The paradox of barren ancient rocky shores in the western mediterranean

Resumen

Encrusting coralline red algae cover significant proportions of the surface of rocky walls, blocks, and boulders in the mid- and infralittoral zones in present-day temperate seas. Despite the common occurrence of their living counterparts, examples of fossil coralline plants attached to rock surfaces in ancient temperate shores are very scarce and can be considered cases of exceptional preservation. In the Mediterranean region, however, the most frequent encrusting species have existed at least since the Miocene; they comprise heavily calcified plants which, a priori, should have a high preservation potential. In fact, fossil representatives of these species are relatively common as components of rhodoliths (algal nodules) and bioclastic deposits. In addition, palaeocliffs and ancient rocky shores are widespread in Neogene and Quaternary basins around the Mediterranean Sea. The key feature explaining the low preservation potential of encrusting corallines as in-situ growths, based on observation of algae on submarine rock surfaces in the Cabo de Gata area in SE Spain, seems to be the adhesion mechanism. The organic adhesion substance decays after death, and the plant is then removed by grazers or simply falls to the bottom. No abiotic or biologically induced cementation prevents detachment of a dead plant and there is no accretion of the coralline cover on the rock surface. The plant debris is incorporated into the sediment around the rocky substrate and the algal fragments undergo later taphonomic processes asloose bioclasts.