EVOLUTIONARY PALEOECOLOGY OF ORDOVICIAN THROUGH DEVONIAN CRINOIDS

Although many studies have addressed the paleoecology of Paleozoic crinoids, relatively few have examined patterns of environmental change among major crinoid clades, especially for the lower Paleozoic. The recognition of such changes is necessary for generating and testing hypotheses of mechanisms that drove the evolutionary paleoecology of Paleozoic crinoids.

To identify patterns in the environmental distributions of lower to middle Paleozoic crinoids, we assigned most Ordovician-Devonian species to one of twelve 10-million-year time bins and at least one of fourteen depositional environments. In addition, each species was assigned to one of eight taxonomic groups that reflect differences in stalk attachment and filter morphology, traits that have been hypothesized to influence crinoid distribution.

Preliminary results suggest that camerates maintained the highest overall genus and species diversity throughout the study interval, especially in shallow carbonate environments. Disparids exhibited higher diversity than other groups in shallow clastic environments from the Lower Ordovician-Lower Devonian, while dendrocrines dominated deeper clastic environments. The relative diversity of dendrocrines increased in shallow clastic environments, and that of flexibles and camerates increased in deeper clastic settings, by the Middle Devonian. Interestingly, camerates show a gradual trend of increasing relative diversity in deep clastic habitats over the study interval.

Disparids reached their peak relative diversity in the Lower Ordovician and generally became less dominant through the middle Paleozoic. Conversely, dendrocrines and flexibles show a gradual but steady increase in relative diversity during the same interval. Hybocrinids reached their highest relative diversity in the Middle to Late Ordovician, especially in relatively deep clastic and carbonate environments. Cyathocrines exhibited their highest relative diversity in carbonate environments from the Middle Silurian to Middle Devonian.

Although taxonomic and environmental assignments are in need of refinement, these data reveal clear trends in the spatiotemporal distribution of Ordovician-Devonian crinoids that promote deeper understanding of the evolutionary paleoecology of Paleozoic crinoids.