PHYLOGENY AND EVOLUTIONARY MODULARITY OF A TRILOBITE FAMILY OVER THE ORDOVICIAN RADIATION

Trilobites are organized in packages; the adult trilobite body plan is composed of a cephalon (head), thorax (midsection), and pygidium (tail). These packages, or modules, are composed of traits that 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, the hallmark of an adaptive radiation, as evolutionary rates among modules can vary along phylogenetic branches. Trilobites provide an unmatched fossil record, due to their biomineralized exoskeleton, to test the long-term relationship between macroevolutionary diversification and modularity.

However, the lack of a phylogenetic framework for major trilobite groups hampers the study of macroevolutionary questions. For instance, the trilobite family Pterygometopidae diversified during the Middle and Late Ordovician of Baltica, Avalonia, Laurentia, and Siberia. This group is traditionally comprised of four subfamilies with strong biogeographic signal; however, relationships between and within subfamilies remain unresolved. To address questions regarding the relationship between macroevolution and modularity, phylogenetic relationships must be resolved in this trilobite group. To do so, we constructed a comprehensive character matrix comprising >240 discrete and meristic characters. Analyses include taxa from all 36 genera assigned to Pterygometopidae and include Ordovician exemplars from families within the suborder Phacopina. We ran Bayesian phylogenetic analyses to produce trees that co-estimate topology and evolutionary rates using the fossilized birth-death model.

Further, we quantified the 3D morphology of the trilobite head using high-density geometric morphometrics for exemplar taxa within Pterygometopidae to identify the structure and degree of modularity of the trilobite head in this group. Further work will assess evolutionary rates for the trilobite head and, importantly, evolutionary rates of individual modules over the Ordovician Radiation to determine an increase or decrease in modularity over this diversification event.