Synapsid Diversity and the Rock Record in the Permian-Triassic Beaufort Group (Karoo Supergroup), South Africa

Abstract

This study investigates diversity patterns of Synapsida in the Permian-Triassic sequence of the Karoo Basin, South Africa. Permian-Triassic synapsids represent the dominant terrestrial tetrapods of their time and play a central role in assessing the impact of the end-Permian mass extinction on terrestrial ecosystems. On the regional scale of the Karoo Basin, synapsid diversity shows a mid-Permian extinction and a pronounced extinction event at the end of the Permian, whereas the subclades of Synapsida exhibit clade-specific diversity patterns. Taxonomic diversity estimates (TDEs) of Synapsida and its subclades are not significantly correlated with outcrop area for the complete time series. However, after exclusion of the Lystrosaurus Assemblage Zone from all data series, the TDEs of the majority of synapsid subclades show statistically significant strong positive correlations with outcrop area. Nonetheless, diversity residuals, resulting from modeled diversity estimates, exhibit clade-specific patterns with varying support for a mid-Permian event and strong support for an end-Permian extinction. The results confirm studies at the global scale and imply that synapsid diversity in the Karoo Basin is at least partially biased by the Permian-Triassic terrestrial rock record. Moreover, Anomodontia, the most speciose clade of non-mammalian synapsids, is not the sole driver of the synapsid diversity signal. Instead, there seems to be a general synapsid pattern, with each subclade diverging from this pattern to varying degrees for clade-specific reasons. Thus, despite the obvious rock record bias, the end-Permian extinction maintains its major impact on synapsid diversity and therefore on the composition and structure of past and present terrestrial ecosystems.