While most fossils are several thousands to several billions of years old, there is no minimum age for a fossil. Fossils vary in size from microscopic, such as single cells, to gigantic, such as dinosaurs. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous exoskeletons of invertebrates. Preservation of soft tissues is exquisitely 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 feces of a reptile. These types of fossil are called trace fossils (or ichnofossils) as opposed to body fossils. Finally, past life leaves some markers that cannot be seen but can be det