This form of preservation is rare in most of the geologic column, but becomes more frequent in younger sedimentary rocks. Types of unaltered preservation where even the soft body parts are preserved include: (i) mummification, (ii) encasement in tar, (iii) encasement in amber, (iv) encasement in sediment, and (v) freezing. More frequently, however, only the hard skeletal material is preserved after removal of soft body parts.

Examples of unaltered preservation include the skeleton of a horseshoe crab, whose shell is composed of interlocking plates and jointed appendages which quickly disarticulate after death; cockle bivalved molluscs, whose outer-most shell layer has been removed by abrasion, yet the original shell material of the inner layers remains; an ammonoid from the Cretaceous period in which you should note the pearly luster which is original aragonite shell material; and an insect encased in amber .


This general class of preservation entails making "replicas" of the skeletal hard parts of organisms. In general, a mold is an impression in the sediment of a skeleton or shell. Once encased in lithified sediment, the dissolution of skeletal material leaves behind the impression or mold of original skeletal form. Thus, a mold is a "mirror image" of the original skeleton. An internal mold (sometimes called a steinkern) is the impression of the inside surface of skeletal hard parts. An external mold is the impression of the outside surface of skeleton or bone. An example of both types of molds can be seen in this image of a trilobite .

A cast is formed by the filling-in of a mold. It is thus a true replica (not a "mirror image") of the original skeleton or shell. By this definition, the cast one gets for a broken limb is not really a cast at all but an external mold.

A graphical representation of the formation of casts and molds is provided in Figure 2 below.

Figure 2 - Different diagenetic processes leading to different preservational styles in skeletal materials.
* Note that molds are produced directly as imprints of the shell and casts are produced from molds.

Modified from McRoberts (1998)


This common form of preservation involves chemical and/or physical alteration or replacement of original skeletal material. To properly identify replacement and recrystallization, one must know what the original constituents of the organism's skeleton were. These are provided in Figure 1.3. Replacement occurs often by the filling in (by various minerals) of the void space after dissolution of original skeletal material. Sometimes, the replacement occurs on a molecule by molecule basis. Common replacement minerals that you should be able to recognize include Silica (SiO2) as shown in the coral, and Pyrite (FeS2) shown in the ammonoid.

Recrystallization involves the physical re-arrangement of crystalline structure of skeletal material. This is a common phenomenon in shells which were originally aragonite and/or calcite (both forms of calcium carbonate- CaCO3). Examples, both of which are now calcite, include a gastropod which was originally aragonite and a brachiopod which was originally calcite.


As organic remains decompose in sediments, volatile constituents such as oxygen, hydrogen, and nitrogen are slowly lost to the surrounding sediments frequently leaving behind a carbon film. This process is carbonization (or sometimes called distillation), and occurs most frequently in oxygen deficient, organic-rich environments such as basinal black shales, and coal swamps. The carbon films often show exquisite details of plants and soft-body parts of animals not readily preserved, and can often be recognized by a dark gray or black film with a metallic sheen such as these fern-like fossil plants.


Permineralization involves the filling-in of pore and/or void spaces in shell or bone by secondary mineral matter in solution. With permineralization, the tiny pore spaces in the fossil are filled and the original skeletal material is still retained. However, it is often common for other types of preservation (e.g. replacement) to occur during and/or after permineralization. Because of its porous nature, bone and wood is especially prone to permineralization.

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