In paleontology, a zombie taxon (plural zombie taxa) or the zombie effect refers to a fossil that was washed out of sediments and re-deposited in rocks and/or sediments millions of years younger. That basic mistake in the interpretation of the age of the fossil leads to its title, in that the discovered fossil was at some point mobile (or "walking") despite the original organism having been long dead. When that occurs, the fossil is described as a "reworked fossil".