Palaeoclimatic implications of the occurrence of the arcoid bivalve Anadara trapezia (Deshayes) in the Quaternary of Australasia

The arcoid bivalve Anadara trapezia (Deshayes, 1840) is a eurythermal estua rine mollusc that flourishes at present in eastern Australia between Port P hillip Bay, Victoria and Townsville in northern Queensland. A. trapezia fir st appeared in the Australian Quaternary fossil record during Oxygen Isotop e Stage (OIS) 7. In New Zealand, where it is now extinct, the earliest know n occurrences are in the Rangitawa "fossil beds" and in uppermost Castlecli ffian strata west of Wanganui, North Island (OIS Il, ca. 400 ka). The speci es had an extensive distribution during the Last Interglacial Maximum (OIS 5e) in both Australia and New Zealand and appears to have had a slightly wi der than present geographic range in southeastern Australia during the Midd le Holocene, as shown by records from Tasmania and western Victoria, where it no longer lives. Apart from an isolated population inhabiting Oyster Har bour, Western Australia,A. trapezia has not been confirmed living in coasta l waters west of Port Phillip Bay, Victoria. It became extinct in New Zeala nd sometime after OIS 5e. The extensive distribution and abundance of A. tr apezia and other fauna of subtropical affinity during the Last Interglacial Maximum, in areas where both are now extinct, appears to be associated wit h an enhanced Leeuwin Current at that time, coinciding with higher, less se asonally concentrated levels of precipitation and river discharge. The arri val of A. trapezia in Australasian coastal waters may have been a consequen ce of planktonic dispersal from southern South America. A. bravardi del Rio , from the Middle Miocene Puerto Madryn Formation, on the Valdes Peninsula, Argentina, may be a possible ancestor for A. trapezia. (C) 2000 Elsevier S cience Ltd. All rights reserved.