BRACHIOPOD ABUNDANCE AND EXTINCTION IN THE LATE ORDOVICIAN AND EARLY SILURIAN: BACKGROUND EFFECTS DURING A MASS EXTINCTION
We test the relationship of abundance and extinction among articulate brachiopod genera within seven third-order depositional sequences spanning the Late Ordovician to Early Silurian (Katian-Aeronian) of the Cincinnati Arch, USA. We measure abundance at three scales of observation: mean among samples, mean within depositional sequences, and total occupancy in the entire data set. We compare each of these measures to global duration in the fossil record and survival during the end-Ordovician mass extinction.
Abundance and duration display no correlation at the coarser scales, but the correlation is strongly negative at the sample level. Comparing the results across the three scales of analysis suggests that these non-positive correlations are not an artifact of incomplete sampling along histories of rise and fall in occupancy, but may be attributable to the emergence of simultaneously rare, yet cosmopolitan fauna in the Silurian.
Conversely, a comparison of survivors and victims indicates that survivors were more abundant than victims, particularly when measured in terms of occupancy. This positive relationship between abundance and survivorship during the extinction suggests that, within the context of Jablonski’s alternating regimes model, the end-Ordovician extinction was characterized by a background-type selectivity regime. Previous studies have also suggested that although the end-Ordovician extinction was unusually large, selectivity was more typical of background periods.
Our finding that abundance is not positively correlated with duration, yet is correlated with survivorship across the Ordovician/Silurian boundary, provides further evidence of changing selectivity patterns through time.