COMPARING TAXONOMIC AND GEOGRAPHIC SCALES IN THE MORPHOLOGIC DISPARITY OF ORDOVICIAN THROUGH EARLY SILURIAN LAURENTIAN CRINOIDS

Understanding the effects of different scales of study on estimates of morphological disparity is important in the interpretation of morphologic radiations and macroevolutionary patterns. The effects of varying taxonomic (species and genus) level and geographic (regional and local) scale on on disparity estimates are examined in a broad study of Ordovician though Early Silurian crinoids. Using discrete morphologic characters, we examined the disparity of 421 crinoids from 65 Laurentian assemblages. The choice of taxonomic level does not have an effect at the local level. However, at the regional level, the two taxonomic scales can produce different results because of variation in the number of species per genus through time and the amount of morphologic variation contained within individual genera. Crinoid disparity also differs when analyzed at different geographic scales. Regardless of the fluctuations observed in regional crinoid disparity, average local disparity was static throughout the Ordovician deviating only during the Silurian, because of the proliferation of the morphologically aberrant myelodactylid crinoids. Environmental differences, such as depth, substrate, amount of sedimentation and type, do not appear to play a major role in the level of disparity between localities. These differences often play major roles in crinoid communities, but the major morphotypes are present in most biofacies even if their greatly differing relative proportions indicate strong environmental pressures.