Australian cretaceous terrestrial faunas and floras: biostratigraphic and biogeographic implications

Abstract

Nonmarine Cretaceous sediments representing fluvial/lacustrine deposits occur in 22 of Australia's 23 Mesozoic depositional basins. They are associated with open to marginal marine sediments whose enclosed planktic faunas and floras provide tie points to the Tethyan and European stages. Integration of marine and nonmarine sequences has been effected through spore-pollen biostratigraphies which in turn are linked to macroplant zones, vertebrate ranges and radiometric dates. This evidence is reviewed and it is concluded that the C. hughesii-P. pannosus spore-pollen Zones define Aptian to Albian ages and that their zonal boundaries are isochronous across Australia; the P. mawsonii-F. longus spore-pollen Zones in south-eastern Australia are datable as Turonian-Maastrichtian. However, evidence for a Cenomanian age for the intervening A. distocarinatus Zone is not indubitable. Also, the position of the Jurassic-Cretaceous boundary and the temporal significance of several index taxa of the ?latest Jurassic-Barremian C. australiensis-F. wonthaggiensis Zones in eastern Australia and the B. eneabbaensis-lower B. limbatus Zones in Western Australia is not certain. Isochronous incomings of individual spore-pollen taxa on a regional scale relate to the mode of dispersal of the source plant, and to the distribution of environments suitable for dispersal. For the Aptian-Albian, when Australia was inundated by shallow seas and lake/river systems that fed into them, several index taxa sourced from aquatic and strandline plants appear to have temporal significance. By contrast, anemophilous pollen are stratigraphically useful and temporally significant in the Turonian-Maastrichtian of south-eastern Australia. Throughout the Cretaceous, Australia supported a succession of coniferous forests. Early Cretaceous podocarp/araucarian/ Ginkgo canopy associations were modified in the Late Cretaceous by the loss of Ginkgo and the introduction of rainforest Proteaceae and Nothofagus. Regional variations in the vegetation reflect topographic, edaphic and climatic variations between the disparate sedimentary basins. Floral migration within Australia and between associated land masses occurred in a step-wise fashion, and was mostly west to east. Two dinosaur faunas are recognized: one, in the Aptian-early Albian of Victoria, consists of small ornithopods and theropods; and the other, in the middle Albian-Cenomanian of Queensland, comprises ankylosaurs, sauropods and theropods. The Victorian herbivorous dinosaurs were low feeders and probably shared a single mode of feeding, whereas those from Queensland may have had several feeding modes and fed up to 6 m from the ground. The small ornithopods, mostly from Victoria, were hypsilophodontians with grinding dentitions; their food source may have been lycopods or diaspores of podocarps and Ginkgo. Ankylosaurs seem to have preferred soft vegetation, some possibly aquatic. The Queensland sauropods may have fed on living and dead vegetation, perhaps including fleshy seeds of podocarps/taxads and gleicheniaceous ferns that apparently proliferated in the region. The lungfish had grinding toothplates and were probably omnivorous like extant Neoceratodus forsteri. Victorian faunas contain several relicts including the youngest known temnospondyl, the theropod Allosaurus, the lungfish Ceratodus avus, and the freshwater mussel Mesohydridella ipsviciensis. Australian earliest Cretaceous climates were temperate with cool temperatures (0–12°C) and high precipitation levels (750–1150 mm/year) in southern regions where winter freezing may have occurred during the Aptian. A warming trend during the Albian is indicated by land vegetation and verified by isotope palaeotemperatures (12–16°C) from the marine realm. Cool to warm temperate climates prevailed in the south-east during Turonian-Maastrichtian times; sea water temperatures were 16–28°C, and the vegetation is indicative of high humidities.