AN EXAMINATION OF WEIGHTED DISPARITY WITHIN EARLY PALEOZOIC CRINOID COMMUNITIES

The quantification of disparity is an important tool in deciphering morphologic constraints and innovation as well as evolutionary and ecological patterns. Morphologic disparity is relatively understudied in relation to taxonomic diversity, especially at finer geographic scales. The study of local disparity attempts to capture the range of morphologies present in a community of organisms, but biased sampling can have a severe effect and this metric does not capture the functionality of the community. Weighting local disparity by abundance combines both the range of morphology and community structure thus giving a better view of the relationship between morphology and ecology. This approach is similar to the study of functional ecology, which centers on determining the function role that individual species play within an ecosystem. The results of using the different metrics of local disparity are examined in a broad study of Ordovician through Early Silurian crinoids. The disparity of 421 crinoids from 65 Laurentian communities was examined using discrete morphologic characters. The within community abundances are based on published counts, museum collections, and field surveys. Average weighted disparity at the community level shows similar patterns to that of the local disparity, but weighted disparity has a much greater degree of variation between individual localities. Biofacies with a low ratio of weighted and unweighted disparity display the patterned community structure (based on aerosol filtration theory) that is often reported in crinoid assemblages. Tentative results indicate that these structured communities began appearing no later than the Late Middle Ordovician.