Populations of the freshwater mussels Unio crassus and U. tumidus have declined significantly over many decades. The river Vantaanjoki, at their northern limit in southern Finland, has yielded a number of empty hinged shells of U crassus (39 shells) and U. tumidus (71 shells), which have been studied visually for taphonomical features. All of the shells are to some degree affected by taphonomical processes, particularly dissolution and fragmentation. These death assemblages of Unio shells are time-averaged as suggested by their radiocarbon (C-14) dates, with consequences for the expected temporal resolution of both death and fossil assemblages. Although the shells arc of different ontogenetic and post-mortem ages, there are no distinct differences in their preservation. Considering both taphonomical features and C-14 dating, time-averaged death assemblages, which might illustrate the time-evolving anthropogenic changes of the river environment over the last centuries, should with some caution be compared with modern populations for conservation evaluations.