Fossil Preservation – Virtual Teaching Collection

This Virtual Teaching Collection corresponds with Chapter 1.3 of the Digital Encyclopedia of Ancient Life: Types of fossil preservation.

Unless otherwise indicated, each 3D model was created by Emily Hauf using specimens at the Paleontological Research Institution, Ithaca, New York. Visit our complete collection of models on SketchFab

Note that each model may be viewed at full screen size by clicking the corresponding button in the lower right after the model has been loaded.

Last updated: August 30, 2018.

Unaltered remains

Unaltered fossil remains are comprised of the original materials—and sometimes tissues—produced by an organism when it was alive. These materials have not changed into something else over geological time (i.e., they have not been altered).

Skull of a Pleistocene cave bear, Ursus spelaeus (PRI 50009). The mineral composition of the skull has not been modified relative to that produced by the animal when it was alive.

Unaltered fossil specimen of the gastropod Ecphora quadricostata from the upper Pliocene Yorktown Formation of Hampton County, Virginia (PRI 70750). This shell is composed of the mineral calcite and is 37.7 mm in length.

Unaltered specimen of the gastropod Conus spurius from the upper Pliocene Tamiami Formation of Sarasota County, Florida (PRI 70626). This shell is composed of the mineral aragonite and is 62.3 mm in length. 

Unaltered specimen of the bivalve scallop Argopecten gibbus from the Neogene (Pliocene) of Sarasota County, Florida (PRI 50125). This shell is composed of the mineral calcite and has a width of approximately 6 cm.

Permineralization and Petrification

Most fossil bones and some fossil plants exhibit permineralization. Bone is a highly porous material because space must be available inside to hold bone marrow and other tissues. After a bone is buried, the pore spaces may be filled with minerals (such as calcite or silica) that precipitate out of ground water, forming a cement. (The original organic material, however, is not removed; thus, that material could be classified as unaltered.) This process effectively changes bone to stone. While they may have similar shapes, it is easy to tell apart a modern bone from a permineralized fossil bone because the fossil bone will feel much denser when lifted. This is because there are no longer any empty spaces inside of it.

Fossil plants are also sometimes preserved as permineralizations because, like bones, they often also have numerous pore spaces that may be filled with minerals following burial. Petrified wood takes the process of permineralization one step further: minerals fill the pore spaces in the wood (the air that fills these pores is what makes modern wood float in water) and the original organic material (i.e., the wood) is replaced with minerals. Thus, no original wood remains in petrified wood samples, but these fossil nevertheless tell us a lot about ancient forested ecosystems.

Permineralized skull of the oreodont Merycoidodon culbertsoni from the Oligocene of South Dakota (PRI 50653).

Permineralized or petrified wood from the Cretaceous of Seymour Island, Antarctica. Longest dimension of specimen is approximately 9 cm.


Fossil specimens exhibiting replacement do not preserve the original body parts produced by the organism when it was alive. Instead, a different, secondary material replaces the original material shortly following the death of the organism. The mineral pyrite (“fools gold”) sometimes replaces calcite, leading to golden colored fossils that are said to be “pyritized.” 

Pyritized specimen of the brachiopod Paraspirifer bownockeri from the Devonian Silica Shale of Lucas County, Ohio (PRI 76796). Width of specimen is approximately 4 cm. 


External molds

External molds are impressions. Imagine pushing a modern shell into Play-Doh, then removing it. The impression left behind is an external mold. 

External mold of the ammonoid Gunnarites sp. from the Cretaceous Lopez de Bertodano Formation of Snow Hill Island, Antarctica (PRI 61543). Diameter of specimen (not including surrounding rock) is approximately 9 cm.  Click here to view the shell that formed this mold.

External mold of the leaves of the horsetail Annularia stellae preserved in a concretion from the Pennsylvanian of the Mazon Creek area of northeastern Illinois (PRI T-1762). Length of surrounding rock is approximately 15 cm.

The crinoid Melocrinus williamsi from the Devonian Ithaca Formation of Cortland County, New York. Specimen preserves mineralized hard parts (stem segments) and external molds of stem segments and the calyx. Rock slab is approximately 29 cm in length. 

Internal molds (or, Steinkerns)

Imagine a snail shell that has been buried in soft sediment. After the animal decays (poor snail!), sediment fills the inside of its spiral shell. Eventually this sediment hardens. Now suppose that at some later time, the unaltered snail shell dissolves away. All that may be left is the lithified coil of sediment (perhaps consisting of siliciclastic sediment) that filled the inside of the snail’s shell. Such internal molds (or, steinkerns) tell us what the inside of the snail’s shell looked like, but not much about the external morphology of the shell. Internal and external molds are often found in association, however, particularly if slightly acidic ground water has dissolved away the calcitic or aragonitic shells, but left behind the surrounding siliciclastic sedimentary rock.

Internal mold of a strombid gastropod from the Pliocene Mao Adentro Formation of the Dominican Republic (PRI 76798). Length of specimen is approximately 11.5 cm. 

Internal mold of the cephalopod Actinoceras beloitense from the Ordovician Platteville Limestone of Ogle County, Illinois (PRI 70326). Length of specimen is approximately 9 cm. 

Internal mold of the brachiopod Pentamerus oblongus from the Silurian Waukesha Formation of Jackson County, Iowa (PRI 70760). Specimen is approximately 5.5 cm in length.

Specimens showing both internal and external molds

A small slab full of brachiopod fossils, showing preserved shell, internal molds, and external molds. Longest dimension of specimen is approximately 13 cm.

Internal (and portion of external) mold of the gastropod Cassidaria mirabilis from the Cretaceous of Snow Hill Island, Antarctica (PRI 58468). Specimen (not including surrounding rock) is approximately 6 cm in length. 

Internal (and portion of external) mold of the bivalve Cucullaea sp. from the Cretaceous of Antarctica (PRI 61558). Length of specimen (not including surrounding rock) is approximately 4.5 cm. 


Casts form when an external mold (or the void between an external and internal mold) is filled by sediment that later lithifies, reproducing the form of the original organic structure. 

If you were to press Play-Doh into an external mold (see above), and then let it harden, you would have a near-perfect reconstruction (a cast!) of what the outside of the ancient organism's body looked like. But, it would be made of Play-Doh, rather than calcium carbonate. Most dinosaur skeletons that you see on exhibit in museums are artificial casts made from actual fossil bones; plaster casts are much easier to work with (in part because they are much lighter) than actual bones. The model below shows a natural cast of an ancient camel’s brain (in this case called an “endocast”). How did it form? Sediment filled the deceased camel’s brain case (within the skull) and this sediment later hardened, reproducing the space once occupied by the brain itself.

Fossil specimen of a Poebrotherium sp. camel skull exhibiting an endocast of the brain; specimen is from the Oligocene of Wyoming (PRI 49382). Open model to see annotations.


Carbonized fossil remains (also called carbonizations) may result when organisms are rapidly buried, especially in low-oxygen conditions. Carbonized remains are thin, approximately two-dimensional films of carbon preserved on a flat surface of rock. They are most often black in color, reflecting the fact that they composed mostly of carbon (as is coal, which is also black in color). Most fossils that exhibit “soft part” preservation are carbonizations. Examples include many plant fossils (also known as compressions), insect fossils, and the famous fossils of the Burgess Shale.

Carbonized specimens of the graptolite Monograptus clintonensis from the Silurian Williamson Shale of Rochester, New York (PRI76765). Maximum dimension of rock is 11.5 cm.