Paleontologic Evidence For Continental Drift

Continental drift has affected the distribution of ancient animals and plants (Briggs, 1987) by creating barriers to

Areas of tropical coal forests at 300 Ma which some 50 Ma later became vast hot deserts

Areas of glaciation between 300 and 250 Ma with arrows indicating known directions of ice movement

Figure 3.9 Use of paleoclimatic data to control and confirm continental reconstructions (redrawn from Tarling & Tarling, 1971).

their dispersal (Hallam, 1972). An obvious example of this would be the growth of an ocean between two fragments of a supercontinent which prevented migration between them by terrestrial life-forms. The past distribution of tetrapods implies that there must have been easy communication between all parts of Gondwana and Laurasia. Remains of the early Permian reptile Mesosaurus are found in Brazil and southern Africa. Although adapted to swimming, it is believed that Mesosaurus was incapable of travelling large distances and could not have crossed the 5000 km of ocean now present between these two localities.

Oceans can also represent dispersal barriers to certain animals which are adapted to live in relatively shallow marine environments. The widespread dispersal of marine invertebrates can only occur in their larval stages when they form part of the plankton (Hallam, 1973b). For most species the larval stage is too shortlived to exist for the duration of the crossing of a large ocean. Consequently, ancient faunal province boundaries frequently correlate with sutures, which represent the join lines between ancient continents brought into juxtaposition by the consumption of an intervening ocean. The distribution of Cambrian trilobites strongly suggests that in Lower Paleozoic times there existed several continents separated by major ocean basins. The similarity between ammonite species now found in India, Madagascar, and Africa indicates that only shallow seas could have existed between these regions in Jurassic times.

Paleobotany similarly reveals the pattern of continental fragmentation. Before break-up, all the Gond-wana continents supported, in Permo-Carboniferous times, the distinctive Glossopteris and Gangamopteris floras (Hurley, 1968; Plumstead, 1973) (Fig. 3.8), which are believed to be cold climate forms. At the same time a varied tropical flora existed in Laurasia (Fig. 3.10). After fragmentation, however, the flora of the individual continents diversified and followed separate paths of evolution.

A less obvious form of dispersal barrier is climate, as the latitudinal motions of continents can create climatic conditions unsuitable for certain organisms.

Indeed, relative continental movements can modify the pattern of ocean currents, mean annual temperature, the nature of seasonal fluctuations, and many other factors (Valentine & Moores, 1972) (Section 13.1.2). Also, plate tectonic processes can give rise to changes in topography, which modify the habitats available for colonization (Section 13.1.3).

The diversity of species is also controlled by continental drift. Diversity increases towards the equator so that the diversity at the equator is about ten times that at the poles. Consequently, drifting in a north-south direction would be expected to control the diversity on a continent. Diversity also increases with continental fragmentation (Kurten, 1969). For example, 20 orders of reptiles existed in Paleozoic times on Pangea, but with its fragmentation in Meso-zoic times 30 orders of mammals developed on the various continents. Each continental fragment becomes a nucleus for the adaptive radiation of the

Taylor Continental Drif
of identical reef-forming corals followed later by tropical coal forests

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

Figure 3.11 Correlation of invertebrate diversity with time and continental distribution. A, earlier Pangea; B, fragmentation of earlier Pangea producing oceans preceding Caledonian (I), Appalachian (2), Variscan (3), and Uralian (4) orogenies; C, suturing during Caledonian and Acadian orogenies; D, suturing during Appalachian and Variscan orogenies; E, suturing of Urals and reassembly of Pangea; F, opening of Tethys Ocean; G, fragmentation of Pangea. a, Gondwana; b, Laurasia; c, North America; d, South America; e, Eurasia; f, Africa; g, Antarctica; h, India; i, Australia (after Valentine & Moores, I970, with permission from Nature 228,657-9. Copyright I970 Macmillan Publishers Ltd).

Figure 3.11 Correlation of invertebrate diversity with time and continental distribution. A, earlier Pangea; B, fragmentation of earlier Pangea producing oceans preceding Caledonian (I), Appalachian (2), Variscan (3), and Uralian (4) orogenies; C, suturing during Caledonian and Acadian orogenies; D, suturing during Appalachian and Variscan orogenies; E, suturing of Urals and reassembly of Pangea; F, opening of Tethys Ocean; G, fragmentation of Pangea. a, Gondwana; b, Laurasia; c, North America; d, South America; e, Eurasia; f, Africa; g, Antarctica; h, India; i, Australia (after Valentine & Moores, I970, with permission from Nature 228,657-9. Copyright I970 Macmillan Publishers Ltd).

species as a result of genetic isolation and the morphological divergence of separate faunas. Consequently, more species evolve as different types occupy similar ecological niches. Figure 3.11, from Valentine & Moores (1970), compares the variation in the number of fossil invertebrate families existing in the Phanerozoic with the degree of continental fragmentation as represented by topological models. The correlation between number of species and fragmentation is readily apparent. An example of such divergence is the evolution of anteating mammals. As the result of evolutionary divergence this specialized mode of behavior is followed by different orders on separated continents: the antbears (Edentata) of South America, the pangolins (Pholidota) of northeast Africa and southeast Asia, the aardvarks (Tubulidentata) of central and southern Africa, and the spiny anteaters (Monotremata) of Australia.

Continental suturing leads to the homogenization of faunas by cross-migration (Hallam, 1972) and the extinction of any less well-adapted groups which face stronger competition. Conversely, continental rifting leads to the isolation of faunas which then follow their own distinct evolutionary development. For example, marsupial mammals probably reached Australia from South America in the Upper Cretaceous along an Antarctic migration route (Hallam, 1981) before the Late Cretaceous marine transgression removed the land connection between South America and Antarctica and closed the route for the later evolving placental mammals. Sea floor spreading then ensured the isolation of Australia when the sea level dropped, and the marsupials evolved unchallenged until the Neogene when the collision of Asia and New Guinea allowed the colonization of placental mammals from Asia.

How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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