Before the ice: Biogeography of Antarctic Paleogene molluscan faunas

摘要

New identifications are listed of Antarctic Paleogene molluscan faunas (Lopez de Bertodano and Sobral Formations, Seymour Island, Paleocene; La Meseta Formation, Seymour Island, Eocene; McMurdo Sound erratics, Eocene; and King George Island, Oligocene-Early Miocene). These show that both cosmopolitan and Antarctic-subantarctic genera increased regularly as percentages of faunas with time; 15% of Paleocene genera, 30% of Eocene genera and 59% of Oligocene-Early Miocene genera still live in the Antarctic-subantarctic region. The present major Antarctic families of larger molluscs have remained the most diverse ones since Middle Eocene time, and the Antarctic fauna was well established by then. However, Mytilidae and Veneridae have decreased markedly in diversity, Mactridae and oysters have disappeared from the Antarctic, and Antarctic Trochidae, Volutidae and Conoidea are now distinct guilds of more offshore taxa. This reflects the present Antarctic ice cover; shallow-water taxa that are no longer able physically to live in Antarctica provide an index of Antarctic glaciation. Glaciation was not extensive on the Antarctic Peninsula by Late Eocene time, and a few genera of shoreface molluscs still inhabited the King George Island area during Oligocene-Early Miocene time. Subduction has left only sparse localities around the margins of the Pacific and Indian/Australian plates containing shallow-water early Paleogene macrofossils; almost none are known from the central regions of these plates. Few data points are available, so apparent indications of taxa originating in Antarctica are not interpretable. New names are provided for two junior homonyms (Gaimardia zinsmeisten, replacing Gaimardia flemingi Zinsmeister, 1984; Prosipho stilwelli, replacing Aeneator huttoni Stilwell and Zinsmeister, 1992).