Page 109 - Introduction to Paleobiology and The Fossil Record

96 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

derived from a raw data matrix of the presence or absence or numerical abundance of fossils at each
site. Cluster analysis is most commonly used in ecological studies and there is a wide range of both
distance and similarity measures, together with clustering techniques, to choose from. R-mode analy-
sis clusters the variables, in most paleoecological studies the taxa, whereas Q-mode analysis clusters
the cases, usually the localities or assemblages (Fig. 4.17).
For example, Late Ordovician brachiopod-dominated assemblages from South China have been
investigated by cluster analyses (Hammer & Harper 2005) and fall into a number of ecogroups.
These data are available at http://www.blackwellpublishing.com/paleobiology/.

0 0 0 0 0 0 0 0 0 1 1 1 1 1 Lingula
1 1 0 0 0 1 1 0 0 1 1 1 1 1 Eocoelia

1 1 0 0 0 0 0 1 1 1 1 1 1 0 Stricklandia
0 1 0 0 0 1 1 1 1 1 1 1 1 0 Pentamerus

1 1 1 1 1 0 1 1 0 1 1 0 0 0 Clorinda

Leptostro Streptola Clorinda Glassia Aegiria Pentameru Eoplectod Atrypa Stricklan Tentaculi Eocoelia Bivalve Stegerhyn Lingula

Figure 4.17 In a two-way cluster analysis, an R-mode clusters the genera (bottom) and a Q-mode
clusters the community type (right). The original data matrix is in the center of the diagram. The
data indicate the reality of a shallow-water biofacies (Lingula and Eocoelia communities), and
mid to deep shelf (Pentamerus and Stricklandia communities) and outer shelf to slope (Clorinda
community) assemblages.

building up to a climax community in equi- may evolve, become extinct or migrate out of
librium with its environment. There is still the immediate area during intervals of climate
some discussion among ecologists about change thus destroying the community struc-
whether communities conform to Eltonian ture. They may, however, return and recom-
models of change (predictable over long bine to form the original communities during
periods of time), Gleasonian models (short- intervals of more favorable climate (Bennett
term, rapid change and instability) or perhaps 1997). Nevertheless, paleocommunities domi-
even both. Evidence from Quaternary, mainly nated by incumbent taxa such as the dino-
Holocene, communities suggests them to be saurs during the Jurassic and Cretaceous or
rather ephemeral (Davis et al. 2005). Species pentameride brachiopods during the Silurian