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CONTINENTAL DRIFT  63



            suggests that in Lower Paleozoic times there existed   Indeed, relative continental movements can modify the
            several continents separated by major ocean basins. The   pattern of ocean currents, mean annual temperature,

            similarity between ammonite species now found in   the nature of seasonal fluctuations, and many other
            India, Madagascar, and Africa indicates that only shallow   factors (Valentine & Moores, 1972) (Section 13.1.2).
            seas could have existed between these regions in Juras-  Also, plate tectonic processes can give rise to changes
            sic times.                                   in topography, which modify the habitats available for
               Paleobotany similarly reveals the pattern of conti-  colonization (Section 13.1.3).
            nental fragmentation. Before break-up, all the Gond-  The diversity of species is also controlled by con-
            wana continents supported, in Permo-Carboniferous   tinental drift. Diversity increases towards the equator
            times, the distinctive  Glossopteris and  Gangamopteris   so that the diversity at the equator is about ten times

            floras (Hurley, 1968; Plumstead, 1973) (Fig. 3.8), which   that at the poles. Consequently, drifting in a north–
            are believed to be cold climate forms. At the same time   south direction would be expected to control the

            a varied tropical flora existed in Laurasia (Fig. 3.10).   diversity on a continent. Diversity also increases with
            After fragmentation, however, the flora of the individ-  continental fragmentation (Kurtén, 1969). For


            ual continents diversified and followed separate paths   example, 20 orders of reptiles existed in Paleozoic
            of evolution.                                times on Pangea, but with its fragmentation in Meso-
               A less obvious form of dispersal barrier is climate,   zoic times 30 orders of mammals developed on the
            as the latitudinal motions of continents can create   various continents. Each continental fragment
            climatic conditions unsuitable for certain organisms.   becomes a nucleus for the adaptive radiation of the











































            Figure 3.10  Present distributions of Pangean flora and fauna (redrawn from Tarling & Tarling, 1971).
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