SPATIO-TEMPORAL VARIATIONS IN LAND-SNAIL ASSEMBLAGES RECOVERED FROM A PLEISTOCENE CINDER CONE

Species compositions of fossil assemblages may fluctuate through space and time in response to environmental and/or taphonomic factors, but these relationships are sometimes difficult to diagnose. In Quaternary assemblages, which are generally well-preserved and contain mostly extant species, these kinds of assessments are feasible because of excellent preservation and potentially high temporal resolution. Here, we investigated whether compositions and spatio-temporal patterns in a sequence of Quaternary land-snail assemblages preserved in the Canary Islands were influenced by taphonomic bias, climate change, or both.

Fossil assemblages were sampled at Montaña Negra, a Pleistocene cinder cone volcano in Tenerife, dated to Marine Isotope Stage 8. Two separate beds (Scoria Unit A and B) rich in fossil land snails spanning ~2ka (from 302-299.9ka) were sampled by hand-picking shells in one and a half meter increments laterally to determine the extent and nature of compositional variation within each bed, as well as differences between the two beds.

Statistical comparisons between the two fossil units illustrated significant differences in body size, species composition, and abundances of individuals. Unit B (11 species) showed a significantly higher diversity and density than Unit A (three species). Interestingly, large body size species (maximum shell length >10mm) dominated Unit A whereas small body size specimens (maximum shell length <10mm) were present at Unit B.

Although environmental change may have driven, at least partially, changes in species composition, density of shells, and body size, these differences are likely to have also been impacted by taphonomic (preservational) bias. The two investigated fossil units differ notably in their matrix. Unit A is characterized by larger clasts of scoria while Unit B contains small to medium sized scoria and fines upward to an unconsolidated paleosol. Accordingly, we propose that clast size will substantially affect land snail species preservation, with larger clasts favoring the interstitial preservation of large body size taxa. Ongoing stable isotope analyses of the shells will allow us to test the hypothesis that these assemblages may also differ because of climatic changes.