Fossilized shark teeth are a common find for beachcombers, yet they often contrast sharply with the white teeth of a living shark. The deep black color is not an inherent trait of the animal but results from a chemical transformation that occurs over vast stretches of time. This pigmentation is a direct consequence of the geology and chemistry of the burial environment, which dictates the minerals that infiltrate and preserve the tooth.
From Tooth to Fossil: Mineralization
A shark tooth begins fossilization immediately after it is shed and settles into the seabed sediment. Modern shark teeth consist primarily of calcium phosphate, specifically apatite, which forms the hard enameloid and dentin structure. Rapid burial by sediment is necessary for preservation, as it creates an environment with minimal oxygen that prevents decomposition.
The process of fossilization that preserves the tooth’s structure is known as permineralization. Over thousands to millions of years, the organic components within the tooth, such as collagen, break down and are slowly replaced by surrounding minerals carried in groundwater. This mineral replacement fills the microscopic pores and spaces within the original tooth material, stabilizing the structure and turning it into rock. The tooth’s original form is retained, but its chemical composition is fundamentally altered by foreign compounds from the sediment.
Manganese and Iron: The Pigment Source
The black color of fossil shark teeth results directly from the minerals available in the surrounding sediment during permineralization. The two primary elements responsible for this coloration are manganese and iron. These elements, often present as oxides or sulfides, are abundant in certain marine and riverbed sediments where fossilization occurs.
As mineral-rich water flows through the buried tooth, compounds containing manganese and iron are absorbed into the porous structure. Manganese oxides are particularly effective at producing a jet-black hue when they precipitate and crystallize within the tooth’s dentin and enameloid layers. This infiltration acts like a natural dye, permanently coloring the fossil. Phosphate, a mineral often found in marine deposits, can also contribute to the intense, dark coloration observed in some specimens.
Environmental Factors Affecting Final Color
While black is common, fossil shark teeth exhibit a wide spectrum of hues, including brown, gray, tan, and reddish-orange. The final color is entirely a function of the local geological environment where the tooth was preserved. The specific chemistry and oxygen level of the sediment dictate which minerals are absorbed into the tooth structure.
The deepest black colors form in oxygen-poor, or anoxic, environments rich in organic matter, such as deep riverbeds or coastal sediments. In contrast, a tooth fossilizing in an iron-rich but oxygenated environment results in a reddish or orange color due to the formation of iron oxides, which are essentially rust. Teeth buried in sediments composed primarily of gray clay or limestone may absorb minerals that yield lighter colors, such as gray, tan, or white. Post-fossilization factors, like the leaching of minerals by groundwater, can also lighten the color of a preserved tooth, sometimes resulting in a near-white appearance.