Shark teeth are captivating objects. Sharks continuously shed their teeth throughout their lives, making these common discoveries for beachcombers and fossil enthusiasts. These shed teeth vary significantly in age, ranging from recently lost specimens to ancient relics preserved over millions of years. Understanding their durability and how their age is determined offers insight into marine biology and geology.
The Durability of Shark Teeth
Shark teeth are remarkably durable, persisting long after being shed. Their resilience stems from their unique composition, primarily calcium phosphate. Their outer layer is covered by a hard, enamel-like enameloid. This enameloid is denser and more resistant to degradation than human tooth enamel.
This dense, crystalline structure, particularly its enameloid, is crucial for long-term preservation, enabling it to withstand marine and geological pressures. Unlike softer bone or cartilage, which decay quickly, a shark tooth’s hard structure greatly enhances its fossilization potential.
From Mouth to Ancient Relic
A shark tooth’s journey to an ancient relic involves fossilization, specifically permineralization. After a tooth is shed, it must be rapidly buried in sediment (sand, mud, or clay). This swift burial helps protect the tooth from decomposition and scavenging, especially in low oxygen environments.
Over time, minerals from surrounding sediment seep into the porous tooth. These minerals, including iron, manganese, or silica, gradually replace the original organic material. This mineral replacement turns the tooth into stone, preserving its form and structure. The color of a fossilized tooth often provides clues about the minerals present; for instance, phosphate can result in black teeth, while iron may lead to brown hues.
Unlocking the Age of Fossil Teeth
Determining the age of fossil shark teeth relies on indirect methods, as they are not suitable for direct radiometric dating. Scientists assess their age based on the geological layers (strata) in which they are discovered. This often involves relative dating.
Stratigraphy, applying the principle of superposition, is a key method. This principle states that in undisturbed rock sequences, older layers are found beneath younger ones. By identifying the stratum where a tooth is embedded, scientists estimate its age relative to the surrounding rock. The presence of well-dated index fossils within the same layer also aids in pinpointing the tooth’s age. Index fossils are species that lived for a short geological period but were geographically widespread, making them excellent time markers.
While not directly dated, surrounding rock layers may contain materials suitable for absolute dating techniques. For example, layers containing volcanic ash can be dated using radiometric methods, such as potassium-argon dating, which measures the decay of radioactive isotopes. This provides an absolute age range for the layer and any shark teeth found within it. Through these combined analyses, fossil shark teeth can range from thousands to tens of millions of years old, depending on the species and geological context.
Identifying Old vs. New Teeth
Distinguishing between modern and fossilized shark teeth involves several visual and tactile characteristics. Color is a primary indicator. Modern shark teeth are typically white or off-white. Fossil teeth are usually dark (black, gray, brown, or reddish) due to mineral replacement during fossilization.
Texture and feel are other distinguishing features. Fossil teeth tend to be heavier and stony, reflecting their transformation into mineralized rock, while modern teeth are lighter and feel more like bone or enamel. The root’s presence and condition also offer clues. Modern teeth often retain some or all of their root, which is porous and fragile. Fossil teeth, even with a mineralized root, often show erosion or are entirely absent, leaving a clean, flat base.
Finally, discovery location provides insight into a tooth’s age. Modern teeth are commonly found on beaches, in shallow waters, or where sharks currently inhabit. Fossil teeth are typically unearthed in geological formations, exposed riverbeds, or construction sites where ancient marine sediments have surfaced. These differences allow for practical assessment of a shark tooth’s likely age.