THE EFFECTS OF RARITY AND ABUNDANCE DISTRIBUTIONS ON MEASUREMENTS OF LOCAL MORPHOLOGICAL DISPARITY IN ORDOVICIAN CRINOIDS

Understanding the relationships between morphological disparity and environment, geography, and scale require examination at the local level. Even with disparity metrics that are inherently sample size independent, the nature of rare species and the segregation of common and rare species within morphospace can create substantial sampling biases. Eight well-sampled Late Ordovician crinoid assemblages were examined for potential biases in the study of local disparity. The disparity is based on the ordination of discrete characters. The rare and common species within these assemblages contributed equally to disparity. In spite of this pattern, rare species in some localities occupy a distinct and separate area of morphospace in relation to common species, causing disparity to vary greatly with sampling intensity. Morphological rarefaction based on the number of specimens shows that disparity weighted by abundance is constant past a sample size of approximately 30 individuals. This metric is dependent on the evenness within an assemblage as well as the abundance within subgroups in morphospace. Weighted disparity gives a view of the functional disparity of an assemblage that is more applicable in studies of local disparity although unweighted disparity is still preferred in regional scale studies and in investigations of morphospace filling through a clade's history. Compensating for biases in local disparity is a critical prerequisite to interpreting patterns of disparity across scales and through time, space, and environment.