Southwest pacific faunal palaeobiogeography in mesozoic and cenozoic times: A review

Abstract

The changing affinities of Southwest Pacific Permian—Jurassic benthic marine faunas primarily reflect changes in the position of Gondwanaland relative to the South Pole, and, in post-Jurassic time, fragmentation and rotation of Southern Hemisphere landmasses as Gondwanaland broke up. During the late Palaozoic the affinities of Southwest Pacific benthic marine faunas fluctuated between Palaeoaustral (cool-temperate) and Tethyan (warm-temperate), related to glacial and interglacial periods then being experienced by Gondwanaland. After the Permian, Gondwanaland moved away from the South Pole. In Triassic and early Jurassic time many marine animals of high vagility achieved a cosmopolitan distribution and extended into the Southwest Pacific. Nonetheless, throughout this time migration of less vagile benthic forms was impeded and New Caledonia and New Zealand formed a distinctive “Maorian Province”, probably equivalent to the Palaeoaustral province of the late Palaeozoic. In both New Caledonia and New Zealand the Maorian faunas gave way in middle and late Jurassic to migrations of Tethyan and Indo-Pacific benthic invertevrates. This marked faunal changed coincided with onset of the Rangitata Orogeny and start of fragmentation of Gondwanaland. These events ended isolation of the Southwest Pacific, by establishing shallow-water migration routes and ex-tropical warm currents along which moved waves of Tethyan and Indo-Pacific immigrants. Accompanying terrestrial routes also existed, along which migrated ancestors of the “Archaic” elements now present in the modern land biota of New Zealand and New Caledonia. The warm-temperate Tethyan and Indo-Pacific immigrants that entered the Southwest Pacific during middle and late Jurassic time achieved a wide distribution in the Southern Hemisphere. Palaeoaustral faunas (cool-temperate) were not apparent, suggesting that at this time the southern lands with Jurassic sediments now preserved were grouped in mid-latitudes, away from the Jurassic South Pole. However, in the early Cretaceous, Palaeoaustral faunas appeared in New Caledonia, New Zealand, Australia, southern India, western Antarctica and Patagonia, a development interpreted as reflecting southward movement of these lands following fragmentation of Gondwanaland, and their positioning in the cool-temperate climatic belt. At the same time Tethyan and Indo-Pacific migrations into the Southwest Pacific gradually declined, indicating progressive loss of shallow-water migration routes northwards from New Caledonia and New Zealand, resulting from rifting movements preceding opening of the Tasman Sea. Strong Palaeoaustral faunal affinities continued into the late Cretaceous, Paleocene and Eocene indicating that although new sea floor had begun to be created in the Southern Ocean 80 m.y. ago, freely accessible cool-temperate shallow-water migration routes continued to exist around the margins of Antarctica. The cutting of shallow-water and terrestrial migration links by sea-floor spreading in the Tasman Sea and Southern Ocean left the Southwest Pacific region isolated, with remnants of the once wide-spread Gondwana terrestrial biota preserved on New Zealand and New Caledonia. All later migrants had to travel across open ocean via subtropical oceanic currents originating in the Malayo-Pacific and Australian regions or, following the separation of Antarctica and Australia, the cool-temperate Circum-Antarctic Current and West Wind Drift.