Successive assemblages of upper Permian vertebrates in the upper Madumabisa Mudstone Formation of the Luangwa Basin, Zambia

Abstract

The mid-Luangwa Basin has a continuous area of Permo–Triassic fossiliferous exposure, in contrast to the faulted outcrops of the northern basin where most fossils of the 20th century were collected. The richest deposits are in the upper Permian upper Madumabisa Mudstone Formation, which preserve a diverse therapsid assemblage with accompanying pareiasaurian reptiles, temnospondyl amphibians, actinopterygians, and elasmobranchs. In recent analyses of biogeography and ecosystem structure, the upper Madumabisa Mudstone fauna has been treated as a single operational unit, biostratigraphically correlated to the therapsid-dominated Cistecephalus and/or Daptocephalus assemblage zones of the South African Karoo Basin. However, increased sampling, a revised stratigraphic framework, and substantial geological observations in 2018–19 now allow us to delineate at least two distinct vertebrate assemblages from the upper member of the Madumabisa Mudstone Formation. We also identify four lithofacies including fluvial, floodplain, lacustrine, and alluvial plain facies in the lower and upper Madumabisa Mudstone. The assemblages mirror turnover events seen in the Karoo Basin between the upper subzone of the Endothiodon Assemblage Zone and the overlying lower subzone of the Daptocephalus Assemblage Zone (AZ), which bracket a shorter and relatively less well defined Cistecephalus AZ. The lower assemblage is dominated by cryptodonts such as Oudenodon and contains records of Endothiodon and Odontocyclops, whereas the upper assemblage preserves the dicynodontoids Daptocephalus, two species of Dicynodon, two novel lystrosaurid species, and the cynodont Procynosuchus. Successive assemblages in the Luangwa Basin offer a geographically distinct study system for ecosystem changes leading up to the end-Permian mass extinction.

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