GEOGRAPHIC RANGE AND SPECIES RICHNESS OF POST-PALEOZOIC MARINE ANIMAL GENERA ORIGINATING IN RECOVERIES FROM MASS EXTINCTIONS

Marine animal genera originating immediately after the major mass extinctions of the late Permian and Mesozoic have significantly longer durations than genera originating at other times. Previously, we suggested that this difference might arise because genera in these recovery cohorts have more species or broader geographic ranges. We use occurrence data from the Paleobiology Database (http://paleodb.org) to explore these possibilities. We assign collections to international stages; we tabulate the number of species per genus per stage, scaled to the total number of collections in the stage; and we measure geographic range as the number of equal-area grid cells in which a genus occurs in each stage, scaled to the maximum range it could have given the areal extent of the data.

We find several peaks in mean geographic range, for genera first appearing in the early Triassic, early Jurassic, and early Cretaceous. Only the early Triassic shows a comparable peak in species richness. Geographic range within a stage predicts genus duration, but this relationship is strongest and most consistent among established genera; it is generally weaker among the new cohort of genera originating in a stage. Thus, geographic range does not consistently explain within-cohort variation in duration. However, because the recovery cohorts have both broader ranges and longer durations, it does help explain among-cohort variation in mean duration. More generally, for our Permian-Cretaceous focal interval, there is a significant correlation between the mean geographic range and mean duration of a cohort.

If we control statistically for geographic range and species richness, we find that genera that originate in a stage are generally less likely to survive to the next stage than are genera that carry over from the previous stage (see Finnegan et al., 2008, Paleobiology 34:418). There are two conspicuous exceptions, however; during the early Ordovician radiation and the early Triassic recovery, new genera are actually more likely to survive to the next stage. Because this cannot be explained solely by geographic range and species richness, there may be additional characteristics of the products of these major radiations that promote their evolutionary longevity.