INFERRING EXTINCTION DYNAMICS FROM STRATIGRAPHIC DATA: LESSONS FROM THE HOLOCENE FOSSIL RECORD

Stratigraphic patterns of last occurrences (LOs) of fossil taxa potentially fingerprint mass extinctions and delineate rates and geometries of those events. However, the record of extinction events preserved in local stratigraphic sections may be distorted by sea-level driven shifts in environmental conditions, sedimentation rates, and fossil abundance. Whereas most studies explicitly correct for random truncation of stratigraphic ranges (Signor-Lipps effect), the biasing role of depositional architecture has been largely neglected. Using the fossil record of extant mollusk species preserved in the Holocene succession of the Po Plain (Italy), we demonstrate that stratigraphic architecture exerts a strong control on the position of LOs, and readily produces sudden and stepwise apparent extinction events.

Instead of a gradual backward smearing of LOs expected from the Signor-Lipps effect, we observed a strong clustering of apparent extinctions in the late transgressive and early highstand systems tracts, coincident with abrupt facies shifts. The position of LOs is controlled by both the bathymetric preferences of species and vertical trends in fossil density. Because rare species are more readily captured in fossil-rich horizons, non-random stratigraphic distribution of skeletal concentrations can produce clusters of LOs that mimic sudden or stepwise extinction patterns, even in the absence of any facies specificity of individual taxa. Moreover, empirically calibrated numerical simulations indicate that gradual backward smearing of LOs would occur only under unrealistic conditions of continuous and uniform sampling of species with facies-independent distribution. Such assumptions underlie many commonly used methods of inferring extinction dynamics from stratigraphic data. These methods fail to correctly reconstruct the actual extinction pattern among Po Plain mollusks.

The results demonstrate that facies bias and stratigraphic architecture play an overriding role in controlling the apparent timing of LOs. Reliance on traditional methods correcting the Signor-Lipps effect, without a thorough consideration of environmental and sequence stratigraphic biases, may lead to incorrect inferences on the timing, duration, and selectivity of mass extinctions.