[3] Six formations and one informal unit are currently recognized in the Songea Group (Ruhuhu basin) rocks range in age from Pennsylvanian to Anisian, including the Idusi (K1), Mchuchuma (K2), Mbuyura (K3), Mhukuru (K4), Ruhuhu (K5), and Usili (K6) formations and the informal Manda Beds, which include the Kingori Sandstone (K7) and Lifua Member (K8).

Comparison of Usili tetrapods with those of the lower Beaufort Group has suggested a broad biostratigraphic correlation with the Cistecephalus, Dicynodon, and Tropidostoma assemblage zones.

Sidor et al. (2010) recognized only one undivided tetrapod faunal assemblage in the Usili Formation, which includes Aenigmastropheus, temnospondyls, pareiasaurs, gorgonopsians, therocephalians, cynodonts, and dicynodonts, whose remains were collected from various localities.

This suggests that several therapsid genera have unequal stratigraphic ranges and temporal durations in the Ruhuhu and Karoo basins.

Except for the burnetiid MAL 240, which is unique to the Chiweta Beds, the Usili Formation hosts identical genera, including Aelurognathus, Dicynodontoides, Rhachiocephalus, Endothiodon cf.