Assessing Integration and Regulation in Ancient Developmental Systems: An Example from a Cambrian Trilobite

Increasing developmental canalization and integration are hypotheses to explain macroevolutionary trends in intraspecific variation and diversification (e.g., the “genomic hypothesis” regarding the Cambrian radiation of metazoans). Techniques for assessing such developmental attributes in shape data are becoming increasingly refined and powerful, but have not yet been applied to fossils.

Landmarks and semilandmarks sampling cephalic morphology were digitized from well-preserved, noncompacted, silicified specimens of the trilobite Olenellus gilberti from the Combined Metals Member (Pioche Formation, uppermost Dyeran, traditional Laurentian “Lower Cambrian”). Deviations from the mean form (computed from multiple replicates of all specimens) were partitioned into main effects of “individuals” and “sides” in a modified two-factor, mixed model ANOVA; the interaction term measures fluctuating asymmetry (FA). A significant correlation between the structure of FA and among-individual variation suggests that integration was structured by direct interactions along or between developmental pathways. Hypotheses predicting the structure of integration on functional and developmental grounds were tested by comparing observed to expected correlations. We find strong evidence for an occipital module, associated with cephalic-trunk articulation. However, we do not find evidence for discrete glabellar and ocular modules. Rather, integration between the ocular lobes, anterior glabella, and occipital module is stronger than integration between anterior and posterior portions of the glabella. The overall level of developmental integration of the cephalon (measured as variance in eigenvalues) is relatively high.

Application of such methods to fossils should offer unparalleled insight into ancient developmental systems, and can rigorously test hypotheses of changes in the regulation and integration of developmental systems as constraints in macroevolutionary diversification.