THE L-SHAPED DISTRIBUTION OF SHELL AGES: PRESERVATION OF DIVERSITY IS FACILITATED BY STOCHASTIC BURIAL THAT RESETS THE LOSS RATES OF INDIVIDUALS

In the fossil record, information loss caused by the disintegration of individuals with low preservation can be compensated by the accumulation of multiple generations (time averaging), which effectively increases per-species preservation rates (not just by increasing sample size, but also by increasing the evenness of multi-species communities). The degree of this compensation is determined by the magnitude of per-individual preservation rates, which are poorly understood owing to the short duration of laboratory and natural experiments. Here, we use a likelihood-based framework to identify the dynamic of fossil preservation operating across a wide range of temporal scales. We apply this framework to molluscan skeletal remains exposed from the Southern California Bight (where old shells up to 20 ka are present but most shells are < 100 years old; i.e., L-shaped distribution). We find that the postmortem age-distributions of two common species are best explained by a two-phase model where an exponentially-distributed episodic event such as burial significantly reduces per-individual disintegration rate. Such a model allows for millennial time averaging even when preservational half-lives are decadal. Mechanistically, although burial (net sediment aggradation) is on average very slow and shells disintegrate rapidly on and near the sediment-water interface, some shells are quickly buried to zones where disintegration rates are slower by more than one order of magnitude. Burial provides shells a respite from destruction and diagenesis might even strengthen them, permitting their persistence during the short-term reworking events that promote the time-averaging of generations. Such two-phase models of shell loss capture regional diversity of molluscan communities as well as models that sample diversity uniformly from all individuals produced over the course of time averaging, and match empirical diversity more closely than simple exponential models. The older cohorts of individuals in death assemblages, although relatively scarce, can thus retain a significant portion of the past species pool, exceeding the diversity of assemblages collected alive. Chance (stochastic) burial within the window of time-averaging anoints a subset of individuals with high preservation.