TIME-AVERAGING: DEAD SEAFOOD AND TIME

Walker and Bambach (1971), in one of the most cited GSA abstracts ever, proposed the term “time-averaging” to denote subtle temporal mixing that affects even those fossils that are found in unsuspicious-looking deposits. Since then, “time-averaging” has become an integral part of the paleontological vocabulary, a cornerstone of taphonomy, and a major research area. Many subsequent advances in our understanding of processes that control the temporal resolution of the fossil record came from increasingly sophisticated studies of post-mortem history of marine invertebrates: time-averaging of dead seafood.

Dating efforts in modern depositional systems have demonstrated that common marine invertebrates (mollusks and brachiopods) are time-averaged over multi-centennial or even multi-millennial time scales. However, the temporal mixing is not uniform. Instead, shells are increasingly infrequent in older age classes following weibull-type distributions. Often, exponential distributions (a special case of a weibull distribution) are observed. Despite that time-averaged records tend to be remarkably complete, with most age classes represented by shells. However, the scale and structure of time-averaging both vary as a complex function of net accumulation rates, vagaries of biological productivity, intensity of taphonomic processes, and historical contingencies.

With the growing knowledge of the scale and structure of time-averaging, our understanding of its consequences has increased as well. Time-averaging allows for a more effective capture of alpha diversity in local communities, but also suppresses somewhat spatial heterogeneity in community composition (beta diversity). Also, time-averaged shell accumulations provide us with historical diaries of the most recent history of aquatic ecosystems. Thus, thanks to time-averaging processes, the most recent fossil record can yield unique data of direct interest to environmental sciences. This relatively new direction of research is known as “Conservation Paleobiology”. It is remarkable that the conceptual revolution that transformed our understanding of the quality of the fossil record and enabled paleontology to become a societally relevant scientific discipline was initiated by a GSA abstract.