Kristopher M. Kusnerik

Stratigraphic signatures of mass extinctions: ecological and sedimentary determinants

Stratigraphic patterns of last occurrences (LOs) of fossil taxa potentially fingerprint mass extinctions and delineate rates and geometries of those events. Although empirical studies of mass extinctions recognize that random sampling causes LOs to occur earlier than the time of extinction (Signor–Lipps effect), sequence stratigraphic controls on the position of LOs are rarely considered. By tracing stratigraphic ranges of extant mollusc species preserved in the Holocene succession of the Po coastal plain (Italy), we demonstrated that, if mass extinction took place today, complex but entirely false extinction patterns would be recorded regionally due to shifts in local community composition and non-random variation in the abundance of skeletal remains, both controlled by relative sea-level changes. Consequently, rather than following an apparent gradual pattern expected from the Signor–Lipps effect, LOs concentrated within intervals of stratigraphic condensation and strong facies shifts mimicking sudden extinction pulses. Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges. Such effects of stratigraphic architecture, co-produced by ecological, sedimentary and taphonomic processes, can easily confound interpretations of the timing, duration and selectivity of mass extinction events. Our results highlight the necessity of accounting for palaeoenvironmental and sequence stratigraphic context when inferring extinction dynamics from the fossil record.

Using the Fossil Record to Establish a Baseline and Recommendations for Oyster Mitigation in the Mid-Atlantic U.S.

Eastern oyster populations throughout the Mid-Atlantic region of the USA have been in decline for centuries due to overharvesting, disease, increased sediment pollution, and habitat destruction. By studying Pleistocene fossil oyster assemblages, it is possible to reconstruct baseline conditions and develop recommendations for oyster mitigation. Fossil assemblages were studied from five Pleistocene sites located in Maryland, Virginia, and North Carolina. Reconstructions of paleosalinity and temperature were used to identify modern and colonial sites with similar environmental parameters for comparison. Shell height and life span in Chesapeake Bay oysters declined significantly from the Pleistocene to today, at the same time that ontogenetic growth rates have increased. This pattern is driven by age truncation, in which both harvesting and disease preferentially remove the larger, reproductively more active and primarily female members of the population. By contrast, Pleistocene oysters from North Carolina did not differ significantly, in shell height, life span, or growth rates, from modern oysters.