MACROEVOLUTIONARY COMPONENTS OF THE BRACHIOPOD LATITUDINAL DIVERSITY GRADIENT

The latitudinal diversity gradient continues to be a major focus of diversity studies but the generating mechanism remains elusive. Recent studies of fossil latitudinal diversity gradients have shown that peak biodiversity has gradually changed position over time; most recently, it shifted over the last few tens of millions of years from Northern mid-latitudes toward the equator. These findings challenge the implicit assumptions that peak biodiversity occurs at the equator and that the source of evolutionary novelty is some factor(s) intrinsic to the tropical environment. Here, we have attempted to provide further deep-time context for these changes by determining the proximal controls of the brachiopod diversity gradient through the Phanerozoic Eon. To do so, we reconstructed sample-standardized geographic patterns of origination, extinction, immigration, and emigration using occurrence data from the Paleobiology Database, grouped into 10-million-year by 10-degree-latitude bins. Our results show that immigration and emigration accounted for a slightly greater proportion of overall diversity change (59%), as compared with in situ origination and extinction (41%; percentages are calculated from standardized coefficients from multiple regression). On average, 29% of genera found in a particular latitudinal bin had persisted from the previous time interval, 28% had originated in situ, and 46% had immigrated from another latitudinal bin. Of these genera, 26% became extinct by the next time interval, and 46% had emigrated to a different latitude (i.e., became locally extinct). There was a weak tendency for mean rates of origination and extinction to be highest at low latitudes, whereas mean rates of immigration and emigration tended to be lowest at low latitudes. However, spatial patterns of these components were highly variable—often deviating significantly from these averages—in contrast to the stability of the diversity gradient. In fact, in most (47 of 54) time intervals, the gradient was simply a holdover from the previous interval and would not have been created anew by the combination of components that existed at that time. These results further establish a baseline understanding of ancient latitudinal diversity gradients and the geographic distribution of modern marine diversity.