The sight of black shark teeth scattered along sandy shores often sparks curiosity. Unlike the white teeth of living sharks, their dark color hints at a deeper, more ancient story. Their unusual color is not natural, but a result of geological processes that transform organic material over vast stretches of time. These black teeth are true fossils, offering a tangible connection to prehistoric marine life.
How Shark Teeth Become Fossils
When a shark sheds a tooth, it typically sinks to the ocean floor. For fossilization to occur, this tooth must be rapidly covered by sediment, such as sand or mud. This quick burial protects the tooth from decay caused by oxygen and bacteria in the water. Without this protection, the tooth would likely disintegrate.
Over thousands to millions of years, as sediment layers accumulate, pressure builds around the buried tooth. Water carrying dissolved minerals seeps through these sediments and into the porous structure of the tooth. This process, known as permineralization, involves the gradual replacement of the tooth’s original organic material with these minerals. The tooth slowly turns into a stone replica, retaining its shape and gaining the properties of the infused minerals. It takes at least 10,000 years for a shark tooth to become a true fossil.
The Role of Minerals in Color
The black color of many fossilized shark teeth results directly from minerals in the surrounding sediment during permineralization. As water seeps into the tooth, it carries various dissolved minerals that are deposited within the tooth’s pore spaces. The specific color a fossilized tooth acquires depends on the type and concentration of these minerals.
Minerals like iron sulfides and manganese dioxide are common in marine environments and impart dark hues. For instance, if phosphate replaces the original tooth material, the fossil will appear black. Other minerals can lead to different colors, like reddish-brown from iron-rich sediments or gray-green from gray clays and limestone. Thus, the black color indicates fossilization in sediment rich in dark-coloring minerals.
Modern Teeth Versus Fossilized Teeth
Modern shark teeth are white or off-white, similar to human teeth. This is because they are primarily composed of fluoroapatite in the outer enameloid layer and dentin in the inner core. The enameloid is a hard, mineral-rich tissue that gives the tooth its natural light color.
In contrast, the black color of beach-found shark teeth is a clear indication of their ancient origin and fossilization. The extensive mineral replacement during fossilization completely alters the tooth’s original composition and appearance. While a modern tooth’s color comes from its biological structure, a fossilized tooth’s color is a geological signature from its long burial history. This transformation highlights the significant difference between a recently lost tooth and one that has undergone a profound change over millennia.