Dermal denticles are small scales that line the skin of sharks. Denticles have a similar composition to teeth, forming a strong and streamlined suit of armour. If you were to pet a shark, its skin would feel smooth in one direction and rough like sandpaper in the other direction. This is due to the serrated peaks of the layer of overlapping denticles that cover the shark’s body.
Denticles are continually shed by sharks and accumulate in marine sediments, where we can find them beautifully preserved – in both on modern and fossil reefs.
Denticles display a diversity of forms and play a variety of functions. For example, fast predatory sharks – such as hammerhead and requiem sharks – are covered almost entirely by thin, highly ridged denticles that reduce drag and make the shark more hydrodynamic. This type of denticle served as inspiration for the controversial Speedo swimsuits in the 2008 Beijing Olympics.
In contrast, demersal (bottom-dwelling) sharks are have thicker, smooth denticles that resemble pebbles. These protect against abrasion, given that their owners live along rocky, sandy, or coralline substrates.
Other demersal or schooling sharks possess spiny, defensive denticles that are thought to discourage the settlement of parasites and epibionts on the skin.
To make sense of the different types of denticle we had to first build a reference collection of modern denticles from museum collections of sharks. That way, when we find a denticle in a fossil reef we have some idea of which type of shark it could have come from and the way the shark lived.
One big problem is that denticles are rare and that is why we have been collecting so many hundreds of kilograms of fossil and modern reef sediments. Once back in the lab, we will dissolve the calcium carbonate that forms much of the sediment in our bulk sample bags using a process pioneered in Richard Norris’ lab. Picking them is arduous for they are usually less than half a millimetre in size. We hope to find enough to be able to rigorously compare the assemblages of denticles across our sites.
Erin and Félix carry off heavy bags of samples for analysis.
If we get lucky, the denticles from modern Caribbean reefs could tell us about the presence of sharks in areas where fish surveys have failed to report them because of their rarity, yet dive shops and fishermen confirm their presence.
Even more exciting is the possibility that we will uncover enough denticles from the fossil reefs to be able to reconstruct shark populations 7000 years ago, before major human impact.
It’s hard work but we hope that these massive bulk bags of reef sediment will uncover enough tiny denticles to help paint a picture of shark communities past and present.