 For remains such as molluscan seashells, which frequently do not change their chemical composition over geological time, and may occasionally even retain such features as the original color markings for millions of years, the label "subfossil" is applied to shells that are understood to be thousands of years old, but are of Holocene age, and therefore are not old enough to be from the Pleistocene epoch.Unfossilized or partially fossilized remains can include bones, exoskeletons, nests, skin imprints, or fecal deposits.Subfossils of vertebrates are often found in caves or other shelters, where the remains have been preserved for thousands of years.