101-3 Shifts in Evolutionary Modularity Over the Late Ordovician Mass Extinction in a Trilobite Family

Abstract:

Animals are organized in packages. These packages, or modules, are composed of traits that have the potential to evolve semi-independently such that change in one module does not necessarily beget change in another. At the macroevolutionary level, this decoupling and relaxation of evolutionary constraints is thought to promote evolvability. Thus, it is thought that modularity facilitates rapid diversification in diverse evolutionary directions as evolutionary rates among modules may vary along phylogenetic branches. The extent to which modularity mediates organismal response to environmental change, especially during biotic crises, is unknown. Trilobites provide an unmatched fossil record, due to their biomineralized exoskeleton, to examine modularity at the macroevolutionary level in deep time, particularly in the context of mass extinction. Specifically, modularity in the trilobite cephalon (head shield) is of interest due to it being a complex structure composed of several fused segments which served multiple functions. The Ordovician trilobite family “Pterygometopidae” well-expresses this segmentation in the glabella, or medial lobe of the cephalon. Previous Bayesian phylogenetic work resolved relationships within this family and related taxa showing that Pterygometopidae is paraphyletic with Phacopidae nested within the subfamily Eomonorachinae, which survives the Late Ordovician mass extinction to radiate post-extinction in the Silurian. Pre-extinction, the best fit model of modularity in the cephalon is one where the eye constitutes a module with the posterior part of the glabella contrasting with modularity patterns observed at the species level. Post-extinction, the structure of modularity (number and arrangement of modules) is relatively unchanged, yet the degree of modularity between partitions shifts such that within-module integration is stronger. Rates of evolution vary by subfamily and, along branches, by module. High-disparity/high-rate subfamilies within Pterygometopidae become extinct by the end of the Ordovician. Interestingly, Eomonorachinae has relatively low disparity and low evolutionary rates leading up to the Late Ordovician mass extinction which its descendant Phacopidae survives. Further, disparity and evolutionary rates remain relatively low in Phacopidae as it radiates in the Silurian. Modularity patterns of surviving lineages may have influenced recovery over a macroevolutionary bottleneck following a mass extinction. Future work will examine the extent to which extinction itself may be a driver of modularity patterns through time, particularly in the context of ongoing biotic crises, and its effects on intrinsic forces governing trait (co)variation.

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