Dinosaur and tree-line invasion of southeastern Australia during Cretaceous greenhouse spikes

Abstract

The paleosol record of the Early Cretaceous Wonthaggi and Eumeralla formations of Victoria reveals a suite of non-calcareous Histosols and Ultisols, as well as stump casts, indicative of humid temperate ecosystems. At some stratigraphic levels, frigid temperatures are indicated by clastic dykes (ice wedges), load casts of mud (periglacial convolutions), and coal-mantled stone rolls (aapamires). The largest tree stumps and thickest paleosols correspond to known Early Cretaceous greenhouse spikes, as revealed by stomatal index of fossil ginkgo leaves, by black shales of oceanic anoxic events, and both marine and non-marine carbon isotopic anomalies. Temperature and CO2 spikes coincide with appearance of broadleaf deciduous forests, including Ginkgo australis, with dinosaurs, monotremes, placentals, early birds and angiosperms. In contrast, permafrost paleosols represent fern-lycopsid tundra and coniferous taiga vegetation. Early Cretaceous CO2 greenhouse spikes allowed southward migration of tree lines and polar biomes several times during the Cretaceous, which was not continuously a hothouse period. The same greenhouse spikes are recorded by maximal marine transgressions in the Great Artesian Basin of Queensland, and the frigid intervals by dropped pebbles and glendonites in South Australia, New South Wales and Arctic regions.