CLOSING THE GAP: MASS EXTINCTIONS AND THE DURATIONS OF SEDIMENTARY HIATUSES

Many features of the highly volatile history of taxonomic first and last occurrences in the marine animal fossil record, including the major mass extinctions and the frequency distribution of genus longevities, can be predicted on the basis of the temporal continuity of preserved sediments. This result is consistent with two non-mutually exclusive hypotheses: 1) macroevolutionary patterns have been distorted by preservation biases (bias hypothesis), or 2) temporal continuity in sediment accumulation has been driven by processes that also influence evolution (common cause hypothesis). If preservation bias causes the rock-fossil agreement, then gap durations should be positively correlated with apparent rates of turnover because longer gaps encompass more true extinctions and originations. If the common cause is responsible, gap sizes should have no relation to observed patterns of turnover because true extinctions and originations are influenced by the magnitudes of environmental change associated with gaps, not the durations of the gaps themselves.

Here, I tabulate the times of hiatus initiation and termination in the U.S. sedimentary rock record at roughly substage temporal resolution. Mean/median gap sizes, grouped by their times of initiation and termination, vary considerably over the Phanerozoic, but there is no correlation between the duration of gaps and apparent extinction rates in the preceding interval or apparent origination rates in the subsequent interval. Moreover, mean and median gap sizes at the major mass extinctions are not large in comparison to most other non-extinction intervals and are not long enough to accommodate the preservation bias hypothesis.

Although facies and taxonomic biases are related to the temporal continuity of sedimentary rock and may therefore drive much of the agreement between the sedimentary and fossil records, the lack of correlation between gap durations and taxonomic turnover conflicts with the hypothesis that macroevolutionary patterns are simply artifacts caused by sampling failure at major unconformities. Instead, these results support the notion that the macroevolutionary history of marine animals has been dominated by environmental changes related to the expansion and contraction of marine environments.