THE EFFECT OF RANDOM RANGE TRUNCATIONS ON PATTERNS OF EVOLUTION IN THE FOSSIL RECORD

Both fossil preservation and sampling methods affect perceived patterns of biotic diversity. Artificial range truncations, for example, may lead to incongruences between apparent- and actual-diversity curves. Thus, a catastrophic extinction event may appear gradual. Recent advances in biostratigraphic-gap analysis provide models for the distribution of gap lengths between fossil occurrence horizons and provide methods to place confidence intervals on local taxon ranges and remove the biases caused by artificial range truncations. Confidence intervals for a set of local taxon ranges may then be evaluated collectively to test a hypothesis of co-extinction/co-emigration or co-origination/co-immigration. In the case of terminal Cretaceous ammonities from Seymour Island, range-chart data are compatible with an abrupt extinction event, although the test statistic is not minimized at the stratigraphic horizon that was suggested by Macellari (1986).