| Riassunto: | Although a division of the Phanerozoic climatic modes of the Earth into "greenhouse" and "icehouse" phases is widely accepted, whether or not polar ice developed during the relatively warm Jurassic and Cretaceous Periods is still under debate. In particular, there is a range of isotopic and biotic evidence that favours the concept of discrete "cold snaps", marked particularly by migration of certain biota towards lower latitudes. Extension of the use of the palaeotemperature proxy TEX<sub>86</sub> back to the middle Jurassic indicates that relatively warm sea-surface conditions (26-30°C) existed from this interval (~160 Ma) to the Early Cretaceous (~115 Ma) in the Southern Ocean. The Jurassic and Cretaceous "cold snaps" represent falls of only a few degrees. Belemnite δ<sup>18</sup>O data gives palaeotemperatures that are consistently lower by ~14°C than does TEX<sub>86</sub> and these molluscs likely record conditions below the thermocline. Such long-term warm climatic conditions would only be compatible with the existence of continental ice and if appreciable areas of high altitude existed on Antarctica, and/or in other polar regions, during the Mesozoic Era.
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