Brasiliano/Pan-African belts " L Grenvillian belts
-¡m Pre-Grenvillian (Transamazonian/Birmian/
Eburnian) orogenic belts W////,' Pre-Grenvillian (without Transamazonian/
^ Birmian/Eburnian) orogenic belts
Cambrian. At its widest in the late Cambrian, possibly extending as much as 4000 km across, only floating graptolites were similar on both sides of the Iapetus. But as the ocean closed, swimming organisms such as the conodonts could next cross the seaway (McKerrow & Cock 1976), and later so could the mobile and eventually the fixed benthos, the trilobites and brachiopods (Fig. 2.16b). By the late Silurian, as the Iapetus Ocean narrowed to only a few hundred kilometers, benthic ostracodes scuffled their way across. By the Devonian, when the ocean was almost completely closed, freshwater fishes were similar in Europe and North America. In a refinement to the original model, Cocks and Fortey (1982) described the ocean in terms of a three-plate model with oceans separating Gondwana, Baltica and Avalonia. The smaller Avalonia broke away from Gondwana during the late Cambrian-earliest Ordovician and, together with Baltica, headed north towards Laurentia (Fig. 2.16c, d). Neuman (1984) placed islands within the Iapetus Ocean, small suspect terranes with peculiar faunas, not seen elsewhere. Even more intriguing, Baltica spun anticlockwise as it moved towards the equator picking up these various terranes on the edge of the continent (Torsvik et al. 1991). Both cladistic and phenetic techniques have been used to analyze the large amount of distributional data from within and around the
Figure 2.16 (opposite and this page) Changing ideas on the development of the Early Paleozoic Iapetus Ocean and its faunas: (a, c, d) paleogeographic reconstructions; (b) the mobility of organisms across a closing ocean; (e) a cluster analysis of the Iapetus and related Early Ordovician brachiopod faunas (tinted blocks in descending order indicate low-latitude, high-latitude, low-latitude marginal and high-latitude marginal provinces); and (f) the possible movement of the Precordilleran terrane in three stages, 1-3. A dataset of early Ordovician brachiopod distribution across the Iapetus terranes is available at http://www. blackwellpublishing.com/paleobiology/. These data may be analyzed and manipulated using a range of multivariate techniques including cluster analysis (see also Hammer & Harper 2005). (a-d, from Harper, D.A.T. 1992. Terra Nova 4; f, based on Finney 2007.)
Iapetus Ocean, all confirming in broad terms current paleogeographic reconstructions of this complex ocean system (Harper et al. 1996) (Fig. 2.16e). Finally in this apparent confusion, some terranes, such as the Argentine Precordillera, have faunas that have even switched provinces as their terranes drifted across latitudes (Astini et al. 1995) (Fig. 2.16f). But this evidence has been disputed. The view of fauna switching is not entirely supported by a geochronometric study of detrital zircons that shows that the Precordillera had an origin in Gondwana, where the basement rocks that supplied the zircons probably occur (Finney 2007). Perhaps on this occasion the faunal data require an alternative explanation.
Careful paleogeographic study has shown that some continents have been put together from numerous formerly separated strips of land. Geological mapping may highlight major fault zones, lines of disjunction between unmatched rock units on either side, but it is, in fact, the fossils that can pin down where each continental slice, or terrane, came from in the first place. A classic example is the North American Cordillera, which is a mosaic of terranes, now plastered onto the west coast of the continent, but probably originating at lower latitudes. Paleontologists have recognized so-called Boreal (northern, low-diversity) and Tethyan (southern, high-diversity) faunas of marine invertebrates in the separate terranes in the Mesozoic. In an east-west traverse across the North American Cordillera, there is a progressive northward displacement of Tethyan-type faunas of Early Jurassic age. Some of the more exotic, far-traveled terranes may have moved over 1300 km (Fig. 2.17).
Biogeography and climatic gradients have driven patterns of changing biodiversity. In broad terms, low latitudes support high-diversity faunas, and biodiversity decreases away from the tropics towards the poles. Studies on modern bivalve, bryozoan, coral and forami-niferan faunas show marked increases in diversity towards the equator, and since many cool-water species breed later in life, polar and temperate-zone animals are sometimes larger than their tropical counterparts. But this is only plausible if the growth rates are the same in both regions; they may not be. What is true today is true in the past (Box 2.7).
Many authors have suggested that changing plate configurations, oscillating between fragmentation and integration, have affected biodiversity through time. For example, the huge Early Ordovician radiation of marine skeletal faunas may be related to the breakup of Gondwana, while the end-Permian extinction event coincides with the construction of Pangaea. More recent diversifications have occurred during the late Mesozoic fragmentation of this supercontinent (Fig. 2.18).
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