Unusual preservation of crustaceans and microbial colonies in a vadose zone, northwest Morocco

Abstract

Exceptional fossil preservation is observed in self-sealing microcavities in limestones where lichens, cyanobacteria and fungi together entombed themselves and organic walled microfossils, crustaceans and their eggs. Preservation has been enabled by calcite coating, lining and impregnation of the exoskeleton of the crustaceans, which had a high original calcium content and acted as a nucleus for precipitation. The good preservation was facilitated by the microcavities, the surrounding limestone, the seasonality and rapidity of precipitation, the microbial colonies living on the specimens, and the fluxing of vadose waters through the karst. The microbes and the crustaceans probably became trapped by the very high rate of calcite precipitation promoted within the cavities. Entrapment in mucous-secreting mats of cyanobacteria and fungi preceded the destruction of some of the soft parts of the crustaceans. By the time the mats and the incorporated biota were sealed into the cavities in the limestone, the mucosic mats and their bacterial communities had mediated production of a range of calcite cements promoting preservation of the refractory tissues. This process has important implications for cementation studies in arid zones (and especially in the Martian subsurface), since a range of microbes are involved in progressive biomineralization leading to fossilization within a perched, vadose karst.