Abstract

Faunal lists were compiled for the presence or absence of 94 genera from 16 regions. Four similarity coefficients (Simple Matching, Dice, Jaccard, and Otsuka) were computed for the data matrix in Q mode. The similarity matrices were clustered by both the unweighted pair group method and by single linkage. The dendrograms based on the Simple Matching coefficient are not readily interpretable: coefficients that consider negative matches are unsatisfactory for biogeographic studies. The remaining dendrograms have very similar major structure. Elimination of rare genera, those known from only one locality, reduced the number of genera to 64, but the resulting dendrograms differed little from those based on the full data matrix. With the present data set, the methods used are relatively robust. Ordination of the Dice similarity matrix by principal coordinates analysis and nonmetric multidimensional scaling was also employed. The latter technique produced the best model of the relations between the localities as assessed by correlation between the original distances in multidimensional space and in the reduced 3-dimensional space. In this respect, it is superior to principal coordinates and to both of the clustering techniques that were used. All the methods, however, revealed a broadly similar structure to the data.

The nonmetric multidimensional scaling model can be readily interpreted. It confirms the previous division of faunas into these characteristic of the restricted shelf environment and these that inhabited the shelf margin—open-ocean regions. It allows recognition of a North American faunal province that may be divided into two subprovinces, one characteristic of the restricted shelf, the other typically found in the northeastern part of the continent and bearing a shelf margin—open-ocean fauna. We have no strong evidence of a homogeneous faunal belt peripheral to the entire cratonic restricted shelf subprovince. The available evidence shows that the faunas inhabiting this shelf-margin-open-ocean environment were, in contrast, relatively heterogeneous. Dependent on their geographic position, these faunas display, to varying extent, the effect of Asiatic and European influences.

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