UC Santa Barbara

Shark populations worldwide have been depleted by overfishing, yet empirical evidence of shark abundance and diversity before human impact is scarce. Without long-term data to document the timing and magnitude of shark declines, it is challenging to understand how the loss of these high-level predators has reshaped ecosystems and set meaningful management targets informed by natural variability. In this dissertation, I use fossil shark scales (dermal denticles) to reconstruct shark communities on coral reefs over the last several millennia. After finding denticles to be well-preserved in reef sediments, I refined an extraction method and built a reference collection to facilitate denticle classification. By measuring denticle shedding in captive sharks, I showed that shedding rates vary across sharks with different life modes, thereby influencing how shark communities are represented as denticle assemblages in the fossil record. I then calibrated the denticle record by comparing denticle assemblages preserved in surface sediments with wild shark populations on a remote, unfished atoll. I found that denticle accumulation rates were positively correlated with shark abundance and that denticle assemblage composition was consistent with commonly observed shark species. Building on this foundation, I recovered denticles from mid-Holocene and modern reef sediments in Caribbean Panama to define a local shark baseline before major human impact and quantify shifts in the modern shark community from this historical reference point. Denticle assemblages indicated that shark communities in this region declined threefold since the mid-Holocene and were functionally restructured by both long-term harvesting and habitat modification. Together, this work demonstrates that the denticle record can reveal changes in shark communities over long ecological timescales, helping to contextualize contemporary abundances and inform shark management and ecology.