Can shallow- and deep-water chemoautotrophic and heterotrophic communities be discriminated in the fossil record?

Abstract

The recognition of marine chemoautotrophic paleocommunities has added a new dimension to the interpretation of the fossil record. We compared the death assemblages formed by chemoautotrophic and heterotrophic molluscan faunas of the continental slope with their counterparts in shallow water and with a fossil chemoautotrophic analogue from the Tepee Buttes of the Pierre Shale (Cretaceous) to determine whether differences between them could be ascribed to differences in community structure or taphonomy. Evaluation of guild and tier structure and size-frequency distribution using abundance and energy flow (paleoproduction and paleoingestion) suggests that differences between inshore and offshore Recent heterotrophic assemblages reflect autecologic differences in the living communities forming them rather than differences in preservation. In contrast, the degree of similarity between chemoautotrophic assemblages is a function of their taphonomic and autecologic history. The differences observed amongst Recent chemoautotrophic assemblages issue from changes in the abundance and preservability of the small heterotrophic fauna, relatively poorly preserved offshore, and the presence of a large heterotrophic fauna, essentially restricted to shallow water. Taphonomic removal of the small heterotrophs (and juveniles of larger species) and the autecologic absence of large heterotrophs could produce an assemblage equivalent to that of the Cretaceous Tepee Buttes from any one of the Recent inshore or continental slope chemoautotrophic assemblages. Our analysis indicates that shelf communities represent the shallowest communities likely to be identified as fossil cold-seep faunas. Farther inshore, community attributes normally used to distinguish chemoautotrophic and heterotrophic faunas are no longer discriminative. Nearshore communities may have included a dominant chemoautotrophic guild, but recognition would rely solely on the identification of known chemoautotrophic taxa. Community trophic structure has classically been evaluated on the basis of numerical abundance and species composition. Comparing these chemoautotrophic and heterotrophic assemblages demonstrates the importance of evaluating community structure on criteria that more clearly document the way energy flowed through the community, namely paleoproduction and paleoingestion. Many of the unique structural attributes of these assemblages only became obvious using these measures.