THE ACTUALISTIC PERSPECTIVE ON TAPHONOMIC IMPACT OF LITHIFICATION: COMPARISON OF MOLLUSCAN ASSEMBLAGES FROM UNLITHIFIED AND RECENTLY CEMENTED STORM BEACH CARBONATE SANDS, SAN SALVADOR, BAHAMAS

Many previous taphonomic studies have addressed potential losses of paleontological information involved in the transition from living communities to surficial death assemblages. In contrast, post-burial and early diagenetic processes governing the formation of fossil assemblages have attracted much less attention. Several recent studies, focused on large scale regional analyses of Cenozoic siliciclastic successions, suggest that lithification represents a major taphonomic filter distorting the fossil record of biodiversity. However, impact of lithification on smaller-scale, community level patterns, especially in carbonate settings, has remained largely unexplored.

Rapid cementation of storm beach sands in modern tropical carbonate environments offers us an opportunity to address this issue using actualistic data. Mollusk assemblages from shell concentrations occurring in unlithified and poorly to moderately cemented storm beach deposits of Sand Dollar Beach (NW San Salvador, Bahamas) were compared in terms of diversity and faunal composition using various analytical methods including rarefaction, resampling and ordination techniques.

Lithification with related changes in sampling methodology appears to influence relative abundance patterns of mollusk taxa. This bias is distinguishable from effects of size sorting and is more pronounced in finer grained concentrations dominated by small gastropods. However, although assemblages from lithified samples may be compositionally distinct from those extracted from unlithified samples, both types of deposits record very similar rarefied species richness and evenness estimates. This encouraging result suggests that diversity estimates obtained using different sampling procedures (sieving vs. mechanically breaking up of bulk samples) may be methodologically comparable in case of carbonate deposits characterized by low to moderate degree of cementation. Poorly lithified carbonate rocks – present in many Neogene successions, may potentially yield biodiversity data comparable to those recorded by unlithified sediments.