Why Are Ancient Rocky Shores so Uncommon?

Abstract

The geologic history of rocky shore biotas has been neglected by paleontologists. Under present conditions of high continentality, rocky shores account for more than 33% of the world's coastline. They are generally located on shores facing narrow continental shelves, where the pattern of global plate tectonics exerts much influence over their regional concentration. The highest frequency of rocky shores occurs on islands associated with hot spots, island arcs, and convergent-plate margins associated with subduction. Rocky shores situated in these settings are subject to recycling, so their remains are rare in the stratigraphic record. Some passive continental margins, including those along newly rifted oceans and those with low rates of sedimentation, exhibit frequencies of rocky shores as high or higher than those found on many convergent-plate margins. These have a good chance of being retained in the stratigraphic record for geologically significant periods of time. Epicontinental flooding reduces the overall extent of rocky shores during major high stands in sea level, but unconformable surfaces produced by the fluctuation between marginal seas and epicontinental seas are a common feature in the stratigraphic record. These surfaces are the only stable archives of rocky-shore biotas. Ancient rocky shores are rarely described in the paleontological and geological literature because unconformable surfaces are seldom excavated in three-dimensional relief. Important data are easy to overlook in cross-sectional slices of limited extent.