EVOLUTIONARY LABILITY OF INTEGRATION IN CAMBRIAN PTYCHOPARIOID TRILOBITES

Phenotypic integration is revealed by the structure of covariances among traits. Integration can constrain evolutionary rate and direction by channeling variation into a few dimensions: lineages characterized by strong and invariant integration structures should therefore have little capacity to generate morphological novelty, especially if integration arises through direct interactions (DI) among developmental pathways. Here we compare the level and structure of integration (including the contribution of DI) in two species of exquisitely preserved Cambrian ptychoparioid trilobites from the Pioche Formation of Nevada. A total of 21 landmarks and 136 semilandmarks were digitized on 35 mature, silicified cranidia of “Eokochaspis” metalaspis (Combined Metals Member, uppermost Dyeran, traditional “Lower” Cambrian) and of “E.” nodosa (Comet Shale Member, basal Delamaran, traditional “Middle” Cambrian). The correlation structure of variation among individuals and of random deviations from bilateral symmetry (fluctuating asymmetry, FA) reveals the structure of phenotypic integration and DI, respectively. The level of integration among nine anatomical partitions of the cranidium in “E.” nodosa was markedly higher than that in “E.” metalaspis . The species are not even significantly similar in either the structure of phenotypic integration or that of DI. Phenotypic integration in “E.” nodosa is moderately but significantly influenced by DI among developmental pathways, but these play little role in patterning phenotypic integration in “E.” metalaspis . These differences in level and structure of integration are surprising given the close morphological similarity and recent common ancestry of the species. Developmental systems were evolutionary labile in these early ptychoparioids. While macroevolutionary implications of a two-species comparison are obviously limited, it is intriguing that a higher level of integration and greater contribution of DI are detected in the stratigraphically younger species. Such study of integration level and structure through time offers the potential to discover whether and how developmental constraints shape macroevolutionary diversification.