Patterns and processes of shell fragmentation in modem and ancient marine environments

Shell fragments are important components of many Recent and fossil marine benthic ecosystems and can provide crucial information on past and present environmental conditions. Interpreting such fragments requires integrated knowledge in various fields and the information potential is therefore rarely optimally utilized. This paper uses the definition of a fragment as being a piece of shell having less than 90% of its original form. It then outlines the potential characteristics, pathways, and fates that shells and their fragments can have. Fragmentation is a key factor shaping the shelly part of death assemblages, but it is difficult to interpret because it can be broadly caused by ecological, biostratinomic or diagenetic processes and also depends on shell strength. Strength, in turn, reflects multifunctionality during ontogeny and depends on a complex set of skeletal and taphonomic factors. Therefore, no particular shell parameter clearly determines strength, but thickness, microstructure type and degree of organic matrix have the strongest influence on pre- and post-mortality strength. Size measurements are usually less important for shell strength than thickness, although ecologically complex size refuges from predation do exist. Similarly, shell shape, sculptural features and specific aperture types (in gastropods) provide various defence strategies rather than increasing strength per se. Key ecological factors of fragmentation include predation due to crushing, peeling, along with mistaken predation, self-inflicted damage during predation and burrowing, and more physical aspects such as impacts by stones. Modem studies must consider damage by benthic commercial fisheries or dredging by scientific vessels. Key biostratinomic factors include transport-induced abrasion (littoral zone of surf-washed beaches), bioerosion and dissolution (mainly sublittoral environments). Diagenetically, fragmentation during compaction mainly occurs when shells are in direct contact with one another or with coarser grains; taphonomic features (e.g., drillholes) have only minor influence. A key step in interpreting fragments is to categorize breakage into repaired versus unrepaired, diagnostic versus nondiagnostic, and severe versus localized damage. Quantifying the above categories can then provide useful information on breakage patterns and underlying processes. Understanding the many characteristics of shells and their fragments is a significant interpretive tool in maximizing the information gain in palaeoecological and taphofacies analyses. (C) 2003 Elsevier Science B.V. All rights reserved.