NETWORK-BASED BIOSTRATIGRAPHY FOR THE LATE PERMIAN-MID TRIASSIC BEAUFORT GROUP IN SOUTH AFRICA ENHANCES BIOZONE APPLICABILITY AND STRATIGRAPHIC CORRELATION

The vertebrate assemblage zones (AZs) of South Africa’s Karoo Basin have become a standard for local and global correlations of Permo-Triassic strata. However, temporal, geographical and methodological limitations challenge the reliability of these biostratigraphic units. We analyzed a unique dataset comprising 1408 fossils of 115 species grouped into 19 stratigraphic bin intervals spanning the Cistecephalus, Daptocephalus, Lystrosaurus declivis, and Cynognathus AZs. Using network science, we compare frameworks based on historical data (Broom, Rubidge) and modern schemes incorporating recent lithostratigraphic, radiometric, and paleontological data (Viglietti, Formation, Member). We also test another framework suggesting diachroneity of the Daptocephalus/Lystrosaurus AZ boundary (Gastaldo) to determine if it is an improvement over other frameworks. By modelling fossil occurrence data as bipartite networks, we show that historical frameworks identify meaningful AZs and can be useful in corroborating frameworks that identify more unique AZs. None support the Cistecephalus AZ, which likely comprises two discrete communities. The Lystrosaurus declivis AZ is traced across all models, despite many shared species with the underlying Daptocephalus AZ. This suggests the extinction event across this interval is not a statistical artifact. An AZ shift with few shared species at the Katberg/Burgersdorp formation boundary may indicate a depositional hiatus. This has important implications for regional correlations, and Mesozoic ecosystem evolution. Analysis of meter-level occurrence data indicates that 20-50 m sampling intervals adequately capture Karoo AZs. Despite its different temporal groupings, the Gastaldo model still identifies the Lystrosaurus/Daptocephalus AZ shift, does not significantly improve recent AZ models, and highlights important issues with some AZ studies. Localized bed-scale lithostratigraphy (sandstone datums), and singleton fossils cannot be used to reject patterns shown by hundreds of fossil specimens, and regional (> 100 km) stratigraphic markers of the Karoo foreland basin. Our results unify the use of meter-level placements of singleton fossils to delineate AZ boundaries, and improves Karoo AZ applicability for correlation globally.