Mollusk maxima and marine events in the Middle Ordovician of Baltoscandia

Abstract

Except for the scattered cephalopod conchs that have given the rock type its name, mollusk fossils are relatively rare in the Middle Ordovician ‘orthoceratite limestone’ of Sweden. However, an interval in the Darriwilian stands out as being unusually rich in various types of mollusks. Throughout southern Sweden, gastropods became relatively abundant close to the Volkhov–Kunda Baltoscandian Stage boundary, and their numbers increase significantly in the Kundan. This increase is most apparent in the microscopic realm. A distinct peak in abundance is seen toward the middle Kundan, across the boundary between the regionally recognized Asaphus expansus and Asaphus raniceps trilobite zones, close to the Lenodus variabilis–Yangtzeplacognathus crassus conodont Zone boundary (Dw1–Dw2 transition). Hyoliths, which are otherwise exceptionally rare, are also more common in these beds, as are putative bivalves. Moreover, macroscopic cephalopod conchs occur in unusually high numbers. Gastropods and other small mollusks become relatively rare again in the upper half of the Kundan, but cephalopods are concentrated in some beds. The mollusk-rich intervals coincide with diverse assemblages and multiple morphotypes were distinguished. Point counting of skeletal grains in thin sections showed that the relative abundance of gastropods fluctuates in a pulse-like (cyclic?) manner throughout the studied succession. The concurrent paleontologic and sedimentologic development suggests a correlation to sea level, and thus that gastropod abundance can be used as a paleobathymetric proxy. Abundance peaks are associated with inferred lowstand intervals. Closely similar abundance patterns throughout southern Sweden suggest that the increase in mollusks was a geographically widespread phenomenon, perhaps even global in extent.