ONTOGENETIC MODULARITY AND METAMORPHOSIS IN AN ORDOVICIAN TRILOBITE

Some trilobites underwent drastic morphological transformations through their development. For example, the Ordovician trilobite Lonchodomas chaziensis transforms, within a single molt, from a nonadult-like globular protaspid larvae to a drastically different adult-like meraspid juvenile. This metamorphosis is interpreted to be related to a complete shift in lifestyle from a free-floating planktonic life mode into a bottom dwelling life mode. Ontogenetic modularity provides a framework to study and better understand this morphological transformation. By focusing on (co)variation within multiple individuals of a single species at multiple ontogenetic stages, shifts in patterns of covariation through development can be identified. Here we explore patterns of covariation coincident with the restructuring of the trilobite head which can be conceptualized in terms of modules which are semi-autonomous packages of highly correlated traits. A change in the number of modules or shift in modular configurations may have facilitated the restructuring of the trilobite head during this transition in morphology and lifestyle (including feeding and locomotion). Exquisitely preserved silicified specimens of L. chaziensis allows rigorous quantitative analysis in 3D. Protaspid and meraspid instars immediately before and after metamorphosis were microCT scanned and shape was quantified using three-dimensional landmark-based geometric morphometrics. Preliminary modularity analyses based on a priori hypothesis testing show that more complex models of modularity, with greater number of modules, best fit the patterns of covariation of the protaspid while simpler models of modularity, with fewer modules, are better supported for the meraspid. Thus, the drastic morphological transformation and inferred shift in life mode undergone by this trilobite is associated with an increase in integration (decrease in modularity) of the trilobite head. Beyond trilobites and arthropods, these results are consistent with studies of other animal groups that undergo metamorphosis, namely facultative paedomorphic salamanders, where integration increases post-transformation.