When walking along a beach, small, dark, triangular objects are often found. These intriguing finds are fossilized shark teeth, and their distinct black coloration frequently sparks curiosity. Understanding why these ancient relics appear so different from the pearly white teeth of living sharks reveals a fascinating story of geological processes and transformation over vast stretches of time.
The Mineral Transformation
The black color of many fossilized shark teeth is a direct result of the fossilization process itself, specifically the replacement of organic materials within the tooth by surrounding minerals. Shark teeth are primarily composed of calcium phosphate. When a tooth is shed and becomes buried in oxygen-poor sediment, water carrying dissolved minerals begins to permeate its porous structure.
This process, called permineralization, involves minerals from the sediment seeping into the microscopic spaces within the tooth. Over thousands to millions of years, these infiltrating minerals, such as iron sulfides, manganese dioxide, or phosphates, gradually replace the original organic components of the tooth. The specific type and concentration of these minerals dictate the final color of the fossil. For instance, sediments rich in iron or manganese often lead to the characteristic black or dark gray hues observed in many fossil shark teeth.
The Journey to Fossilization
For a shark tooth to undergo fossilization and turn black, a specific sequence of events and environmental conditions must occur after it is lost. Sharks continually shed their teeth throughout their lives. Once shed, the tooth must quickly sink to the seafloor and be covered by sediment. This rapid burial is crucial as it protects the tooth from decay caused by oxygen and bacteria, and from physical damage or scavenging.
Over vast periods, layers of sediment accumulate above the buried tooth. The increasing pressure from these overlying layers, combined with the presence of mineral-rich water seeping through the sediment, facilitates the permineralization process. This geological environment, particularly marine sediments rich in organic matter and low in oxygen, creates the ideal conditions for the absorption of dark-coloring minerals like manganese oxide, resulting in the deep black appearance of the fossilized tooth.
Beyond the Fossil: Modern Shark Teeth
Not all shark teeth are black; teeth from living sharks exhibit a much lighter coloration. Modern shark teeth are typically white or cream-colored. This is because they are made of dentin and an outer layer of hard enameloid, which gives them their natural, lighter appearance. These teeth have not yet undergone the geological processes of mineral replacement that lead to fossilization and color change.
Sharks continuously replace their teeth throughout their lifespan. New teeth constantly grow behind functional ones, pushing older teeth forward until they are shed. This continuous tooth replacement makes shed teeth readily available for the fossilization process if the conditions are right. The contrast in color between a freshly shed, white tooth and a fossilized, black tooth highlights the impact of deep time and geological forces on these durable remnants of ancient marine life.