Subfossil refers to remains whose fossilization process is not complete, either for lack of time or because the condition in which they were buried were not optimal for fossilization.
Subfossil remains that date back into the Mesozoic are exceptionally rare, usually in an advanced state of decay and consequently much disputed. The vast bulk of the material comes from Quaternary deposits.
The main importance of subfossil vs. fossil remains is that the former contain organic material, which can be used for radiocarbon dating or extraction and sequencing of DNA, protein, or other biomolecules. Additionally, isotope ratios can provide much information about the ecological conditions under which extinct animals lived.
Subfossils are often found in depositionary environments, such as lake sediments, oceanic sediments, and soils. Once deposited, physical and/or chemical weathering can alter the state of preservation, and small subfossils can be ingested by living organisms.
Subfossils are useful for studying the evolutionary history of an environment, and can be important to studies in paleoclimatology.