Why Are Shark Teeth Black?


Shark teeth are black because they absorb color from minerals during fossilization.

Have you ever wondered why shark teeth are black? The dark color comes from the fossilization process. Here is an explanation of how shark teeth become fossils, why they are colored, and how to find shark teeth.

What Kind of Shark Has Black Teeth?

Fossilized shark teeth are often black, but it’s reasonable to wonder if any sharks have naturally black teeth. Colored teeth are not unknown in the animal kingdom. Beavers have orange teeth from the iron incorporated in their enamel. Anglerfish have translucent teeth (like the creature in the Alien movie). However, shark teeth of all species are white or creamy, much like human teeth. The only type of shark with black teeth is one that has been dead for thousands or millions of years.

How Shark Teeth Become Fossils

Shark teeth become fossils through a process called permineralization. When a tooth is lost or a shark dies, it sinks to the bottom of the ocean and becomes buried by sediment (if not buried by sediment, it eventually disintegrates). The sediment lacks oxygen, preserving the specimen. As more and more sediment accumulates, pressure builds up and forces mineral-bearing water into the tooth. Depending the pH of the water, some compounds in the tooth may slowly dissolve. The minerals crystallize within pores in the tooth and form a fossil. The most common minerals are calcite and silica, but other minerals form fossils, too. The process takes a lot of time, so fossilized shark teeth are at least 10,000 years old and could be millions of years old. Most fossilized shark teeth date from the Late Cretaceous (100.5 to 66 million years ago) and Tertiary periods (66 to 2.6 million years ago). C. megalodon teeth date from 28 to 1.5 million years ago.

Chemicals That Cause Produce Colors

Fossilized shark teeth aren’t always black. They can be gray, brown, beige, or even red, orange, blue, green, or yellow. Bleaching and leaching after fossilization can also return the tooth to a white color.

The color of a shark tooth or other fossil reflects the chemical composition of the sediment that formed it. Iron-rich sediment typically produces reddish, orange, or brown fossils. Phosphate produces jet black fossils. Gray clay and limestone yield grayish green to graying yellow fossils.

The pH affects fossilization, too. Acidic sediments decalcify tooth enamel and expose the collagen-rich dentine and cementine. Alkaline sediments destroy the organic part of dentine. The interplay of chemical reactions on different parts of the tooth, perhaps followed by periods exposed to fresh water, results in a fossil that can be any color.

How to Tell If a Shark Tooth Is a Fossil

If you find a shark tooth embedded in a rock, it’s a safe bet that it’s a fossil. Shark teeth collected on a beach might be normal teeth or fossils. Color is an indicator of shark tooth age, but not a reliable one. While most fossilized teeth are dark or colored, sometimes water leaches out minerals, leaving a white or pale fossil. The composition of the root and crown differs, so they may be different colors in a pale fossil. Both the root and crown are white normally. Finally, identifying the shark species offers a clue to tooth age. Some teeth come from shark species that have been extinct for millions of years.

The root and crown of a shark tooth may be different colors.
The root and crown of a shark tooth may be different colors. (photo: Tom Bullock)

Why Shark Teeth Are So Common

While it’s easy to find a shark tooth, especially if you live near a beach, it’s harder to find other fossils. Why? Partly it’s because sharks have been around so long. Unlike many animals in the fossil record, they haven’t gone extinct, so they continue to drop teeth that become fossils. Another reason is because sharks have so many teeth. An individual may shed thousands of teeth in its lifetime. The teeth are dense, so they quickly sink into sediment, which protects them from degradation. Finally, teeth consist largely of the mineral hydroxyapatite (crystalline calcium phosphate), so they are less susceptible to damage and degradation than other types of tissue.

How to Find Shark Teeth

People have found shark teeth nearly everywhere on Earth, but the strategy for finding them is different on land versus near a beach. On land, your best chance of finding a shark tooth is to visit an area where other teeth have been found. Shark teeth and other fossils occur only in sedimentary rock. So, teeth are often found near river beds, beaches, and sand pits. Phosphate pits, such as Bone Valley in Florida, and kaolin pits are other excellent sites for shark teeth.

It’s easiest to find shark teeth on a beach at low tide. Sift through sections with small shells and coarse sand, looking for triangular or needle-like shapes. The color of the teeth depends on location. For example, around Myrtle Beach, most teeth are either white (fresh, not fossils) or black (fossilized teeth). Black shell fragments also occur. To tell shark teeth from shell, dry off your find and hold it up to the light. Shark teeth look glossy, while shell appears rippled and maybe iridescent. Also, shark teeth often retain some structure, such as ridged cutting edges.

References

  • Cappetta, H. (1987). “Mesozoic and Cenozoic Elasmobranchii”. Handbook of Paleoichthyology. 3B. München, Germany: Friedrich Pfeil. ISBN 978-3-89937-046-1.
  • Fernandez-Jalvo, Y.; Sanchez-Chillon, B.; et al. (2002). “Morphological taphonomic transformations of fossil bones in continental environments, and repercussions on their chemical composition”. Archaeometry. 44 (3): 353–361. doi:10.1111/1475-4754.t01-1-00068
  • Fisher, Daniel C. (1981). “Taphonomic Interpretation of Enamel-Less Teeth in the Shotgun Local Fauna (Paleocene, Wyoming)”. Museum of Paleontology Contributions. The University of Michigan. 25 (13): 259–275.
  • Hay, O. P. (1901). “Bibliography and Catalogue of the Fossil Vertebrata of North America”. Bulletin of the United States Geological Society (179): 308.
  • McNamara, K.J. (1996). “Dating the Origin of Animals”. Science. 274 (5295): 1993–1997. doi:10.1126/science.274.5295.1993f

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