An early Miocene extinction in pelagic sharks

Sharks populations have been decimated in recent decades due to over-fishing and other anthropogenic stressors, however the long-term impacts of such changes in marine predator abundance and diversity are poorly constrained. We present evidence for a previously unknown major extinction event in sharks that occurred in the Early Miocene, approximately 19 million years ago (Ma). During this interval, sharks virtually disappeared from open-ocean sediments, declining in abundance by >90%, and morphological diversity by >70%, an event from which they never recovered. This abrupt extinction occurred independently from any known global climate event, and ~2-5 million years prior to diversifications in the highly migratory, large-bodied predators that dominate today, indicating that the Early Miocene was a formative period of rapid, transformative change for open ocean ecosystems.

Methods
Ichthyoliths (shark dermal scales and fish teeth) were isolated from deep-sea sediments, following Sibert et al (2017). Ichthyoliths larger than106 µm were picked out of the sediments and placed on cardboard microfossil slides. The slides were imaged on the Hull Lab Imaging System at Yale University and a VHX-6000 Keyence Digital Microscope at Harvard University. Individual ichthyoliths were identified using the AutoMorph software developed by Hsiang et al (2017), and each object was made into a single image with a unique object number in the filename. Images with denticles were isolated (versus images with teeth or images with no fossils picked up by the software as 'noise'), and each denticle image was classified into a distinct denticle morphotype.

References:

Hsiang, A. Y., K. Nelson, L. E. Elder, E. C. Sibert, S. S. Kahanamoku, J. E. Burke, A. Kelly, Y. Liu and P. M. Hull (2018). "AutoMorph: Accelerating morphometrics with automated 2D and 3D image processing and shape extraction." Methods in Ecology and Evolution: 605-612.

Sibert, E. C., K. L. Cramer, P. A. Hastings and R. D. Norris (2017). "Methods for isolation and quantification of microfossil fish teeth and elasmobranch dermal denticles (ichthyoliths) from marine sediments." Palaeontologia Electronica 20(1): 1-14.