ASSESSING THE STRUCTURE OF INTEGRATION IN THE CEPHALON OF THE MIDDLE CAMBRIAN AGNOSTINE ARTHROPOD PERONOPSIS BRIGHAMENSIS

Morphological traits within an organism can be functionally or developmentally coupled into integrated units. Such modules are empirically recognized as sets of traits that have stronger interactions within the set and fewer, weaker interactions between sets. Evolutionary change to one trait within a module begets changes to the other traits within that module, leading to a channeling of variation into a few directions. The strength and structure of integration may therefore play a role in macroevolutionary patterns, constraining the direction of evolution along lines of least resistance and, depending on how those axes of variation align with selective pressure, affecting the rate of evolution. This study investigates the structure of integration in the middle Cambrian agnostine arthropod Peronopsis brighamensis. Silicified cephalic sclerites of this species were recovered from the Spence Shale unit of the Langston Formation in northeastern Utah. In order to assess the structure of integration, geometric morphometric methods were used to determine the structure of fluctuating asymmetry (FA)--random small deviations from perfect symmetry--that can be used to delimit modules and infer the structure of integration that arises from direct interactions among developmental pathways. The cephalon was divided into nine anatomical partitions, including six partitions of the glabella. Each specimen was photographed and digitized twice to assess measurement error, which was determined to be negligible. Exploratory analyses were conducted to determine the strength of covariation in FA between these partitions. Preliminary results show a highly integrated glabella in P. brighamensis with a structure strongly defined by anteroposterior position on the sclerite. This suggests that local morphogenetic controls may have a strong effect on the structure of integration in this species. This study serves as a starting point to compare other closely-related species, culminating in an understanding of whether and how the evolution of this clade was constrained by integration.