KING FOR A DAY: RISE AND FALL IN SITE OCCUPANCY AND GEOGRAPHIC RANGE IN MOLLUSCAN SPECIES AND GENERA

The ecological footprint of a taxon can be assessed in a number of ways, including geographic range, frequency of occurrence in paleontological collections, and, for supraspecific taxa, species richness. Within marine invertebrates over the course of the Phanerozoic, analysis of data from the Paleobiology Database has shown that all these measures rise and fall symmetrically, on average, over the lifetime of a genus. Genera tend to take millions of years to reach their maximum footprint, and they decline over millions of years prior to their time of last appearance.

Some previous work has suggested that species, in contrast to genera, may achieve their maximum geographic range relatively rapidly. To assess the generality of this postulated difference, we have analyzed occurrence data on shelfal molluscs from a comprehensive, taxonomically vetted database of field-based paleontological collections covering the Cenozoic of New Zealand (The Fossil Record File, a.k.a. FRED). We measured occupancy as the proportion of collections in which a taxon occurs, and we measured geographic range as the number of 1:50000 maps in which a taxon occurs and as the greatest distance between its occurrences. For both measures of geographic range, we scaled to the maximal possible range, given the distribution of fossiliferous outcrop.

For both genera and species, we find a distinct pattern of increase to relatively short-lived peak occupancy mid-duration, followed by decline towards extinction. This "king-for-a-day" model stands in contrast to that of ecological incumbency. This general pattern in the history of occupancy and geographic range, and the unexpected congruence between species and genera, provide important constraints for models of macroecological dynamics on ecological and geological time scales.