In order to interpret the distribution and consequent diversification of the Me-sozoic reptiles, one has to realise that continents are by no means static. During Cambrian times, some 500 mya (million years ago), they were distributed along the equatorial belt of the world. They included Laurentia, Siberia, Baltica, Avalonia and the enormous Gondwanaland, the only one to extend into temperate latitudes. During the course of Silurian and Devonian periods, approximately 400 mya (Table 1), Gondwanaland moved southwards while Siberia shifted towards the north, causing major changes in the circulation of the oceans. Later, in the Carboniferous, there was a single supercontinent in the south, Gondwanaland, which comprised what is now South America, Africa, Australia, India and Antarctica, and another, Laurentia, to the north. The latter consisted of North America, Greenland, north-western Europe, and Russia west of the Urals. It is possible that these two vast landmasses may have been connected across the Equator. They became fully united to form an even larger supercontinent, named Pangaea during the Late Triassic and Early Jurassic or Lias, only to split apart again during the Cretaceous period. Finally, with the further break up of both supercontinents, the Atlantic opened up, India began its long migration from Antarctica to Asia, and the earth slowly assumed its present appearance. These 'tectonic' movements, caused by convection currents of molten magma in the earth's mantle (below the solid crust of the planet) were responsible for continental drift, and many of the climatic changes outlined above (Benton 1996).
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