Large-scale spatial biostratinomic patterns along the Eastern Indian coasts: Identification of a generalized siliciclastic beach taphofacies applicable in analogues (paleo)environments

Abstract

Studies on biostratinomy of modern molluscs are biogeographically (mostly from the Americas and Europe) and environmentally (mostly from subtropical and temperate carbonate settings) restricted, documenting local variation, while large-scale spatio-environmental patterns from the siliciclastic settings are less understood. Here, 8670 dead bivalve specimens were collected from several sandy, sandy-rock mixed, and sandy-clay mixed beach, and beaches associated with tidal flat and sea-grass habitat. Sampling stations were located along a stretch of about 2500 km covering several ecoregions along the eastern Indian coast, a large-scale, biogeographic pattern in biostratinomy is studied. Each shell was assigned seven grades based on shell damages intensities in reference to seven biostratinomic attributes (shell completeness, surface bioerosion, secondary rounding, abrasion, ligament loss, and external and internal wear); the summation of these values indicate the Total Taphonomic Grade (TTG) of individual specimens. Our results suggest, TTG values do not vary systematically with ecological factors (e.g., body size and shape, shell microstructure, etc.), although environmental (e.g., substrate and energy conditions) factors could play some role (e.g., high energy sandy beaches have higher TTG values); no large-scale biogeographic gradient is observed. Based on the characteristics of damage profiles, a siliciclastic beach taphofacies, common in all locations, is proposed: abrasion is the main damage type, whereas, shell fragmentation or biotic damages are negligible. Based on these, a biostratinomic pathway of shells is inferred: after death, shells are immediately locally transported to the beach, reducing exposure time for biotic damages; on beaches, constant swash and backwash cause external and internal abrasion. These taphofacies and biostratinomic pathway models can be used to identify the biostratinomic conditions of analogous modern and fossil assemblages.