Fossils, named from the Latin word "fossus" meaning "having been dug up" are the preserved remains or traces of animals, plants, and other organisms from the remote past. Fossils vary in size from microscopic, such as bacterial cells, to gigantic, such as dinosaurs and trees many meters long and weighing many tons. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralised during life, such as the bones and teeth of vertebrates, or the chitinous exoskeletons of invertebrates. Preservation of soft tissues is rare in the fossil record. Fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or faeces (coprolites) of a reptile. These types of fossil are called trace fossils (or ichnofossils), as opposed to body fossils. Fossil ferns were widespread in the carboniferous period (320 million years ago) and are often found in coal seams.