Manda Formation
Geologic formation in Tanzania
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Through Stratum
The formation was historically considered to be from the Anisian (first stage of the Middle Triassic), as it shares some fauna with the Cynognathus Assemblage Zone of South Africa. Many recent studies cast doubt on this age, suggesting that parts of it may be more recent, potentially as young the early Carnian (first stage of the Late Triassic). The dicynodont Dinodontosaurus is known from this formation as well as formations in South America which are directly dated to the late Ladinian (last stage of the Middle Triassic) and early Carnian.
History of study
One of the first to study rocks of the Manda Formation was British geologist G. M. Stockley. In 1932, Stockley explored the geology of the Ruhuhu Basin in Tanzania. He called a series of layers dating from the Late Carboniferous to the Middle Triassic the Songea Series and divided it into eight units labelled K1-K8. Stockley was also the first to describe fossils from these rocks, naming an older layer the "Lower Bone Bed" and a younger layer the "Upper Bone Bed".
In 1957, paleontologist Alan J. Charig described many more fossils from the bone beds in his Ph.D. thesis for the University of Cambridge. Charig renamed the youngest of Stockley's units in 1963, calling unit K6 the Kawinga Formation, K7 the Kingori Sandstones, and K8 the Manda Formation. Fossils were identified in many strata, invalidating Stockley's division into two distinct bone beds. Since Charig's description, the Kawinga Formation has been renamed the Usili Formation, the Kingori Sandstones have become the Kingori Sandstone Member of the Manda Formation, and Charig's original Manda Formation has become a subunit of the formation called the Lifua Member. 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).
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Age and correlations
The upper Manda Beds have been assigned to the Perovkan LVF based on reports that Eryosuchus, Shansiodon, Angonisaurus, and Scalenodon were present. Angonisaurus does seem to tie the Manda Beds to subzone C of the Cynognathus Assemblage Zone in the Karoo Basin. However, the relations of the other Tanzanian taxa are more ambiguous. Purported Tanzanian "Eryosuchus" and "Shansiodon" specimens likely represent new genera unrelated to their supposed namesakes, while Scalenodon may be endemic to Africa due to the uncertain relations of non-African "Scalenodon" species. One Upper Manda cynodont, Aleodon, has also been found in the Dinodontosaurus assemblage zone of the Santa Maria Formation in Brazil.
Paleoenvironment
The Lifua Member sandstone was deposited by rivers. Meandering, probably sandy rivers were present. In the middle to upper Lifua bone accumulations, evidence suggests "that ponds and springs were a much more common occurrence on the Ruhuhu Basin floodplains at this time than before or after"; the local climate likely became warmer and wetter then. The increase in water would have promoted perennial vegetation, which became diversity hotspots for tetrapods, resulting in more bones being deposited here.