Fossilization and the fossil record

We would much rather have a living organism to study than a fossil, which is incomplete and ugly by comparison. The living biota, however, gives us only the vanishingly thin time slice of the modern world. To appreciate the processes that made that biota, we have to study the imperfect remains of ancient creatures.


Definition: A fossil is any trace of an organism's body or behavior that becomes part of the rock record.

  • Two types of fossil:

    Modes of Preservation: Generally only the hard parts of organisms are preserved (although there are some exceptions). Despite what you have heard, organismal remains do not have to be altered in any way to be regarded as fossils. And yet, they often are chemically altered. Major modes of preservation include:

  • Soft tissue preservation: Rare but interesting when it happens.

    Taphonomy - The process of becoming a fossil: From the birth of the organism to discovery by a paleontologist, fossils go through four general stages.

    Biotic stage - the realm of paleoecology: Birth to death. The organism grows whatever tissue is capable of being preserved then dies. It has done its part. Whether it will become part of the fossil record depends on whether any parts of its carcass can remain intact long enough to be incorporated into the rock record. The quicker the burial, the better because.....

    Interment stage - the realm of biostratinomy: Death to final burial. The carcass is exposed to:

    Diagenetic stage - the realm of sedimentology: Final burial to discovery. Once buried, the remains are officially fossils, however their existence is still perilous. Diagenesis results in:


    Field excavation from Bureau of Land Management
    Investigative stage - the realm of Sociology: Discovery to ultimate destruction. Every day, fossils are unearthed by erosion, only quickly to be destroyed by it. To enter the fossil record, as scholars understand it, a fossil must be exhumed (usually by natural processes), discovered, and described. As with the previous stages, this imposes filters and biases.

    Filters and Biases

    The selection of fossils for inclusion in the fossil record is non-random. Each stage imposes its filters.

    Fossils are bound to the study of the rock record in two ways:

    Recall that fossils were crucial to the establishment of the Geologic Time Scale. Are all fossils equally useful? No, you have to use the right fossils: