MORPHOLOGIC CONSTRAINTS AND THE RISE OF THE MYELODACTYLIDS; A REEXAMINATION OF EARLY PALEOZOIC CRINOID DISPARITY

Studies of crinoid morphology have been pivotal in understanding the constraints on the range of morphology within a clade as well as the patterns of disparity throughout the Phanerozoic. Newly discovered and described faunas and recent study of Early Paleozoic crinoid diversity provide an ideal opportunity to reanalyze Ordovician through Early Silurian crinoid disparity with more complete taxonomic coverage and finer stratigraphic resolution. The updated Early Paleozoic crinoid disparity curve has a much higher initial disparity during the Early Ordovician, equaling that of the Late Ordovician, and a pronounced rise in disparity during the Silurian recovery. The Silurian morphospace expansion occurs primarily within disparids and coincides with the origination and proliferation of the myelodactylids. Crinoid disparity did not remain static during the Early Paleozoic and, although less frequent than during the initial radiation, new body plans evolved following the Ordovician Extinction (e.g. the myelodactylids). Therefore, crinoid disparity increased during the Silurian due to a large expansion of one clade of disparids along with the retention of the other major clades of crinoids. Alternatively, crinoid disparity decreased following the Permian extinction with the extinction of three of the four major clades of crinoids. However, the clade that survived expanded past its Paleozoic limits just as disparids did during the Silurian. Thus, the decrease in disparity following the Permian extinction was probably due to a winnowing of species as opposed to the tightening of genetic constraints. These types of constraints may play a significant role in crinoid morphological history. However, the loss of morphologic potential most likely occurred during the Early Ordovician, and the later constraints on disparity were more likely due to ecology and the contingencies of lineage terminations rather than developmental or genetic canalization.