Interhemispheric biodiversity peaks of living brachiopods coinciding with warm-temperate zones and correlated to a multitude of biotic, abiotic and evolutionary factors

Applying Locally Weighted Scatter-plot Smoother (LOWESS) regression, we analysed the global latitudinal diversity patterns of 394 living brachiopod species. We found a consistent bimodal pattern dominated by two mid-latitude peaks located respectively at 35–40°S and 30–35°N (∼warm-temperate zones, WTZs). The bimodality was found to be largely persistent and consistent irrespective of taxonomic, bathymetric and spatial scales. We tested the strength of seven ecological hypotheses thought to potentially explain the classic unimodal latitudinal diversity gradient. The Rapoport's rule was found to be the strongest model fitting the bimodal pattern, followed by the Species-Energy hypothesis. The mechanisms through which the mean latitudinal ranges and thermal niches of living brachiopods may have combined to produce the bimodality are further linked to two other factors: 1) most living brachiopods seem to have a strong adaptation to the mesothermal conditions of the WTZs, and 2) for living brachiopods, the WTZs appear to represent both a source of speciation indicated by a relatively higher per-genus diversification rate and a ‘sink’ of biodiversity due to biogeographic mixing and a concentration of species range-end points.