The vertebrate fossil record of the Permian Usili Formation (Ruhuhu Basin, Tanzania): recent advances and historical collections

The main Karoo Basin of South Africa is synonymous with the study of terrestrial late Permian ecosystems due to its near-continuous succession of diverse and well-preserved fossil vertebrate assemblages. However, this preeminence in the literature could lead local patterns to have an outsized impact on large-scale interpretations of the fossil record. Comprehensive descriptions of Permian basins outside the Karoo are necessary for refinement of these interpretations. This study provides a comparison for late Permian-aged vertebrates outside of South Africa by synthesizing data from nearly a century of vertebrate paleontology in the Usili Formation of the Ruhuhu Basin in Tanzania. We construct a new stratigraphic range chart, identify voucher specimens, and provide descriptions of four fossiliferous intervals within the Usili Formation, designated U1–U4. Although our new range chart indicates a general correlation between the Usili Formation and the Cistecephalus–lower Daptocephalus assemblage zones of the Karoo Basin in South Africa, at this time we lack sufficient data to support recognizing more than one biozone within the Usili Formation. This situation is due in part to lack of sampling higher in the section, the absence of taxa common in the Karoo (e.g., Diictodon, Cistecephalus), and the presence of endemic taxa (e.g., Kawingasaurus, Peltobatrachus). That the biozonation system of the Karoo Basin may not be perfectly applicable to other upper Permian basins of southern Africa, despite their relative geographic proximity, highlights the need for further comprehensive analysis of the terrestrial Permian outside South Africa.