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.

Sharks aren’t easy to spot on many, if not most, Caribbean reefs these days so figuring out what species remain and in what amount is a daunting task. Even though sharks are rare, they still leave traces behind — dermal denticles, their tiny tooth-like scales. Finding these can tell researchers like STRI fellow Erin Dillon what a shark community looks like, no matter how small it is.

But finding dermal denticles is only slightly less difficult than seeing a shark on a reef dive. They are so small that most can only be seen under a microscope and even relatively large concentrations are equivalent to about 25 per 10-kilogram sack of sediment. So identifying the correct site to sample is essential. These spots are often what scientists call low-energy sites, meaning they are not subject to strong wave action or currents, which sweep tiny particles like denticles away.

When a potentially apt site is identified, it’s time to get to work. Today, Erin and Mauro Lepore dug up some 100 kilograms of sediment. Doing so required that they carry more weight than usual in order to stay put on the sea floor when pushing into the hard surface. Otherwise, each thrust of the trowel would push them up from the bottom. It’s also important to avoid losing any fine sediments by digging too vigorously. When each sample bag is full, Félix Rodríguez, the O’Dea lab manager, drops a line down with a mesh bag and hoists the bags to the surface.

Erin, Mauro and Félix are just beginning their trek around the reefs of the Dominican Republic. Over the next few days they aim to gather another half ton of sediments. And while that might sound like hard work, once the samples are back at the lab of STRI scientist Aaron O’Dea in Panama, it will take a solid year (and an intern or two) to digest all the material (a process that involves copious amounts of acid). This, they hope, will get us one critical step closer to learning just how drastically shark communities on Caribbean reefs have changed since humans became the major driver of their decline.

We’ve traded rock hammers for trowels and are diving into the Caribbean to sample the modern equivalent of our fossil reefs from western Dominican Republic. Today we’re working on the fringing reef at La Caleta, near Santo Domingo, in the first of the #BaselineCaribbean project’s dives.

One of the problems of trying to work out what coral reefs were like before human impact is that the reefs of the past are underneath the reefs of today.

One way to get to these reefs is by coring through the reef and producing a timeline of the coral reef as it grew. This is great for small and common fossils, such as molluscs and fish teeth, but the core itself is only 10cm in diameter and so very large animals (such as large coral heads) and very rare fossils (such as shark dermal denticles) are not captured. This means the complexity of the coral reefs is often missed.

One way to get to the whole reefs of the past, and capture the full variation and natural complexity of the reefs, is wait for an excavation to dig out and drain the modern reef, exposing the fossil reef underneath. In Bocas del Toro we had the good fortune to be able to explore one such massive excavation – see the video here.

The Enriquillo Lake in the Dominican Republic was originally a marine embayment, flanked by hills on either side and open to the ocean to the east. The coral reefs fringing these hills are those we are studying. Around 4,000 years the River Yaqui del Sur deposited a large delta right in front of the entrance to the bay, sealing it off from the open sea. Bbecause the area sits in a rain shadow, evaporation exceeds precipitation and the lake began to dry out. Eventually the lake level dropped to below 40m below sea level and became hypersaline. With the reefs exposed above the lake level, storm channels started to cut through the ancient reefs and it is in these small canyons that we get to see the incredible full sections of coral reefs that grew between 6,000 and 9,000 years ago.

Recently, the lake level has started to rise again and nobody is sure why. Many theories have been proposed from climate change to natural variability. Whatever the cause, the impact on the local communities around the lake is devastating as it eats up the little arable land they have, and drowning the roads that encircle the lake. The government seems to be taking steps forwards helping and have started building a new road 20m or so above the lake level.

The problem for us is that if the lake continues to rise at the steep rate of recent years, within a decade these spectacular fossil reefs could once again be under water, and unavailable for study.