Subfossil bioerosion of mollusc shells from a freshwater lake, Ontario, Canada

Numerous circular, 30 to 270 μm‐diameter bore holes in several species of gastropod and bivalve shells from Julian Lake, southern Ontario, Canada are similar in appearance to the ichnotaxa Oichnus simplex, Oichnus paraboloides, and Tremichnus isp. These ichnotaxa are well described from marine systems but are rarely reported from freshwater environments.The numerous bore holes and their random placement in individual shells from Julian Lake indicate that the borers were non‐predatory. The purpose of boring may have been to scavenge either the organic material contained within the shell microstructure or the shell carbonate to be used as a source of carbonate. The Julian Lake borings may have been created by either freshwater gastropods or bacteria. The identity of the trace maker (s), however, is uncertain. Similar holes have been previously described in lacustrine settings and interpreted as dissolution pits. SEM examination of the microstructure reveals no definitive evidence for dissolution.As with marine systems, the recognition of freshwater borers indicates that bioeroders may be active contributors to carbonate sedimentation in lacustrine settings, and they also diminish the preservation potential of shells in the sedimentary record. Furthermore, as with their marine counterparts, freshwater borings create potential pathways in shells through which diagenetic porewaters can enter and dissolve or recrystallize shell carbonate.