Plate tectonics and the evolution of intertidal and shallow-water benthic biotic distribution patterns of the southwest Pacific

A brief summary is given of recent information on the changing palaeogeography of the southwest Pacific region following the break-up of Gondwanaland and the parallel palaeoceanographic changes that occurred. Present intertidal and shallow-water benthic marine provinces in Australasia can be grouped into tropical—subtropical, warm-temperate, transitional warm-temperate, cold-temperate, transitional subantarctic and subantarctic regions. A notable feature is the marked faunistic attenuation that occurs with increasing latitude in most groups. Historical explanations of biotic distribution patterns fall into two classes: dispersal explanations and vicariance explanations. The problems involved in evaluating the relative roles of dispersal and vicariance in the evolution of the biota of a region are outlined and the conditions which must be met before larval dispersal across wide stretches of ocean can be successful are reviewed. It is concluded that rafting of adults is restricted to a few groups of animals and is of limited importance as a dispersal mechanism. An examination of the external relationships anddfossil record of Australasian intertidal and shallow water biota has identified a number of elements of which the predominant one is a Malayo-Pacific one comprising plants and animals of tropical affinities. The evolutionary history of the biota is re-interpreted in the light of palaeogeographic and palaeoceanographic changes with reference to a series of palaeogeographic reconstructions showing successive positions of Antarctica, Australia and New Zealand which have been used as a geographic base upon which speculative patterns of oceanic circulation and positions of oceanographic fronts have been depicted. Tectonic events, coupled with changes in circulation patterns and sea temperatures and the establishment of the Antarctic and Subtropical Convergences with a series of water masses forming belts from the poles to the tropics have been controlling influences of the distribution and evolution of the marine biota of the region. A major factor has been the migrations that have occurred in the position of the warm-water/cold-water boundary, the Subtropical Convergence. This migration coupled with changing island archipelago nature of the New Zealand land area has played a major role in speciation processes in the New Zealand region. It also led to the extraction of many warm-water genera and species as it moved north. In Australia the larger land mass extending further to the north provided opportunities for species to migrate up both the east and west coasts. This, coupled with the Pleistocene land bridges which connected Tasmania and New Guinea to Australia, led to isolation and subsequent allopatric speciation, followed by sympatry when the derived species migrated along the coast as the Subtropical Convergence retreated south to its present position and the Torres and Bass Straits barriers disappeared with rises in sea level.