As we hiked through cactus and traversed down the side of the canyon toward our first sampling site, I marveled at the terrain and how un-reeflike it appeared at first glance. We then reached the beginning of the reef, or what was a fringing reef 7,000-9,000 years ago. I was blown away. A diverse thicket of corals and molluscs – representing thousands of years of growth – towered over me on the canyon walls. The preservation of details was extraordinary, allowing me to identify coral species as I strolled through the site. Not only was I walking upward through the reef zones – as if diving on a modern reef – but I was peering at the growth of the reef across time. The three-dimensionality of this fossil reef was incredible, and it challenged me think about reefs from an entirely new perspective.
Over the past three days, we collected sediment samples from four fossil reefs. These sites give us unique insight into what healthy reefs were like before people. In particular, I am interested in characterizing the shark communities in the past and understanding how they have changed over time. How can we do this, though, if there aren’t many shark teeth in our samples? Instead, we have found that sharks leave their mark in reef sediments in the form of dermal denticles – tooth-like scales that cover their skin. Denticles are no bigger than a couple strands of hair, yet are well-preserved in the fossil record. They also display a variety of forms that reflect shark ecology, allowing us to reconstruct community composition.
However, this valuable insight comes with its costs. We have found that denticles are rare in reef sediments. This means that we have to collect a large quantity of sediment to obtain a sufficient number of denticles to analyze. We have already collected 300kg of sediment that I will use to extract denticles. Over the course of our trip, we hope to collect 900kg of sediment from the modern and fossil reefs we visit. Processing this sediment will require around 100 gallons of acetic acid to digest the carbonate and months of work sieving and picking out the denticles under a microscope. I expect to find 25-30 denticles in each 10kg bulk bag, yielding about 2,250 denticles. This 900kg of material will thus be reduced down to about a gram of denticles, a pile small enough to easily fit in the palm of your hand. These tiny microfossils can reveal pre-exploitation baselines of sharks on reefs, which are currently unknown. They can also supplement our understanding of sharks on modern reefs in the Dominican Republic and how their assemblages have shifted over time due to human activities. Unlocking this missing information about the status of unfished shark communities is crucial to better conserving them in the future. This unusual trip to the desert allows us to turn to the past to help protect the future of reefs.