, Volume 19, Issue 6, pp 563-581

The Latitudinal Diversity Gradient Through Deep Time: Testing the “Age of the Tropics” Hypothesis Using Carboniferous Productidine Brachiopods

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The latitudinal gradient in global diversity, in which the number of species decreases away from the tropics, is widely recognized and well-studied but no consensus exists as to its cause. The “Age of the Tropics” hypothesis argues that the tropics have more species because young, ecologically dominant, tropical clades have not had sufficient time to disperse and adapt to colder climates; given time, the tropics act as a diversity pump. This hypothesis is tested using Carboniferous productidine brachiopods, which at that time were a young, ecologically dominant clade that originated in the tropics. A database of geographic occurrences indicates that productidines did not manifest a pattern of dispersal away from the tropics through time. A phylogeny of one productidine clade demonstrates that many lineages contracted, rather than expanded, their ranges over time. The results suggest that the hypothesis that the tropics are a diversity pump cannot be generalized across time and taxa.