The stratigraphic record of the therocephalian Theriognathus (Synapsida) and its utility as a biostratigraphic index in Karoo-Aged basins

The stratigraphic record of the whaitsiid therocephalian Theriognathus in the Karoo basins of southern Africa is reviewed, with special emphasis on biostratigraphic correlations of the Ruhuhu (Tanzania) and Luangwa (Zambia) basins. Historically, tetrapod-bearing horizons in the Usili Formation of Tanzania and upper Madumabisa Mudstone of Zambia have been inconsistently correlated to the Cistecephalus Assemblage Zone (AZ), Daptocephalus AZ, or have not been conclusively subdivided to the level of the Karoo biozonation. Recent collecting in the main Karoo Basin reference section helps to resolve the first (FAD) and last (LAD) appearance datum of Theriognathus and may reconcile the differing opinions on these formation’s age assignments. The relative abundance of Theriognathus peaks in the middle third of its range in the Daggaboersnek Member of the Balfour Formation (lower Daptocephalus AZ), but a new specimen collected from the Oukloof Member in 2020 and a review of prior records confirms the FAD of Theriognathus in the middle Cistecephalus AZ. Refinements to the FAD partly help to reconcile inconsistent age assignments for Theriognathus in the literature, especially for its occurrences in the Ruhuhu and Luangwa basins which have been most recently interpreted as Cistecephalus AZ-equivalent. We suggest that the well-documented genus Theriognathus provides a reliable biostratigraphic index for the Usili Formation and upper Madumabisa Mudstone Formation, and, in conjunction with its associated dicynodont fauna, is most consistent with the upper Cistecephalus to lower Daptocephalus AZ which has been dated as late Wuchiapingian in age (ca. 256–253 Ma).