Dermal denticles as a tool to reconstruct shark communities

The last 50 yr of fisheries catch statistics and ecological surveys have reported significant decreases in shark populations, which have largely been attributed to human activities. However, sharks are challenging to census, and this decline likely pre-dated even the longest fishery- dependent time series. Here we present the first use of dermal denticles preserved in reef sediments as a novel tool to reconstruct shark communities. We first built a dermal denticle reference collection and conducted a morphometric analysis of denticle characters to relate denticle form to taxonomy, shark ecology, and denticle function. Denticle morphology was highly variable across the body of an individual shark and between taxa, preventing species-or genus-level identification of isolated denticles. However, we found that denticle morphology was strongly correlated with shark ecology, and morphometric analysis corroborated existing functional classifications. In a proof of concept, we extracted 330 denticles from modern and fossil reef sediments in Bocas del Toro, Panama and found them to be morphologically diverse and sufficiently well-preserved to allow classification. We observed a high degree of correspondence between the denticles found in the sediments and the sharks documented in the region. We therefore propose that (1) denticle assemblages in the recent fossil record can help establish quantitative pre-human shark baselines and (2) time-averaged denticle assemblages on modern reefs can supplement traditional surveys, which may prove especially valuable in areas where rigorous surveys of sharks are difficult to perform.