magnetofossils are the fossil remains of magnetic particles produced by magnetotactic bacteria (magnetobacteria) and preserved in the geologic record. the oldest definitive magnetofossils formed of the mineral magnetite come from the cretaceous chalk beds of southern england, while magnetofossil reports, not considered to be robust, extend on earth to the 1.9-billion-year-old gunflint chert; they include the four-billion-year-old martian meteorite alh84001.