FOSSIL PHYLOGENIES REVEAL SPECIES-LEVEL DIVERSIFICATION DYNAMICS DURING THE ORDOVICIAN RADIATION OF MARINE LIFE

The ability to estimate hypotheses of evolutionary relationships (i.e., phylogenies) and the development of tree-based comparative methods to study character evolution and lineage diversification has greatly expanded research programs in paleontology. An especially fruitful avenue of current research in phylogenetic paleobiology concerns methods of time-scaling cladistic hypotheses to absolute geologic time by incorporating information regarding ancestor-descendant relationships (e.g., “budding” divergence of morphotaxa), incomplete sampling, and general diversification dynamics.

The Crinoidea (Echinodermata) are a species-rich clade of marine invertebrates with a rich fossil record spanning nearly a half billion years. Because crinoids were significant constituents of Ordovician paleocommunities and have a well-sampled fossil record, crinoids form an ideal group for dissecting patterns of lineage diversification during the Ordovician radiation and testing whether these changes were associated with biotic and environmental changes in global ecosystems.

Building on results from recent phylogenetic analyses, we assemble the first comprehensive species-level informal supertree of Ordovician crinoids (N=597). Using this phylogenetic framework, we present results of a Bayesian analysis utilizing a time varying birth-death-sampling model to generate a posterior distribution of time-calibrated phylogenies to assess temporal patterns of lineage diversity and estimate rates of speciation, extinction, and sampling parameters. Results indicate a two-phase diversification. Although net diversification peaks in the early Ordovician, absolute rates increase during the Sandbian-Katian interval, culminating in maximal species-level diversity. Comparison with global taxonomic databases suggests these patterns may transcend the Crinoidea and reflect more general patterns of clade diversification during this important interval of Earth history.