They are the remnants of once-living animals, preserved in rocks or (less often) in sediments, amber, ice etc. The term is normally reserved for remains dating back to before the last Ice Age. Hard parts of animals may be preserved with little change in appearance: among invertebrates these include arthropod exoskeletons, the shells of molluscs and brachiopods (another phylum of shelled animals), echinoderm skeletons and jaws and other hard bits from many phyla. Arthropod exoskeletons may be fossilised whole or preserved as thin films of carbon on rocks. Fossilised soft parts have usually been turned to stone, for example by replacement of organic material by minerals from solutions underground.
Living fossils: this is a potentially confusing description of animals that have changed remarkably little over long periods of time. Examples include the brachiopod Lingula, found in Cambrian fossils and persisting today.
Trace fossils are imprints on the environment of the activity of animals long ago, such as tracks, trails, burrows, coprolites (i.e. fossilised faeces) and impressions on soft substrata which may be made even by animals such as jellyfish.