RELICT SHELLS OF SUB-ANTARCTIC MOLLUSCA FROM THE ORANGE SHELF, BENGUELA REGION, OFF SOUTHWESTERN AFRICA

Subfossil shells of three locally extinct mollusk species of Subantarctic affinity have been found in sediments on the middle Continental Shelf (120-140 m depth) in the Benguela region off southwestern Africa. These are the venerid Tawera philomela (E. A. Smith), the ranellid Sassia (Sassia) philomelae (Watson), and the buccinid Pareuthria fuscata (Bruguiere). All three presently live on the mid-Atlantic islands of Tristan da Cunha and Gough. The abraded, bored, and encrusted state of T. philomela shells, including algal borings, records their exposure as a shell gravel under high-energy conditions in the photic zone. Cores of sediments and a C-14 date of 13.5 kyr B.P. confirm that T. philomela abundantly populated coarse sands in the sample area during the low sea level (approximately -100 m) of the late glacial. The shell condition of P. fuscata suggests it was a contemporary, while that of S. philomela indicates a higher position in the faunal succession of the sample area, most likely due to a greater depth preference. The dispersal of these species from the mid-Atlantic to Africa is consistent with glacial intensification of the circumpolar circulation and accords with previous work indicating enhanced opportunities for Southern Ocean dispersal during glaciations. This zonal dispersal compensates to some extent for Africa's lack of shelf connections to the subpolar benthic fauna. The specific means of dispersal remains speculative, as do the causes for the subsequent local extinction of the dispersed species.