COVER This scanning electron microscope image shows a dermal denticle from a great hammerhead shark (Sphyrna mokarran). Ridges and other features on these toothlike scales reduce drag in fast-swimming sharks. Denticles are shed and preserve in marine sediments, recording past shark assemblages. Fossil denticles from Panama show that Pacific reefs supported 20 times as many sharks as Caribbean reefs before human exploitation and that Caribbean populations have since declined by 75%.