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

Abstract

The arcoid bivalve Anadara trapezia ( Deshayes, 1840) is a eurythermal estuarine mollusc that flourishes at present in eastern Australia between Port Phillip Bay, Victoria and Townsville in northern Queensland. A. trapezia first appeared in the Australian Quaternary fossil record during Oxygen Isotope Stage (OIS) 7. In New Zealand, where it is now extinct, the earliest known occurrences are in the Rangitawa “fossil beds” and in uppermost Castlecliffian strata west of Wanganui, North Island (OIS 11, ca. 400 ka). The species had an extensive distribution during the Last Interglacial Maximum (OIS 5e) in both Australia and New Zealand and appears to have had a slightly wider than present geographic range in southeastern Australia during the Middle Holocene, as shown by records from Tasmania and western Victoria, where it no longer lives. Apart from an isolated population inhabiting Oyster Harbour, Western Australia, A. trapezia has not been confirmed living in coastal waters west of Port Phillip Bay, Victoria. It became extinct in New Zealand sometime after OIS 5e. The extensive distribution and abundance of A. trapezia and other fauna of subtropical affinity during the Last Interglacial Maximum, in areas where both are now extinct, appears to be associated with an enhanced Leeuwin Current at that time, coinciding with higher, less seasonally concentrated levels of precipitation and river discharge. The arrival of A. trapezia in Australasian coastal waters may have been a consequence 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.