A Reexamination of Ordovician through Early Silurian Crinoid Morphologic Disparity

Morphologic disparity is a fundamental aspect of the analysis of the large-scale patterns in the evolution of form. Even though it is related to taxonomic diversity, morphologic disparity can show drastically different patterns through time. 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. Previous studies of crinoid disparity based on the analysis of discrete character data show an abrupt rise in disparity during the Early-Middle Ordovician followed by plateau throughout the Early Paleozoic (Ordovician through Devonian) despite extinctions and originations (Foote, 1995). Early Paleozoic disparity is reexamined in this study using a finer stratigraphic resolution, the inclusion of several newly discovered crinoid faunas, and a closer examination of intrageneric disparity.

The disparity curve is then dissected at a local scale in order to examine whether the regional scale patterns are mirrored within localities. This also provides an opportunity to examine the nature and selectivity of the Late Ordovician extinction and ensuing Silurian recovery within crinoid communities. Preliminary results show that localities within a single time bin (Late Ordovician) show a wide variety of disparity values, possibly related to water depth, environmental conditions, or substrate type. Even with this wide array of local disparities, however, the average disparity at a local scale is comparable to the regional disparity.