Range expansion of tropical shallow-water marine molluscs in the NE Atlantic during the last interglacial (MIS 5e): Causes, consequences and utility of ecostratigraphic indicators for the Macaronesian archipelagos

Abstract

Controlled by ecological and physical factors, marine species distribution may vary due to global climatic changes that result from range expansion or contraction (the latter caused by local disappearances, i.e., extirpations). Spanning from 13° to 39°N, the Macaronesian region encompasses five archipelagos located within warm-temperate to tropical climatic zones and influenced by a surface water current regime that favours a N–S range expansion of marine species. The extensive and well-preserved Macaronesian fossil record makes these islands excellent candidates for evolutionary studies. It documents a northward range expansion of tropical species with a Cabo Verdean-Mauritanian-Senegalese biogeographic provenance during the Last Interglacial (LIG). For the first time, a thorough revision and update of marine molluscs’ checklist from the Macaronesian archipelagos for the LIG is presented. On this basis, we hypothesize that the range expansion occurred during the last phase of glacial Termination 2 and was enhanced by ephemeral sea surface currents that created windows of opportunity for the long-distance dispersal of marine species along sweepstake routes. During these short intervals of time, numerous tropical and subtropical species expanded their geographic range northwards, towards what today are subtropical and temperate archipelagos. Simultaneously, however, a smaller number of northern species expanded their ranges southward, some reaching Cabo Verde. This work yields a set of 24 mollusc “ecostratigraphic indicators” (10 bivalve and 14 gastropod species) that are important to identify fossiliferous deposits from the Last Interglacial epoch in Macaronesian archipelagos. Longitudinal range expansions from West African shores to the Cabo Verde archipelago and the Canary Islands were also detected. These are related to coeval weaker upwelling systems which now constitute effective barriers for the exchange of species/individuals between continental and insular shores (e.g., Senegal/Cabo Verde; Morocco/Canaries). Finally, an ecological filter-effect also existed, which we associate with the increasingly longer distances to cover, as many of the most typical representatives of the warm water fauna with Cabo Verde/Senegalese affinities failed to reach the northern Azores and Madeira archipelagos.