ENVIRONMENTAL CONTROL OF RATES OF GEOGRAPHIC RANGE FLUCTUATION IN MARINE ANIMAL GENERA

The geographic ranges of animal genera vary substantially in size on geologic time scales. Recent work has documented a regular expansion and contraction when absolute time is ignored and genera are superimposed to form a composite average. But individual genera deviate from this average pattern, so it is worth asking whether there is any identifiable reason for their fluctuations in range. Here I test whether variation in range size within individual genera is at least partly controlled by the expansion and contraction of their preferred habitats. Using occurrence data from the Paleobiology Database, I identify genera that have a significant affinity for carbonate or clastic depositional environments that transcends the representation of these environments in the Database. These affinity assignments are not just a matter of arbitrarily subdividing a continuum in preference; rather, genera form distinct, nonrandom subsets with respect to lithologic preference. I tabulate the stage-by-stage transitions in geographic range within individual genera and the stage-by-stage changes in the extent of each environment. Comparing the two shows that genera with a preference for a given lithology are more likely to increase in geographic range, and to show a larger average increase in range, when that lithology increases in areal extent, and likewise for decreases in geographic range and lithologic area. Similar results obtain for genera with preferences for reefal and non-reef settings. Simulations and subsampling experiments suggest that these results are not artifacts of methodology or sampling bias. Genera that have roughly equal preference for carbonates and clastics do not have substantially broader geographic ranges than those with a distinct affinity, suggesting that, at this scale of analysis, extent of preferred environment is more important than breadth of environmental preference in governing geographic range.