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Geographic range and genus longevity of late Paleozoic brachiopods

¹ Institut für Geowissenschaften, Facheinheit Paläontologie, Johann Wolfgang Goethe-Universität, Altenhöferallee 1, 60438 Frankfurt am Main, Germany. powell{at}jhu.edu
² Present address: Department of Geology, Juniata College, 1700 Moore Street, Huntingdon, Pennsylvania 16652

Geographic range size is one of the few traits that promoted survivorship during both mass and background extinctions, but the exact reason (or reasons) why a large geographic range confers extinction resistance remains unclear. Proposed explanations have focused on the roles of dispersal ability, climate tolerance, global abundance, and widespread ranges in predicting taxon longevities. This study uses biogeographic data for late Paleozoic brachiopod genera to test the relative contribution of these traits to genus longevities, using simple but accurate proxy measurements. The results demonstrate a strong positive relationship between genus longevity and geographic range size, which is robust to several potential errors. Further, latitudinal range, which predominantly reflects climate tolerance, was no more important than longitudinal range, which predominantly reflects dispersal ability, in predicting genus longevities. Rather, longevities were an outcome of the total number of occurrences, which estimates global abundance, and the advantages of widespread distribution, regardless of which particular traits were responsible for generating the total geographic range. The advantages of a large geographic range were apparent during both background and mass extinctions of late Paleozoic time. Although not statistically significant, there was a tendency for the greatest selectivity to occur in intervals with the lowest extinction rates. The correlation of genus longevity and geographic range size had a profound consequence for the secular pattern of global brachiopod diversity: because the diversity of genera with small geographic ranges was more volatile owing to their correspondingly shorter longevities, global diversity and mean geographic range size paralleled each other almost exactly. Given that the correlation between taxon longevity and geographic range size has also been demonstrated for other taxonomic groups and at other time intervals, these results suggest that global diversity curves compiled from taxonomic databases dominantly reflect changes in the diversity of genera with small geographic ranges.