When fossilized shark teeth are discovered along coastlines, their striking black coloration is a hallmark of their ancient history. Unlike the white or creamy teeth of living sharks, this transformation occurs over vast stretches of time due to geological preservation processes.
The Original Structure of Shark Teeth
Before fossilization, a shark’s tooth is a complex biological structure built for durability. The outer layer consists of a hard, mineral-rich tissue called enameloid, one of the hardest mineralized tissues found in animals. This robust coating protects the inner core, known as dentin.
Dentin, while also mineralized, contains organic components, including collagen. This internal structure, similar to bone, provides the tooth with strength and resilience.
The Journey to Black: Fossilization
The primary reason shark teeth turn black is permineralization, a common form of fossilization. When a shark loses a tooth, it sinks to the seafloor and must be rapidly covered by sediment like sand, silt, or clay. This swift burial protects the tooth from decay by limiting exposure to oxygen and bacteria.
Over thousands to millions of years, as more sediment accumulates, pressure builds, and groundwater begins to seep through these layers. This groundwater is rich in dissolved minerals from the surrounding environment, including elements like iron, manganese, phosphate, silica, and calcite. These minerals infiltrate the porous structure of the tooth, filling its microscopic spaces.
As minerals permeate the tooth, they undergo chemical reactions, gradually replacing the original organic material within the dentin. The specific minerals present in the sediment dictate the resulting color. High concentrations of iron sulfides, manganese dioxide, or phosphate minerals often result in the characteristic deep black or charcoal gray coloration.
What the Color Reveals
The varied colors of fossilized shark teeth offer clues about the ancient geological environments where they were preserved. The final color is directly determined by the mineral composition of the surrounding sediment during permineralization. This means even two teeth from the same shark species can have different colors if they fossilized in distinct locations.
While black often indicates iron sulfides, manganese dioxide, or phosphate-rich sediments, other hues are common. Brown or reddish tones result from iron oxides, while gray or white teeth fossilized in environments with higher concentrations of phosphate or calcium carbonate, such as limestone. Rarer blue or green shades can occur due to trace minerals like glauconite or copper. The color reflects the geochemical environment, providing insights into whether the tooth came from riverbeds, organic-rich layers, or sandy marine sediments. The color of a fossilized shark tooth does not directly indicate its age.