Not every ancient organism is preserved as petrified bone or original shell material. Often, the original body material is completely gone, leaving only an impression of its form. These fossils, known as molds and casts, are three-dimensional copies that capture the external shape and surface textures of long-extinct life forms. They contain none of the original organic matter.
The Process of Mold and Cast Formation
The formation of a mold and cast fossil begins with the rapid burial of an organism, often a shelled marine creature, by fine-grained sediment like sand or mud. This quick covering prevents the remains from being scattered or consumed. As more material accumulates, pressure and the cementing action of minerals cause the surrounding sediment to harden into solid rock through a process called lithification.
Once encased in rock, the original material, such as a calcium carbonate shell, is exposed to circulating groundwater. Over vast spans of time, slightly acidic groundwater seeps through the rock layers and chemically dissolves the buried shell or wood. This dissolution leaves a hollow space, or cavity, within the surrounding rock that perfectly mirrors the organism’s shape. This void is the fossil mold, which preserves the negative imprint of the organism’s exterior.
Identifying Molds Versus Casts
The mold and the cast represent two distinct phases of preservation. The mold itself is the negative space, or the hollow depression left behind in the rock after the original organism has dissolved away. This impression captures the precise surface details of the ancient life form.
A cast fossil is formed when this mold cavity is later filled with new material, such as mineral-rich water, mud, or volcanic ash. These infilling materials solidify within the mold, creating a positive, three-dimensional replica of the organism’s original shape. Paleontologists differentiate between an external mold, which records the outer surface of a shell, and an internal mold, or steinkern, which forms when sediment fills the interior cavity before the outer shell dissolves.
Common Organisms Preserved as Molds and Casts
This preservation method is common for organisms that possessed hard, durable outer parts. Mollusks, such as ancient clams, snails, and ammonites, are frequently preserved as molds and casts because their calcium carbonate shells dissolve easily. The resulting hollow space is often filled by minerals like quartz or calcite, yielding a stone replica of the shell’s exterior.
Trilobites, extinct marine arthropods, are another group often found as mold and cast fossils, showcasing the intricate segmentation of their exoskeletons. Plant life can also be preserved this way, such as the pith casts of the extinct giant horsetail Calamites. These casts form when the hollow center of the stem fills with sediment before the woody tissue decays. Even trace fossils, like dinosaur footprints or ancient worm burrows, are often found as casts when sediment fills the original impression before hardening into rock.
These impression fossils are a primary source of information, allowing researchers to reconstruct the size, shape, and external features of species that lived millions of years ago.