The river pearl mussel, Margaritifera margaritifera, is becoming an important tool in reconstructing past changes in climate because of its shell growth increments and longevity. Surveys of the Lutto River system and surroundings in northeastern Finland have been conducted to identify the distribution of fossil and subfossil mussel shells. By increasing the findings of mussels may enable an extension of the existing sclerochronologies, that is, temporal records based on shell growth patterns may be achieved. The distribution pattern includes the preservation characteristics of their calcareous (aragonitic) shells and periostraca, indicating which pathways these remains may have taken to survive the transition from the biosphere into the lithosphere. Principally, the distribution of the mussels depends on larval spreading by salmon fish which act as hosts. However, the post-mortem natural preservation of the shells occurs mainly as periostraca in ancient river bank and bar deposits. This is related to acidic river and percolation water. Post-mortem preservation of calcareous shells has probably been ameliorated due to anthropogenic activity. During the past centuries, the river pearl mussel has been sought after by pearl fishers who left shell remains typically around camp-fire sites. As the sites are located on topographically higher levels, the shells became less affected by acidic water. If the shells were hidden in the soil by the hunters, this may have improved their preservation potential due to higher pH values deeper in the podzol soil. Although the outer shell layer tends to be partly dissolved, these shells can be suitable for sclerochronologists. On the other hand, the limited preservation of the shells impedes reconstruction of past geographical ranges of the species, with the danger that severe spatial underestimation of their past spatial occurrence may occur.
Taphonomy of freshwater molluscs in carbonate-poor deposits: a case study of the river pearl mussel in northeastern Finnish Lapland