MORE SHELL MICROSTRUCTURES IN CAMBRIAN MOLLUSCS

Many imprints of shell microstructure have been newly discovered in phosphatic internal molds of molluscs from the Early Cambrian of China and Siberia and the Middle Cambrian of Australia. Because several cases of apparent shell microstructure in Early Paleozoic molluscs are equivocal, new observations of microstructure as well as cases from the literature have been assessed using the following approaches: recording how and where the microstructures are preserved in many specimens of the same species; mapping out the position and orientation of microstructural components in individual specimens; and comparing fossil microstructures with those seen in a variety of modern molluscs and in epoxy molds of modern shells.

The compiled data suggest that shell microstructure characters are useful for phylogenetic studies of Early Paleozoic molluscs, as members of many lineages have strikingly similar shell microstructure patterns. This hypothesis is supported by results of preliminary cladistic analyses of Cambrian molluscs using diverse combinations of character sets. Therefore, shell microstructure data should also be helpful in inferring the relationships of mollusc-like Problematica that originated during the Cambrian such as hyoliths and halkieriids. This will refine our understanding of the magnitude of the Cambrian explosion by providing a better estimate of the number of higher taxa that appeared during that interval. The cladistic analyses also suggest that shell pores were primitive in molluscs, which seemingly contrasts with published observations that modern pore-bearing molluscs differ in their pore systems and in the tissues that fill the pores.

The validity of one type of apparent shell microstructure in Cambrian molluscs has recently been questioned by Ushatinskaya and Parkhaev (2005). They argued the polygonal netting that some have interpreted as the imprint of prismatic microstructure is instead an impression of the epithelial tissue of the molluscan mantle. However, careful observations plus replication of the fossilization process using epoxy have shown that polygonal netting on these internal molds is most likely an impression of shell microstructure, though it could reflect either prismatic or nacreous microstructure.