A paleoichthyological perspective of the Messinian Salinity Crisis

The paleontological record of the Messinian salinity crisis (MSC) is largely incomplete and inadequate to thoroughly define the patterns of paleoenvironmental evolution of the Mediterranean basin in the time interval between 6.974 and 5.33 Ma. Such a remarkable inadequacy of the fossil documentation is certainly related to the nature of the sedimentary record, which testifies the widespread development of stressed environmental conditions. The progressive development of unfavourable environmental conditions begun before the first phases of the MSC and eventually resulted in the apparent extinction of the euhaline benthic biota as well as of the calcareous plankton. During the past four decades, the absence of these components of the aquatic ecosystem was roughly interpreted as the paleobiological evidence of the catastrophic hydrological and geomorphological changes that affected the Paleo-Mediterranean during the uppermost Messinian stage. In concrete terms, because of the catastrophic scenario evocated for the MSC, the sudden disappearance of euhaline benthos and calcareous plankton was implicitly considered as the product of the complete ecological collapse of the Mediterranean marine biome and of the annihilation of the aquatic biota. This approach was based on the assumption that the paleontological record is always reasonably adequate and that the absence of the record necessarily corresponds to the reliable record of the (original) absence. Moreover, the negative paleobiological evidence used to support the catastrophic scenario did not include all the components of the original biota potentially available in the record. In particular, fossil fish remains are relatively common in deposits originated during the MSC, thereby representing a largely unexploited source of paleoenvironmental information about the faunal and ecological structure of the Messinian aquatic biotopes. The study of Messinian fishes dates back to 1832 when Louis Agassiz described the cyprinodontid species Aphanius crassicaudus based on material collected from the Messinian clays outcropping near Senigallia, central Italy. Since that time, a large amount of Messinian paleoichthyological data has been accumulated. Nevertheless, the role of fishes in the paleoenvironmental interpretation of the Messinian event was underestimated or totally neglected. The relevance of fishes in paleoenvironmental studies lies in their bio-ecological characteristics; because of their mobility and migratory behaviour, fishes can provide valuable information about a vast array of contiguous biotopes, whereas their trophic level attribution is unambiguously indicative of the relative size and complexity of the aquatic food webs. A comprehensive analysis of the paleoichthyological record of the MSC revealed the presence of more than 50 localities from Italy, Sicily, Spain, Crete, Cyprus, and Algeria, with an overall diversity of about 75 taxa. The fish assemblages are often oligotypic, documenting different typologies of estuarine or coastal marine biotopes. A detailed stratigraphic calibration of the localities allowed a comparative analysis of ichthyofaunal evolution throughout the MSC. The evolutionary analysis of fish diversity indicates a high degree of ecological homogeneity throughout the three MSC stages (CIESM, 2008), with a continuous presence of marine taxa, both steno- and euryhaline. Such faunal continuity is also suggested by indirect evidences.