The modular organization of the trilobite head in <i>Ceraurus pleurexanthemus</i>

Abstract The trilobite head is a complex structure composed of multiple fused segments which served several functions. Modelling the head as being constituted of modules, or subunits that vary semi‐independently from each other, can reveal underlying patterns of organization. Hypotheses of modular organization based on comparative developmental biology of arthropods were evaluated using geometric morphometrics. Previous work based on two‐dimensional (2D) (semi)landmark data indicated relatively high covariation between the eye and frontmost part of the head to where they form a module in best fit models of covariation patterns. This was interpreted as a developmental signal corresponding to the anteriormost, ocular segment of early arthropods. However, it is possible that the modularity patterns seen were biased by limitations imposed by the use of two‐dimensional (2D) landmarks. Newly collected three‐dimensionally preserved material of the trilobite Ceraurus pleurexanthemus was examined to assess whether these results are consistent in three‐dimensions (3D). 3D models were constructed through micro‐CT scanning and high‐density 3D geometric morphometrics was applied to rigorously quantify the shape of the trilobite head. Results of 3D analyses indicate that the eye, when treated separately from the eye ridge, forms a module with the frontmost part of the head in the best fit hypothesis of modularity. This is consistent with prior 2D analyses, but nuances are evident when treating the eye and eye ridge together as a single partition in analyses. Further, the developmental signal imparted by the ocular segment on modularity patterns may shift over ontogeny.