Page 58 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 45

Box 2.6 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 geologi-
cal 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 coeffi cients are listed:

Dice coeffi cient = 2A/(2A + B + C)

Jaccard coeffi cient = A/(A + B + C)

Simple matching coeffi cient = (A + D)/(A + B + C + D)

Simpson coeffi cient = A/(A + E)

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. pods, trilobites and graptolites were separated
However, paleontological data are now crucial by a major suture running the length of the
to an understanding of the fine details of the modern Appalachian and Caledonian moun-

dance of the continents through time. Wegener tain belts. On this basis, together with a few
suggested that the continents merely ploughed other lines of evidence, Wilson inferred the
through oceanic crust. But during the 1960s, existence of a much older ocean, the proto-
plate tectonic theory with seafl oor spreading, Atlantic (now termed Iapetus), that separated
the subduction of ocean crust under the con- North America from most of Europe prior to
tinents and the collision of the continents an initial collision of these continents and
themselves, provided a mechanism. In the oceanic closure in the Silurian-Devonian.
mid-1960s, during the early stages of the plate Wilson’s classic study depicted a two-
tectonic revolution, Tuzo Wilson (1966) pre- dimensional ocean with opening and closing
dicted that the remains of an ancient seaway between Europe and North America (Fig.
would be found in Lower Paleozoic rocks of 2.16a). The Iapetus Ocean fi rst opened during
the northern hemisphere. North American the Late Precambrian with the breakup of a
and European fossil assemblages of brachio- supercontinent, and developed during the