rock. The fish, around 10 centimetres long, are related to the herrings (Clupea sp.) found in today's seas. They are extremely common in the Green River Shale lakebed formation in Wyoming, USA, and date to the Eocene Epoch of 55-38 million years ago. Changes in the lake climate killed many fish, some of which were preserved as fossils. A fossil is created when an animal is quickly buried in sand, mud or volcanic ash after death, and the normal decay processes do not occur. The minerals of the hard parts of the animal, such as shell or bone, are gradually replaced with rock.

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