Meeting Abstract

59.2  Jan. 7  The esthetal canal system in chitons (Mollusca: Polyplacophora) and homologous structures in Lower Cambrian small shelly fossils. VINTHER, Jakob; Yale University

The esthetal canal system in chitons is one of the most complex canal systems within a mineralized matrix with both a sensory and secretory function. The esthete system is regarded as a distinct feature of the shell-plates in chitons, but it may have already been present in the aculiferan stemgroup. The Sachitida He, 1980 (Lower to Middle Cambrian), where forms like Halkieria Poulsen, 1967 belong, are common constituents of early Cambrian small shelly fossil assemblages. They sometimes exhibit a similar canal system within their sclerites. The significance and function of this canal system, as well as the systematic position of the sachitids within the Metazoa have been debated for some time. A reinvestigation of Halkieria revealed morphological evidence relating sachitids to the molluscs, especially the chitons. This assignment is opposed to earlier suggestions relating the sachitids to the brachiopods, annelids or lophotrochozoans. The morphology of the esthetal canal system in modern chitons is constrained by a number of factors. Acid macerated material of Sinosachites (=Thambetolepis) delicatus from the Lower Cambrian of Australia, which is abundant and preserved in great detail, reveals evidence that the canal system functioned in a similar manner to that in modern chitons. There are a number of remarkable similarities between these two canal systems in external morphology and ontogeny, which further supports the relationship of Halkieria and other sachitids to the molluscs.