Meeting Abstract

100.2  Thursday, Jan. 7  Evolutionary lability of integration in Cambrian ptychoparioid trilobites WEBSTER, M.*; ZELDITCH, M.L.; Univ. Chicago; Univ. Michigan mwebster@geosci.uchicago.edu

Phenotypic integration can constrain evolution by channeling variation into few dimensions: lineages characterized by strong and invariant integration structures should have little capacity to generate morphological novelty, especially if integration arises through direct interactions (DI) among developmental pathways. The level and structure of integration are compared in two species of exquisitely preserved ptychoparioid trilobites. Landmarks and semilandmarks were digitized on mature, silicified cranidia of “Eokochaspis” metalaspis (Lower Cambrian) and “E.” nodosa (Middle Cambrian). The correlation structure of variation among individuals and of fluctuating asymmetry 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 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.