An Overview of the use of Mollusks in Biostratigraphy

One of the prime objectives of the earth scientist is the delineation of the sequential history of the physical and biological evolution of the planet. Biostratigraphy provides one framework of reference for ordering the sequence of events for a large portion of earth history. Representatives of the Phylum Mollusca have played a central role in the development of this frame of reference and indeed in the development of the discipline itself. Molluscs have had an important role since the earliest efforts to utilize fossils for purposes other than as objects of beauty, veneration or commerce. Indeed, William Smith's observations in the 1790's, that certain fossils were characteristically restricted to specific rock units in the vicinity of Bath, England, was primarily a delineation of molluscan species that to this day are recognized as distinctive of the Jurassic Bathonian Stage. Although the term biostratigraphy was not coined by Dollo until the early 1900's, the guiding principles and practices that evolved much earlier through the efforts of such pioneer biostratigraphers as d'Orbigny, Quenstedt, Oppel and Buckman were based primarily upon the study of the stratigraphic and spatial distribution of Jurassic molluscs. The part played by molluscs in these early studies is amply demonstrated by the fact that local Jurassic stages were named after fossil rather than place names by French and Swiss geologists - the Pterocerien and Strombien Stages for snails and the Astartien, Diceratien and Pholadomyen Stages for clams. Although subsequent work has seen the demise of such local nomenclature, these units remain recognizable as defined. In spite of later refinement, the principles and many of the zonations developed by these founders have stood the test of time for over a century and a half.