Investigating the potential of African land snail shells (Gastropoda: Achatininae) for amino acid geochronology

Aragonitic calcium carbonate (CaCO3) terrestrial mollusc shells with complex shell microstructures, such as those in the African subfamily Achatininae, have the potential to be used to build amino acid geochronologies across the African continent. However, as different microstructural shell layers are likely to have different protein compositions, sampling strategies need to be developed to identify the most appropriate shell portion to target. To test possible variability in protein degradation rates between microstructural layers, sampling of a single microstructural shell layer (the ‘nacreous’ layer) was compared to sampling all three aragonitic layers (’3 A L’) in modern and fossil shells. Reliable isolation of the nacreous layer in all samples proved impractical, and additional complications arose due to mineral diagenesis induced by sampling with a drill. Pleistocene fossils of Lissachatina sp. And modern specimens of Achatina tavaresiana Morelet, 1866 were shown to have an intra-crystalline protein fraction. The 3 A L shell portion adhered more closely to closed system behaviour in heated modern, and fossil, samples. The intra-crystalline protein degradation (IcPD) patterns of Achatininae fossil samples were not consistent with IcPD in forced degradation experiments at high temperatures in the laboratory. However, reliable degradation trends were observed in the 3 A L shell portion, demonstrating the potential of fossil achatinids for building relative amino acid geochronologies across Africa. •Achatinid shell has potential for building amino acid geochronologies across Africa.•Different proteins and relative rates of degradation between different shell portions.•Mineral diagenesis may occur during nacreous drilling and caution is required.•Intra-crystalline fraction of the three aragonitic layers recommended for sampling.