Opening of Drake Passage gateway and Late Miocene to Pleistocene cooling reflected in Southern Ocean molluscan dispersal: evidence from New Zealand and Argentina

Thirty-two benthic marine molluscan genus-group taxa are newly recognised as common to New Zealand and Argentina. Previously suggested examples, including Panis, Athlopecten, Austrovenus, large Cirostrema, Lucinoma, Aulacomya and Zygochlamys, are confirmed from new collections. The total of 46 Cenozoic dispersed taxa now recognised falls into four main groups: (a) Oligocene-Early Miocene dispersals from South America to New Zealand (8 taxa); (b) Oligocene-Early Miocene dispersals from New Zealand to South America (22); (c) Latest Miocene-Pliocene dispersals from South America to New Zealand (9); and (d) Pleistocene-Holocene dispersals from South America to New Zealand (at least 7 taxa). Dispersal seems to have occurred in bursts both to and from South America when the Antarctic Circumpolar Current (ACC) initiated, i.e., when Drake Passage opened, late in the Oligocene, while dispersal from South America to New Zealand, only, seems to have occurred again at two later times, (1) during the latest Miocene-Pliocene enhancement of ACC circulation caused by the initiation of West Antarctic glaciation, and (2) during Pleistocene glaciations. However, the incompleteness of the Argentinian record exaggerates the size of the initial burst, and allows no assessment of post-Middle Miocene dispersal to South America. Formation of the ACC through the opening of a passage south of Australia at ca. 43-40 Ma and of Drake Passage at ca. 23 Ma not only led to the glaciation of Antarctica, but also greatly altered Southern Ocean biogeography.