Box Analytic methods

Two main types of biogeographic analysis are widely used and are based on either phenetic or classic cladistic methods (Hammer & Harper 2005). Phylogenetic methods are being increasingly used to study past biogeographic patterns (Lieberman 2000). A third technique, area cladistics, is rapidly developing and converts a taxon-based cladogram into an area cladogram, independently of geological data; in simple terms geographic areas can be mapped onto the branches of a taxon-based tree. Cladistic methods are based on the assumption that an original province has since fragmented with the creation of subprovinces characterized by new endemics, essentially analogous to apomorphies in taxonomic cladistics (see p. 129). This is not always the case since nodes on the cladogram may equally represent widespread range expansion of taxa, perhaps associated with a marine transgression.

The phenetic methods usually start from a similarity matrix between sites based on the presence and absence of taxa, or more rarely the relative abundance of organisms across the sites (see also Chapter 4). There are a large number of distance and similarity measures to choose from. A few of the commoner coefficients are listed:

Simple matching coefficient = (A + D)/(A + B + C + D)

A is the number of taxa common to any two samples, B is the number in sample 1, C is the number in sample 2, D is the number of taxa absent from both samples, and E is the smaller value of B or C.

On the basis of an intersite similarity or distance matrix, a dendrogram can be constructed linking first the sites with the highest similarities or the closest distances. When the distance or similarity matrix is recalculated to take into account the first clusters, additional sites or genera are clustered until all the data points are included in the dendrogram. Clearly the first clusters, with the highest similarities or lower distances, have the greatest significance and less importance is usually attached to later linkages.

widely derided through the 1940s and 1950s. However, paleontological data are now crucial to an understanding of the fine details of the dance of the continents through time. Wegener suggested that the continents merely ploughed through oceanic crust. But during the 1960s, plate tectonic theory with seafloor spreading, the subduction of ocean crust under the continents and the collision of the continents themselves, provided a mechanism. In the mid-1960s, during the early stages of the plate tectonic revolution, Tuzo Wilson (1966) predicted that the remains of an ancient seaway would be found in Lower Paleozoic rocks of the northern hemisphere. North American and European fossil assemblages of brachio-

pods, trilobites and graptolites were separated by a major suture running the length of the modern Appalachian and Caledonian mountain belts. On this basis, together with a few other lines of evidence, Wilson inferred the existence of a much older ocean, the proto-Atlantic (now termed Iapetus), that separated North America from most of Europe prior to an initial collision of these continents and oceanic closure in the Silurian-Devonian.

Wilson's classic study depicted a two-dimensional ocean with opening and closing between Europe and North America (Fig. 2.16a). The Iapetus Ocean first opened during the Late Precambrian with the breakup of a supercontinent, and developed during the

Lower Paleozoic proto-Atlantic

Ma 400

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