STRATIGRAPHIC ARTIFACTS IN EXTINCTION DYNAMICS: AN EMPIRICAL EVALUATION USING 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 sampling artifacts (Signor-Lipps effect), the biasing role of stratigraphic architecture has been largely neglected. Using molluscan assemblages preserved in Holocene deposits 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 with false selectivity patterns.

Instead of a gradual backward smearing of LOs expected from the Signor-Lipps effect, we observed a strong clustering of apparent extinction events, coincident with abrupt facies shifts in the late transgressive and early highstand systems tracts. The sequence of LOs is controlled by the bathymetric preferences of species. If read literally, this relationship could be misinterpreted as a signature of a selective, habitat-specific extinction event. Using empirically calibrated numerical simulations, we also show that gradual backward smearing of LOs would occur only under unrealistic assumptions of continuous and uniform sampling of species with facies-independent distribution. Moreover, even in the absence of any facies specificity of individual taxa, non-random stratigraphic distribution of skeletal concentrations can produce clusters of LOs that mimic sudden or stepwise extinction patterns, because rare taxa are more readily captured in fossil-rich horizons.

The results demonstrate that facies bias and stratigraphic architecture play an overriding role in controlling the apparent timing of LOs. Methods used to evaluate the extinction dynamics typically aim at correcting the Signor-Lipps effect without accounting for stratigraphic artifacts. Reliance on such methods, without a thorough consideration of environmental and sequence stratigraphic biases, may lead to incorrect inferences on the timing, duration, and selectivity of mass extinctions.