Freshwater molluscs

DE FRANCESCO, CLAUDIO GERMÁN

The Encyclopedia of Quaternary Science

Elsevier

Lugar: Amsterdam; Año: 2013; p. 281 - 291

Freshwater molluscs (Phylum Mollusca) are among the most ubiquitous macroscopic remains preserved in continental Quaternary sediments. They are exclusively represented by gastropods (Class Gastropoda) and bivalves (Class Bivalvia). With the only exception of opisthobranchs (Class Gastropoda), all have an external calcareous shell (of one or two pieces, respectively) which is generally the only fossilizable structure in the sedimentary record. Indeed, the freshwater mollusc evidence from the Quaternary usually involves unaltered shells, in contrast to fossils from older deposits that mainly consist of molds or casts. The fossil record of freshwater molluscs is characterized by low diversity compared to the marine record due, in part, to poorer preservation of organisms as well as the ephemeral nature of many freshwater habitats. Fossilization is biased toward lowland and lake deposits, which are low-energetic environments where fossilization is favored. Fossiliferous river deposits are fewer than those of lakes, and the record is negligible or absent for such habitats as upland seepages, springs, and subterranean streams. This incomplete record is also due to the poor preservation potential of the often light, thin shells of most freshwater taxa. In fact, the fossil record of freshwater gastropods is patchy at best and likely to significantly underestimate the age and diversity of freshwater lineages. Despite these difficulties, assemblages of mollusc shells can add important information to paleoecological and paleolimnological reconstructions. They have been widely used to reconstruct ancient local habitats and climatic conditions in combination with other biological and sedimentary proxy records. A number of modern studies have focused on the analysis of stable isotopes and for inferring past temperatures, evaporative effects, and productivity levels. Much recent work has been directed to the evaluation of the processes and agents involved in the genesis of fossil assemblages, such as their compositional fidelity and taphonomy. The aim of this chapter is to review the main research directions associated with Quaternary freshwater molluscs that contribute to make them effective paleolimnological indicators. An overview of the present status of research, advances in methodology, as well as the problems and possibilities of every research direction are presented and discussed. Some case studies are also presented that exemplify various applications in fossil reconstructions.