Modular reorganization of the trilobite <i>Lonchodomas chaziensis</i> demonstrates mosaic patterns of phenotypic change underlie metamorphosis

Some trilobites underwent drastic morphological transformations through their development. The Ordovician trilobite Lonchodomas chaziensis transformed, in a single molt, from a globular protaspid larva to a drastically different adult-like meraspid juvenile. This metamorphosis may be related to a complete shift in lifestyle from a free-floating planktic life-mode into a bottom-dwelling benthic life-mode. By focusing on variation between individuals at distinct ontogenetic stages, shifts in covariation patterns through development can be identified to better understand this transformation. Organisms are composed of parts conceptualized in terms of modules i.e., semi-autonomous packages of highly correlated traits. Modularity is theorized to increase through ontogeny especially in arthropods which undergo metamorphosis. To test this hypothesis, we examine covariation patterns coincident with the restructuring of the trilobite head. Analyses show a decrease in the structure of modularity during metamorphosis and major shifts in the degree of modularity/integration during and preceding transformation. Further, the part which undergoes the most shape change becomes more integrated with other parts. As shape variation is organized in a mosaic fashion through ontogeny, modularity may have played a role in coordinated ontogenetic change among parts leading up to shifts in form and lifestyle occurring during the 'Plankton Revolution' of the early Paleozoic.