Dermal denticles as a tool to reveal pre-exploitation shark communities in Bocas del Toro, Panama

What were shark communities like before humans? Ecological surveys and historical records demonstrate significant declines in shark populations, yet pre-exploitation baselines are nonexistent. Fossil dermal denticles may offer insight into pre-human shark communities. Denticles are well-preserved in reef sediments, but their identification represents a major challenge. To address this problem, we built a large reference collection from museum and local collections comprising 215 denticles from 37 species within 16 families. Morphometric analysis revealed that denticle morphology is loosely tied to taxonomy, making species-level identification almost impossible. We found, however, that denticle traits are strongly correlated with life habit and feeding mode. Quantitative measurements of traits also corroborated existing qualitative functional groupings and refined the boundaries between them. For example, fast, predatory sharks possess thin, ridged ‘drag reduction’ denticles, whereas demersal sharks are characterized by thick ‘abrasion strength’ denticles. In a proof of concept, we extracted 254 denticles from a 7,000-year-old fossil reef and 602 denticles from comparable modern reefs in Bocas del Toro, Panama and classified them using the reference collection. Denticle assemblages in Holocene and modern sediments corresponded well with families documented in the region. We found a significant decrease in the relative abundance of ‘drag reduction’ denticles and an increase in ‘abrasion strength’ denticles over the last ~7,000 years. Denticles in modern sediments can therefore supplement survey data given the rarity of sharks, and denticles in the recent fossil record may shed light on shifts in shark community composition over time.