Regional biotic turnover dynamics in the Plio-Pleistocene molluscan fauna of the Wanganui Basin, New Zealand

Sampling gaps and variable preservation greatly complicate attempts to interpret patterns of biotic change from the fossil record. This study investigates whether observed patterns of first and last occurrences of mollusc species can be explained by poor or uneven sampling of a diverse (>800 species) Plio-Pleistocene marine fauna from the Wanganui Basin, North Island, New Zealand. Estimates of sampling completeness are high, ranging from 78 to 98% (mean 88%) despite several sampling gaps in the succession. Stratigraphic permutation tests suggest that most of the apparent peaks in the local first and last appearances of species are caused by uneven sampling. However, peaks of last occurrences of species during the Late Pliocene (2.5–2.0 Ma), and of first occurrences in the Middle Pleistocene (0.75–0.5 Ma), cannot be classified as artefacts, and appear to represent real biological events. A wide Early Pleistocene gap in the stratigraphic section in the basin prevents the precise estimation of the magnitude, duration or timing of these biotic transitions. Mechanisms for the biotic change remain elusive. Better palaeoenvironmental control and detailed ecological study of selected molluscan clades are required to test alternative causal hypotheses. However, similar turnover episodes have been documented from the temperate and tropical Western Atlantic, suggesting a global biotic perturbation on shallow shelves in response to Plio-Pleistocene global environmental change.