In the Jurassic many families of marine organisms that had evolved during the Triassic were diversifying rapidly; and on land the para-mammals had been replaced by the dinosaurs as the dominant group. The Triassic nothosaurs gave rise to the fish-eating plesio-saurs and pliosaurs, while in the air, the pterosaurs were in full flight. Late in the Jurassic the dinosaurs themselves gave rise to the birds.
The beginning of the Jurassic was a time of marine transgression on a world-wide scale. Broad seaways advanced on to the heartlands of the existing continents (Fig. m). Throughout the period (65 million years) climates seem to have been equable, with an absence of ice-caps even in areas that are thought, on palaeomagnetic grounds, to have been over the poles. The fact that both the north and south poles lay in oceanic areas may explain why ice-caps were unable to become established, as marine circulation would tend to equilibrate temperature differences between areas in the marine realm. This absence of ice-caps contrasted strongly with the extensive glaciations of the Palaeozoic on Gondwanaland. The absence of polar ice-caps probably caused a much less vigorous atmospheric exchange than at the present time.
The Jurassic also saw the beginning of the dispersal of the Pangea supercontinent. This resulted from the westerly extension of the Tethys Ocean and the beginning of rifting in the Central Atlantic (Fig. n ). The separation of various parts of the continent provided greater scope for the development of provincialism in faunas as a result of their geographical isolation but these effects did not become very pronounced until later on in the period.
Through the Jurassic there was a progressive rise in sea levels around the world. This was probably caused by the growth of submarine oceanic ridges that accompanied the widening of the Jurassic oceans. This secular eustatic rise reached its peak in late Jurassic times when vast areas of the continent were flooded. The trend was reversed at the end of the Jurassic when there was a temporary, but widespread, withdrawal of the sea.
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