COMPARING LOCAL AND REGIONAL MORPHOLOGIC DIVERSITY IN ORDOVICIAN THROUGH EARLY SILURIAN LAURENTIAN CRINOIDS

The study of morphologic diversity (disparity) is influential in the understanding of large-scale macroevolutionary trends. However, disparity has rarely been studied on a finer than regional scale. Documenting the relationships between local and regional morphological diversity is important in recognizing the biases in comparing disparity through time, as well as giving insight into the ecological and environmental factors that influence the evolution of form.

Crinoids represent the quintessential group with which to study morphologic disparity, especially in the Ordovician and Silurian. Crinoids are well studied, contain a wide variety of taxa and morphologies, and are present in most Paleozoic marine assemblages. Due to the wide array of morphologic forms, morphology is quantified using discrete characters and morphologic diversity is calculated as the mean pairwise distance between species in an assemblage. Crinoid occurrence data have been compiled from the literature and supplemented with museum, private, and field collections. The data have been binned at the stage level in order to compare local and regional morphologic diversity.

Preliminary results indicate no significant difference between average local and regional morphological diversity in either magnitude or pattern through the Ordovician and Early Silurian. Individual occurrences, however, can be significantly higher or lower than the regional morphological diversity. A fine-scale examination of the type Cincinnatian (Upper Ordovician), which contains both morphologically diverse and depauperate occurrences, indicates that there may be an environmental control on disparity, related to water depth. Shallower assemblages show higher disparities than deeper ones, with similar values through time, despite differences in taxonomic composition.