Shell Microstructures in Early Mollusks

Shell microstructures in some of the oldest known mollusk fossils (from the early to middle Cambrian Period; 542 to 510 million years ago) are diverse, strong, and in some cases unusual. We herein review our recent work focused on different aspects of shell microstructures in Cambrian mollusks, briefly summarizing some of the major conclusions from a few of our recent publications and adding some new analysis. Overall, the data suggest that: (1) mollusks rapidly evolved disparate shell microstructures; (2) early mollusks had a complex shell with a different type of shell microstructure in the outer layer than in the inner one; (3) the modern molluscan biomineralization system, with precise control over crystal shapes and arrangements in a mantle cavity bounded by periostracum, was already in place during the Cambrian; (4) shell microstructure data provide a suite of characters useful in phylogenetic analyses of mollusks and mollusk-like Problematica, allowing better determination of the magnitude of disparity during the Cambrian as well as understanding of how the body plans of extant phyla were built through time; (5) calcitic sem i-nacre, a type of shell microstructure characteristic of brachiopods and bryozoans, occurred in Cambrian mollusks, suggesting a deeper level of homology in the shells of these lophotrochozoan taxa; and (6) laminar shell microstructures, which are the strongest (most fracture resistant) but most energetically expensive and slowest to build, were common in Cambrian mollusks, suggesting predation was a powerful selective force at that time and providing additional evidence that the origin of mobile predators was a contributing cause of the Cambrian diversification event (Cambrian explosion) and the appearance of mineralized skeletons.