EVALUATING THE TAXONOMIC DISPERSION OF ORDOVICIAN PALEOCOMMUNITIES OF THE GREAT BASIN: A NEW APPROACH TO THE ESTIMATION OF ECOSPACE UTILIZATION

Analysis of temporal and environmental patterns of species richness (S) is a standard paleoecological approach. The utility of this method is limited, however. Because S is affected by taphonomic biases, uneven sampling, and heterogeneities in diversity-abundance structure, true species richness is an unknowable quantity. Moreover, S includes no information about the high-level taxonomic diversity of a paleocommunity –it cannot distinguish between a collection consisting of 5 species of the same family (taxonomically clustered) and one consisting of 5 species from five different families (taxonomically dispersed). This information is useful in assessing the ecological diversity of a paleocommunity, as there is broad correlation between taxonomy/morphology and ecological role. We introduce a simple metric ("taxonomic dispersion") which weighs species according to the level at which they are taxonomically unique to a collection. This generates a score which allows quantitative distinction between the two hypothetical scenarios discussed above. Because this score is in part a function of S, it may be normalized by regression. This method is used to analyze a database consisting of ~300 individual collections with minimum richness of 4 from the Upper Cambrian and Ordovician of the Great Basin. Comparison of taxonomic dispersion scores with scores generated by semi-independent ecological weights indicates that there is a strong correlation between taxonomic dispersion and ecological diversity in this dataset. While there is no clear temporal pattern in species richness, there is a striking pattern in the residuals of dispersion regressed on richness: Upper Cambrian and Lower Ordovician collections are characterized by low dispersion, while collections from the Middle Ordovician generally receive high dispersion scores. This finding supports other data which indicate that the Lower-Middle Ordovician boundary is a critical interval of paleoecological change. Combined with analyses of diversity-abundance structure, taxonomic dispersion may provide a useful tool for investigating paleoecological patterns.