The ecological and developmental dimensions of a post-extinction radiation in Ordovician–Silurian crinoids (Superorder Flexibilia)

Abstract

The origin and early evolution of a major crinoid clade, Superorder Flexibilia, remains enigmatic despite over a century of work by paleontologists. Our uncertainty regarding the time and place of origination for members of the clade hinders our ability to characterize the ways in which the group was impacted by (and responded to) the Late Ordovician mass extinction (LOME). However, analyzing the species-level response of this group — taxonomically, morphologically, and ecologically — during its radiation in the aftermath of the LOME provides a novel case study for interpreting the processes shaping macroevolutionary dynamics during an interval of significant environmental and biotic change. In Chapter I, I construct a species-level, time-calibrated phylogenetic tree of Ordovician through early Silurian flexible crinoids using Bayesian tip-dated phylogenetic methods incorporating the fossilized birth–death process. This methodology (1) allows for the explicit testing of likely ancestry for Superorder Flexibilia and (2) provides the most temporally constrained estimates yet for the origination of Flexibilia and its major subclades. I use the paleobiogeographic distributions of flexible species to infer likely locations of origin for ancestral nodes in the tree, and document the spatial patterns of taxonomic diversification within Flexibilia, noting the possible biogeographic implications for extinction selectivity among flexible subclades. In Chapter II, I use a broader set of focal taxa (Ordovician–middle Silurian) and employ phylogenetic comparative methods to fit multiple process-based macroevolutionary models of trait diversification that test alternative hypotheses involving shifts in evolutionary tempo and/or mode across the LOME for three morphological traits directly related to crinoid ecology and development: body size, relative filtration fan area, and calyx complexity. In considering the biological significance of differential model support among the tested traits, we gain a more nuanced understanding of the manner in which biotic and abiotic drivers impacted the early evolutionary history of Superorder Flexibilia.