Modularity at different levels in trilobites

Trilobites are made of parts. These parts, or modules, may vary semi-independently such that change in one does not necessarily beget change in another. This modularity may reveal underlying patterns of organization structured by a combination of developmental and functional drivers. Further, modularity may relax constraints between parts and has implications on microevolutionary and macroevolutionary scales. Focusing on the trilobite head, a complex structure composed of fused segments that performed multiple functions, high-density three-dimensional (3D) geometric morphometrics was used to quantify shape variation and determine patterns of modularity in Ordovician (~460 Ma) trilobites. Depending on the sample, modularity can be studied at different levels of (co)variation including (1) static, (2) ontogenetic, and (3) evolutionary levels. Within a species, (1) static modularity of the trilobite Ceraurus pleurexanthemus shows that development is a major factor in structuring the underlying organization of the trilobite head. Further, within a species (2) ontogenetic modularity of the trilobite Lonchodomas chaziensis shows that abrupt and drastic metamorphosis may be associated with relatively gradual shifts in modularity patterns over ontogeny. Above the species level, (3) evolutionary modularity of the trilobite family “Pterygometopidae” shows that modularity patterns may be structured by functional pressures related to vision, specifically related to the anterior blind spot, as the front part of the head is a separate module from the eye in this group. This contrasts with static modularity patterns observed at the species level, and implies that modularity patterns within species do not necessarily translate directly to modularity patterns among species. Applying Bayesian tree inference and phylogenetic comparative methods, I document that evolutionary rates of modules are variable particularly in the subfamily Eomonorachinae from which the family Phacopidae originates to survive the Late Ordovician mass extinction. These studies of modularity at different levels of variation provide a much-needed deep time perspective, and inform aspects of each other to better understand trilobite evolution during the Ordovician and more broadly.