Mold and cast fossils are common forms of preservation in the rock record. A mold fossil is the negative imprint, or cavity, left in the surrounding sediment after an organism’s original remains have dissolved away. Conversely, a cast fossil is the three-dimensional positive replica created when this mold cavity is filled in by minerals or other sedimentary material. These two forms represent sequential steps in a process that transforms the shape of a once-living structure into stone.
The Necessary Conditions for Preservation
The formation of a mold and cast fossil requires specific environmental circumstances to halt decomposition. The remains, usually a hard part like a shell or bone, must be buried quickly by fine-grained sediment, such as mud, silt, or volcanic ash. This rapid covering shields the structure from scavengers, abrasion, and bacterial decay. Burial must also occur in a low-oxygen, or anoxic, environment. Anoxic conditions significantly slow the rate at which aerobic bacteria break down organic matter, providing time for the surrounding sediment to lithify, or turn into rock.
How the External and Internal Molds Are Formed
Once the organism’s hard part is encased in solid rock, the mold begins to form through a chemical process involving groundwater. Over vast periods, slightly acidic groundwater percolates through the porous sedimentary rock layers. This acidic water dissolves the original material, especially if it is composed of calcium carbonate found in shells and corals. As the structure dissolves, it leaves behind an empty space perfectly shaped like the original organism. If this dissolution happens on the exterior of the buried remains, it forms an external mold, preserving the fine details of the outer surface features.
Internal Molds
A different form, the internal mold, or steinkern, is created when the internal cavities of a shell are filled with sediment before the shell itself dissolves. In this scenario, the sediment hardens. When the outer shell dissolves, the resulting mold cavity reflects the shape of the organism’s interior volume.
The Creation of the Cast
The creation of the cast fossil is the final step, involving the infilling of the negative space left by the mold. This occurs when mineral-rich groundwater continues to circulate through the surrounding rock layers. Minerals dissolved in the water, such as silica, calcite, or iron oxides, precipitate out of the solution and begin to deposit within the empty mold cavity. This deposition slowly fills the void, layer by layer, until the entire space is occupied and the material hardens through lithification. If the surrounding rock is later eroded away by natural forces, the cast fossil is revealed as a distinct, solid object.