DETERMINING ANCIENT SHARK DIVERSITY: USING DERMAL DENTICLES PRESERVED IN DEEP-SEA SEDIMENTS TO STUDY CENOZOIC MARINE PREDATOR DYNAMICS

Sharks play an important role in ancient and modern marine ecosystems, however their cartilaginous skeletons are rarely preserved. Dermal denticles, the scales that cover the bodies of elasmobranchs, are therefore one of the best preserved and most complete fossil records for the group. Microfossil denticles are preserved in nearly all deep-sea sediment cores, providing a unique and unparalleled window into the evolution of open ocean sharks. We developed a novel coding scheme to quantify the morphological disparity of fossil and modern denticles. Denticles can be organized broadly into smooth, spine, linear, or geometric categories based on their ridge patterns, and each unique denticle was given a specific type relating to its code and appearance. We identified fossil analogs from 9 Orders and 24 Families of sharks and rays, however the vast majority of fossil denticles do not have a known modern analog. Linear denticles, those with parallel or single directional ridge patterns, comprised the majority of both fossil and modern samples. However, geometric denticles, characterized by intersecting and branching ridges, comprised a significantly larger amount of fossil, compared to modern, samples and fossil samples had higher diversity of types. Geometric types, for example those found in the families Centrophoridae (gulper sharks), Dalatiidae (kitefin sharks), and Chlamydoselachidae (frilled sharks), were routinely found in the fossil samples. The most commonly represented fossil denticles with modern analogs belong to sharks that inhabit the deep-sea, or which are migratory species known to spend time in the open ocean. Using this framework, we investigate patterns of shark morphological diversity throughout the Cenozoic, revealing a previously unknown major extinction of sharks 19 million years ago. This work provides a framework for studying the dynamics of these important predators throughout their recent history.