The widespread assumption is that every fossil belongs to an animal or plant that is completely extinct. This perception exists because the vast majority of preserved life forms, like dinosaurs or trilobites, are from deep geologic time periods and have no living descendants. However, the connection between fossilization and extinction is not absolute. Specific, scientifically defined exceptions exist, meaning some ancient remains come from species that are still alive today. This nuance is explored by examining the definition of a true fossil, the phenomena of ancient species with continuous records, and the classification of younger, partially preserved remains.
What Defines a Fossil?
A fossil is defined as any preserved remains, impression, or trace of a once-living thing from a past geological age. Preservation involves replacing the original biological material with minerals, a process called permineralization, or creating a mold, cast, or impression in the surrounding rock. This chemical transformation gives fossils their characteristic stony nature, distinguishing them from mere bones or shells.
While mineralization is a defining factor, paleontology also applies a temporal standard. Specimens are considered true fossils if they predate the end of the Pleistocene epoch, which occurred approximately 10,000 years ago. This age cutoff serves as a practical marker, meaning the organism must have lived and died before the start of the current Holocene epoch.
The minimum age requirement means that most organisms studied through fossils are long gone, as biological change over immense timescales typically leads to extinction or significant evolution. This explains why finding a fully mineralized bone that is 50 million years old and belongs to an extant species is exceedingly rare.
The Exception: Fossils of Living Species
Despite the strong link between fossilization and extinction, two distinct categories of true fossils belong to species still living today, known as extant species. The first category involves organisms colloquially called “living fossils.” These represent ancient lineages that have undergone little morphological change over millions of years, persisting in modern ecosystems while maintaining a continuous fossil record.
For example, the horseshoe crab, a marine arthropod, is remarkably similar to its fossilized ancestors from the Ordovician period, over 440 million years ago. Similarly, the coelacanth, a lobe-finned fish, was thought to have gone extinct until a living specimen was discovered in 1938. This confirmed its continuous lineage despite a gap in its recent fossil record.
The second exception is trace fossils, also known as ichnofossils, which are preserved records of biological activity rather than the organism’s body. These include fossilized footprints, burrows, nests, and feeding traces. Ancient trace fossils, such as tracks from certain bird or insect species dating back millions of years, can be indistinguishable from the traces left by their modern counterparts. An ancient burrow pattern, for instance, can be linked to a modern genus of worm or insect that has maintained its behavior and morphology across geologic time.
Remains That Are Not Quite Fossils (Subfossils)
Confusion around this topic often stems from subfossils, which are remains that have not met the 10,000-year age requirement or have not completed mineralization. Subfossils are often preserved remains of recently extinct animals, or more commonly, species still alive today. These remains retain some original organic material, such as proteins, DNA, or collagen, which is typically lost during true fossilization.
The presence of organic matter allows scientists to use techniques like radiocarbon dating or DNA sequencing on subfossils, which would be impossible on a fully mineralized specimen. Subfossils are frequently found in environments that prevent rapid decay, such as peat bogs, dry caves, or permafrost ice layers. Remains of mammoths or bones of modern deer found in a cave are considered subfossils if they are less than 10,000 years old.
The subfossil classification is a crucial distinction, as it includes many recent remains of extant species mistakenly labeled as “fossils” by the public. While remains of an animal still living today found in a recent deposit are not true fossils, their study is significant for understanding recent ecological and climatic changes. This category represents the most common way that remains of modern species are preserved in the geological record.