Palaeobiogeography of the porcelaneous larger benthic foraminifer Borelis de Montfort (Alveolinoidea)

Genera of larger benthic foraminifera with wide distribution and high dispersal potential, such as the porcelaneous Borelis de Montfort (Alveolinoidea), provide the opportunity to evaluate the effect of palaeogeography on shallow-water marine species (palaeo)biogeography. The features diagnostic at species level of B. melo, B. curdica and B. pulchra were assessed by shell architectural analyses of fossil and modern isolated specimens using the micro-computed tomographic system. Stratigraphic, taxonomic and palaeobiogeographic approaches suggest migration and speciation of Borelis species starting from the early Oligocene in the Pacific area. Two lineages characterised by absence/presence of Y-shaped septula were found in the Oligocene-early Miocene in the Tethyan ocean. Borelis inflata, B. pulchra and B. pygmaea lack Y-shaped septula. In the Pacific region, the long-lasting B. pulchra is still widespread. The first species bearing Y-shaped septula, B. philippinensis, appeared in the Rupelian of the Indo-Pacific. Its middle Miocene descendant, B. schlumbergeri, lives in the Indo-Pacific. Westward migrants reached the Mediterranean during the Miocene and gave rise to B. melo and B. curdica. These species became isolated from the Indo-Pacific by the middle Miocene eastern closure of the Mediterranean basin. The upper Miocene Borelis specimens occurred in the last shallow-water coral-reef settings in the Mediterranean. These settings were refugia for B. melo, which subsequently disappeared during the Messinian Salinity Crisis. Before the Pliocene closure of the Central America Seaway, the Pacific B. pulchra reached the Caribbean area from where it colonized the western Atlantic coasts.