AN EXTINCTION OF OPEN-OCEAN SHARKS 19 MA AND ITS IMPACT ON MODERN PELAGIC ECOSYSTEM STRUCTURE

Sharks are significant top predators in nearly all marine ecosystems, however their populations have been decimated in recent decades: shark finning, overfishing, pollution, climate change, and other anthropogenic stressors have pushed 400 million year old clade to the brink of extinction. However, this is not the first time that shark populations have suddenly and dramatically declined in marine environments. Here, we investigate a rapid transition during the Early Miocene, approximately 19-20 million years ago (Ma), when shark fossils, particularly isolated dermal scales (denticles), virtually disappeared from open-ocean sediments, reducing in absolute abundance by >90%. We investigated this dramatic reduction of sharks in the open ocean by developing a morphological framework for description and classification of denticles, and using the system to classify morphological variation in over 1000 fossil denticles from DSDP Site 596, in the South Pacific Gyre and ODP Site 886 in the North Pacific Gyre spanning 40 Ma to present, as well as 130 species of extant elasmobranchs, to create a catalog of modern denticle diversity. We find that there are two major types of denticles in open-ocean sediments: denticles with parallel or near-parallel ridges (‘linear denticles’) that are typically associated with modern sharks, and denticles with complex, intersecting ridge-patterns (‘geometric denticles’). Prior to 20 Ma, geometric denticles were common in assemblages, with a diverse range of morphotypes present, however in sediments younger than 20 Ma, geometric denticles are extremely rare, with only one of the >20 geometric morphotypes persisting through the extinction horizon. Linear denticles also show a reduction in both morphological diversity and abundance across the event, though to a lesser extent than the geometric denticles. This event occurred independently of any major known global climate change event, and approximately 3-5 million years prior to radiations in other large marine predator clades such as beaked whales, tunas, and mysticetes. This significant extinction marks a tipping point for pelagic marine vertebrate communities, redefining how marine top predators make a living in open ocean habitats, and setting the stage for the middle-Miocene radiations in large pelagic clades dominant in the open ocean today.