Statistic approach on taphonomic phenomena in shells ofGlycymeris glycymeris (Bivalvia: Glycymerididae) and its significance in the fossil record

Abstract

Abrasion is a taphonomic process of three-dimensional alteration and destruction of bivalve (and other shelly biota) shell morphology. In this study, abrasion processes inGlycymeris glycymeris and their relationships are presented, quantified and compared with fossil analogues. The Recent material was sampled from sandy beaches of NW France (Normandy). As a basis for describing abrasion patterns and paths, a revised description of the non-abraded morphology ofG. glycymeris is provided. For an analysis of shell microstructure abrasion patterns, fracture samples as well as polished and etched thin-sections were studied by SEM and TLM methods. Based on 524 specimens ofG. glycymeris, abrasion patterns of nine shell characters (valve outline; external shell surface ornamentation;external colour pattern and periostracum; beak/umbo depression; beak/umbo facets;hinge plate;ligamental area; internal shell surface within palliai line and adductor scars; crenulation of the inner ventral margin) were divided into 6–10 abrasion stages, arranged in their successive order. By means of cluster and correspondence analyses, the subjective order of abrasion stages was tested. Particular abrasion stages of certain characters are quite frequent, whereas others are rare. Consequently, some stages are either biased or our subjective stages are positioned within a consecutive path and not well defined. Cluster analysis revealed that certain characters are quite closely correlated; others do not show any relationships. The results derived from the Recent material are compared with two fossil examples,Glycymeris obovata and G. pilosa deshayesi, from the Tertiary of Central Europe.