DIFFERENCES IN EXTINCTION RATES EXPLAIN CONTRASTING REGIONAL DIVERSITY PATTERNS IN MODERN TROPICAL BRYOZOANS

The Indo-Australian Archipelago (IAA) is the global centre of tropical marine biodiversity where species richness of most major animal taxa is several times higher than anywhere else. However, when and why this difference arose is unknown. We have addressed this question using extensive new and museum collections of fossil cheilostomes from the IAA and the tropical western Atlantic (TWA).

Although incompletely studied, cheilostomes are well-preserved, diverse and abundant components of Cenozoic tropical sediments. Middle to upper Miocene cheilostome diversity was strikingly similar in the IAA and TWA, strongly implying that the modern disparity in species richness has arisen within the past 5 million years. However, Miocene cheilostome faunas were ecologically very different across the tropics, with encrusting species dominating in the IAA but erect and free-living species dominating in the TWA. Our results support the hypothesis that the modern biogeographical differences in diversity resulted primarily from differential extinction of erect and free-living species in the TWA rather than higher rates of diversification in the IAA. This view is supported by the well-documented regional extinction of cheilostomes and other major taxa in the TWA, associated with oceanographical changes due to the uplift of the Central American Isthmus.