A mold fossil is a hollow impression left in rock after a buried shell, bone, or plant dissolved or rotted away. A cast fossil is what you get when that hollow later fills with sediment or minerals and hardens into a solid copy of the original’s shape.1Gateway Science Museum, California State University, Chico. Fossil Molds and Casts Neither contains any of the original creature, only its shape, which is why they can be so detailed yet so different from fossils made of actual bone or shell.
How a Mold Fossil Forms
Molds begin when an animal or plant is buried in mud or soft sediment.2Gateway Science Museum, California State University, Chico. Fossil Molds and Casts The sediment presses against every rib, bump, and vein of its surface. As it hardens to rock, it holds a three-dimensional impression of that surface.3U.S. National Park Service. Mold, Casts, and Steinkerns Molds most often form around hard parts like shells, especially those of invertebrates, but they are also known from vertebrates, plants, and footprints or burrows.4U.S. National Park Service. Mold, Casts, and Steinkerns
Then the original disappears. In porous rock, slightly acidic groundwater can seep in and dissolve the shell or tissue.5FossiLab. Casts & Molds What remains is an empty cavity printed with the organism’s outer details.6FossiLab. Casts & Molds Soft material like plants can simply rot away instead, sometimes within weeks in warm conditions.7Elsevier. Review of Palaeobotany and Palynology
Shells made of aragonite, the mineral in many clam and snail shells, dissolve more easily than those made of calcite, a more stable form of the same mineral.8PubMed Central. From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones So in the same rock layer, some kinds of shells may survive while others are left only as molds.9PubMed Central. From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones
A plant mold usually shows as a shallow groove or texture on a rock surface, sometimes stained, so tilting it under low-angle light helps the shape show up. Many molds are found only because the rock split along the layer where the organism once lay, which is why splitting thin, layered rock is a common way to find them. Some molds are partial, showing only the side that pressed into the sediment, so you may find just half an outline.
How a Mold Becomes a Cast
A cast forms when the empty imprint is filled with sediment or with minerals.10U.S. National Park Service. Mold, Casts, and Steinkerns Sometimes new mud or sand washes into the hollow and hardens, taking the place of the plant or animal.11Gateway Science Museum, California State University, Chico. Fossil Molds and Casts In other cases groundwater carrying dissolved minerals passes through, and the minerals grow in the open space as the water evaporates.12Earth Science. How Is a Cast Fossil Different From a Mold Fossil? Common filling minerals are quartz, calcite, and iron oxides, the same minerals that glue sedimentary rocks together.13Earth Science. How Is a Cast Fossil Different From a Mold Fossil?
The filling can even come from the shell itself. In some ancient seabeds, dissolved aragonite shells supplied the calcite that later cemented the rock.14Lethaia. Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas The pattern holds for plants too: upright fossil stems formed when flood sediment made a mold of the stem and later sediment filled it after the stem rotted.15Elsevier. Review of Palaeobotany and Palynology
In hand, a quartz filling may look glassy or sparkly, while iron oxides give rusty red or brown colors. A sediment filling may look almost the same as the surrounding rock, so color differences are often what help you spot where the cast sits.
A cast is a copy, not the original. No shell or tissue remains inside it, and it reproduces only the outer shape the mold recorded. How much fine detail it keeps depends on how finely the filler grains fit the hollow.
Not every mold becomes a cast. A hollow only fills if sediment can wash in or mineral-rich water can flow through it. If it is sealed off within tight rock, it may stay empty, which is why collectors find both empty molds and filled ones in similar rock. An empty mold can still be lost later, since weathering at the surface can wear away its thin rim. Casts, meanwhile, sometimes turn up loose on the ground because the softer surrounding rock eroded away while the harder filling remained.
External Molds, Internal Molds, and Steinkerns
An external mold is an impression of the outside surface of a shell or other hard part.16U.S. National Park Service. Mold, Casts, and Steinkerns On a shell it can show ribs, bumps, hinge lines, and fine ornament.17Prehistoricoregon. Fossil Cast Versus Mold: How to Tell Them Apart
An internal mold forms when sediment or crystals fill a hollow shell before the shell dissolves.18Fossils Window to the past. Casts & Molds The result is a solid copy of the inside, often called a steinkern, German for stone core.19Prehistoricoregon. Fossil Cast Versus Mold: How to Tell Them Apart It can preserve marks from the shell’s inner wall, such as scars where muscles attached.20Fossils Window to the past. Casts & Molds But it usually lacks the outer decoration, showing only the general shape.21Prehistoricoregon. Fossil Cast Versus Mold: How to Tell Them Apart
Two-part shells like clams often close after death, but a gap usually remains, which lets fine mud enter. Because the inner fill records the inside surface, it can show features that never show on the outside. Snails and other coiled shells can produce a steinkern that follows the spiral of the interior chamber.
In a clam, mud inside the closed shell becomes the steinkern, and mud around it becomes the outside mold. When the shell dissolves, it leaves a shell-thick gap between them. If a cast then fills that thin gap, it is a cast of the shell wall itself.22U.S. National Park Service. Mold, Casts, and Steinkerns One fossil site can therefore yield an outside mold, a steinkern, and a cast of the shell wall.
This is where names get confusing. A steinkern is technically an internal mold, though it is often called an internal cast. Naming varies, so museums and books may label the same thing differently.
How to Tell Which One You Found
Start with the direction of the relief. A mold is a negative: features that bulged on the animal appear as grooves or hollows. A cast is a positive, a solid replica showing the external details of the shell.23U.S. National Park Service. Mold, Casts, and Steinkerns The same rule works for tracks. A mold of a footprint is the dip itself, while a cast of it is the raised infill on the layer above.24U.S. National Park Service. Mold, Casts, and Steinkerns
Run a finger over the surface: grooves suggest a mold, ridges suggest a cast. Look for an empty space or a visible boundary of different material. A fossil you can lift free of the rock is often a cast or a steinkern. A split rock may show the mold on one half and the cast on the other.
Light helps too. Under a lamp at a low angle, raised and sunken features throw clear shadows, so you can judge whether ribs bulge or hollow. Turning the fossil so the light comes from different directions can fix the illusion where hollows seem to rise up. If you have a mold, you can also photograph it and invert the image, which can resemble the original, or press clay into it to reveal the original shape.
For shells, a steinkern looks like a smooth stone copy, often with fewer details. If the object looks like a shell but is smooth and slightly small, it may be a steinkern. If the surface carries raised fine decoration the live animal showed on its outside, it is more likely a cast of the outside.
What Molds and Casts Can Reveal
When sediment fills a skull, it can make a natural cast of the brain space.25PubMed. First Natural Endocranial Cast of a Fossil Snake (Cretaceous of Patagonia, Argentina) One snake skull’s cast showed the back of the brain, blood vessels, nerves, and inner ear canals.26PubMed. First Natural Endocranial Cast of a Fossil Snake (Cretaceous of Patagonia, Argentina) Scans of such casts let scientists compare brain shapes across many fossil and living animals.27PubMed Central. Digitized endocasts and brains: a perspective on measurements and historical analyses of the evolution of 172 fossil and extant amniote specimens
A brain cast records the shape of the space inside the skull, so it shows the brain’s size and the pattern of its folds only where the brain pressed against bone. It does not preserve soft tissue, so researchers learn about shape and nerve and blood vessel pathways, not the brain’s living chemistry. Because skulls differ in how tightly the brain fit, some casts are clearer guides than others.
Molds can also capture soft parts. Rapid burial by volcanic ash preserved trilobites as molds showing legs, mouth, and digestive system in 3D.28PubMed. Rapid volcanic ash entombment reveals the 3D anatomy of Cambrian trilobites Some of the oldest large fossils are molds preserved by thin pyrite films that microbes formed within hours to weeks after burial.29Palaios. FRAMBOIDAL PYRITE SHROUD CONFIRMS THE ‘DEATH MASK’ MODEL FOR MOLDIC PRESERVATION OF EDIACARAN SOFT-BODIED ORGANISMS Lab experiments made similar films on decaying tissue within 28 days.30PubMed. The role of iron in the formation of Ediacaran ‘death masks’
Molds do have limits compared with bone or shell. With no original material, there are no chemical clues from the organism itself, and internal structure is lost unless an internal mold formed. Shape alone can still identify many species.
A detailed natural mold can be hard to study while it sits in rock.31Publicación Electrónica de la Asociación Paleontológica Argentina. 3D CASTS FROM NATURAL MOLDS: A CASE STUDY IN FOSSIL FROGS Scientists use silicone rubber to turn it into a full 3D cast free of sediment.32Publicación Electrónica de la Asociación Paleontológica Argentina. 3D CASTS FROM NATURAL MOLDS: A CASE STUDY IN FOSSIL FROGS Such casts serve research, museums, and classrooms.33Publicación Electrónica de la Asociación Paleontológica Argentina. 3D CASTS FROM NATURAL MOLDS: A CASE STUDY IN FOSSIL FROGS That is also why many museum displays are casts: replicas protect fragile originals, let many museums show one rare specimen, and keep the fine detail of the mold.
Look-Alikes That Are Not Fossils
Some disc-shaped markings once thought to be ancient jellyfish were made by escaping gas bubbles and water currents.34Precambrian Research. Precambrian medusoids: The Cyclomedusa plexus and Cyclomedusa-like pseudofossils Holes once identified as dinosaur tracks turned out to be potholes from erosion of volcanic rock.35Ichnos. Re-examining dinosaur track replication in intrusive igneous rocks: a new geological interpretation An alleged human footprint in Utah formed when rock weathered along cracks and flaked off, leaving a foot-shaped mold and matching cast.36Journal of Geological Education. Alleged Human Footprint from Middle Cambrian Strata, Millard County, Utah
A real mold of a shell or animal usually shows features that make anatomical sense, like matching ribs on both sides, repeating features, or symmetric lines. A mimic usually fails close inspection, with irregular spacing, ragged edges, and no anatomical detail. A mark that cuts across rock layers or follows a crack suggests weathering. When unsure, ask a local museum or geology club.
Make a Mold and Cast at Home
Making a model mold and cast is a common lesson for learning the difference.37desertmuseum.org. ACTIVITY 6: MAKE A FOSSIL MOLD AND CAST Flatten some modeling clay and press a shell in firmly, then lift the shell out to reveal the mold. Pour plaster of Paris into the hollow and let it set, then peel away the clay to reveal the cast.
Pressing the shell straight down gives a deeper, clearer mold than a light or tilted press. A shell with clear ridges, like a scallop, makes the contrast between mold and cast easy to see. Mix the plaster smooth and pourable so it flows into small grooves without air bubbles, and let it set fully before peeling the clay so the cast doesn’t break.
Professional fossil replicas are made mainly with silicone rubber, sometimes latex.38Black Hills Institute. How Is a Cast Fossil Different From a Mold Fossil?