THE MODULAR ORGANIZATION OF THE TRILOBITE HEAD

The trilobite head served multiple functions and was composed of several fused segments. Modeling the head as being composed of modules, or subunits which vary and thus have the potential to evolve semi-independently, can reveal underlying patterns of organization. Hypotheses of modular organization based on comparative developmental biology of arthropods were evaluated using geometric morphometrics. The degree and structure of modularity were assessed using the covariance ratio (CR) and the fit of different models compared using an effect size measure derived from the CR. Previous analyses based on datasets of two-dimensional (2D) (semi)landmarks collected from two Ordovician trilobite species, Calyptaulax annulata and Cloacaspis senilis, indicated high covariation between the eyes and the anterior region of the head compared to other modules of the head. These results can be interpreted as a developmental signal corresponding to the anteriormost ocular segment of early arthropods that is retained throughout development.

Newly collected three-dimensionally preserved material of the trilobite Ceraurus pleurexanthemus was examined to assess whether these results are consistent in three-dimensions (3D). Morphologically mature silicified cranidia were picked from sieved residues of dissolved rock and then microCT scanned. To rigorously quantify shape, 3D meshes were produced from scan reconstructions and high-density geometric morphometrics was performed. In the most complex modular hypothesis, landmarks were divided into eight (8) partitions and the degree and structure of modularity was assessed using CR and the associated effect size measure as before. Results of 3D analyses are consistent with prior 2D analyses. The eyes highly covary with the anterior glabella with and without inclusion of the eye ridges. The developmental coupling of the eyes with the anteriormost region of the head may have imposed a constraint on patterns of diversification that may have either impeded or enhanced the rate of evolution depending on its congruence with selective pressures. Assessment of this will require further study of modularity across trilobite groups throughout the Paleozoic.