On the bidirectional relationship between geographic range and taxonomic duration

Geographic range and taxonomic duration are known to be positively correlated in a number of biologic groups; this is usually attributed to the influence of range upon duration rather than the other way about. Here we analyze two distinct components of this correlation within species and genera of marine invertebrates and microfossils by partitioning the total duration into two parts: the time it takes a taxon to attain its maximum geographic range, and the time a taxon persists after attaining its peak range. We find that the longer it takes a taxon to attain its maximum geographic range, the wider is that range. We also find that the broader the maximum range, the greater is the duration after this maximum is attained. These two correlations are equally strong on average. There is thus a reciprocal relationship between duration and geographic range, and there is no compelling evidence that range generally determines duration more or less than duration determines range.