Fossil Type
Mold Fossils
Mold fossils form when an organism's body dissolves away after being embedded in sediment, leaving a hollow impression that preserves its external shape. External molds capture the outer surface, while internal molds (steinkerns) preserve the interior cavity. Though the original organism is entirely gone, mold fossils can reveal remarkable surface detail.
Formation
How Mold Fossils Form
An organism becomes buried in sediment, which hardens into rock around it. Over time, acidic groundwater, pressure, or biological processes dissolve the original shell, bone, or organic material, leaving an empty void shaped exactly like the organism. If the surrounding rock is fine-grained, even delicate surface textures — growth lines, ornamentation, muscle attachment scars — can be preserved in extraordinary detail.
Field Guide
How to Identify Mold Fossils
- 1Hollow impression in rock — looks like a negative or inside-out version of the organism
- 2No original material remains, only the shape
- 3External molds show the outer surface; internal molds show the inner cavity
- 4Fine surface detail often visible (growth lines, ridges)
Examples
Common Examples
Shell molds in limestone, ammonite impressions, internal molds of gastropods (steinkerns), leaf molds in shale.
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