Relationship of isotopic temperatures and faunal realms to Jurassic—cretaceous paleogeography, particularly of the South-west pacific

Abstract

Two major faunal realms, Boreal and Tethyan, may be recognised in Jurassic and Cretaceous macro-faunas. In the Jurassic, Boreal faunas populated the countries adjacent to the modern Arctic Ocean and penetrated southwards to about 50°N. Tethyan faunas populated the Mediterranean and Indo-Pacific regions and penetrated as far south as 75°S. Continental drift, faunal, and isotope studies suggest that, in the Jurassic, Boreal faunas were related to a cool-temperate climatic zone, developed adjacent to the Jurassic North Pole, and Tethyan faunas to a tropical and subtropical zone. A Southern Hemisphere equivalent of the Boreal Realm is not evident in the Jurassic, suggesting that at this time the Southern Hemisphere continents were grouped away from the South Pole. In the Cretaceous, a Boreal Realm can be distinguished, similar in geographic extent, and to judge from oxygen isotope data, also similar climatically to the Jurassic Boreal Realm. But the Cretaceous Tethyan Realm, again probably populated by tropical or subtropical stenothermal animals, had contracted considerably, and in the South Pacific region, an Austral Realm, including Australasia, West Antarctica, and South America, became differentiated. Continental drift, faunal, and isotopic studies suggest that the Austral faunas were related to a cool-temperate climatic zone (i.e., were the Southern Hemisphere equivalents of the Boreal faunas) and that their differentiation was related to the fragmentation of Gondwanaland and subsequent southwards drift of the Southern Hemisphere continents. New Zealand faunas may have been marginal-Tethyan in the Jurassic and marginal-Austral in the Cretaceous, involving a climatic change from warm-temperate (Jurassic) to cool-temperate (Cretaceous).