In the time between speciation and extinction, a species' ecological and biogeographic footprint-its occupancy-will vary in response to macroecological drivers and historical contingencies. Despite their importance for understanding macroecological processes, general patterns of long-term species occupancy remain largely unknown. We documented the occupancy histories of Cenozoic marine mollusks from New Zealand. For both genera and species, these show a distinct pattern of increase to relatively short-lived peak occupancy at mid-duration, followed by a decline toward extinction. Thus, species at greatest risk for extinction are those that have already been in decline for a substantial period of time. This pattern of protracted rise and fall stands in contrast to that of incumbency, insofar as species show no general tendency to stay near maximal occupancy once established.

To interpret changes in biodiversity through geological time, it is necessary first to correct for biases in sampling effort related to variations in the exposure of rocks and recovery of fossils with age. Data from New Zealand indicate that outcrop area is likely to be a reliable proxy of rock volume in both stable cratonic regions, where the paleobiodiversity record is strongly correlated with relative sea level, and on tectonically active margins. In contrast, another potential proxy, the number of rock formations, is a poor predictor of outcrop area or sampling effort in the New Zealand case.