Stratigraphic analysis of the Sterkfontein StW 573 Australopithecus skeleton and implications for its age

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

“…The H. naledi fossils occur without a direct association with non-hominin macrofossil remains, and are found deep inside the difficult to access U.W.101-Dinaledi Chamber (Dirks et al, 2015). The Dinaledi Chamber is characterised by a sedimentary environment that is geochemically and sedimentologically distinct from the rest of the Rising Star Cave (Dirks et al, 2015), and the fossiliferous deposit it contains is profoundly different from other known hominin-bearing cave assemblages in the CoH (e.g., Reynolds and Kibii, 2011; Dirks et al, 2010; Pickering et al, 2011a; Dirks and Berger, 2013; Bruxelles et al, 2014). The fossils occur as a dense bone accumulation in mostly unconsolidated muddy sediment that largely originated from within the cave through weathering of the dolomite host rock (Dirks et al, 2015).…”

“…For the past 3 million years, hominin-bearing deposits in caves formed in broadly similar settings, involving debris cone accumulations near cave openings (Partridge, 1973; Wilkinson, 1985; Brain, 1993; Pickering et al, 2007; de Ruiter et al, 2009; Dirks and Berger, 2013; Herries and Adams, 2013; Dirks et al, 2010, 2016b; Bruxelles et al, 2014; Stratford et al, 2014), with deposits cemented by carbonate-rich waters dripping from cave ceilings (e.g., Wilkinson, 1985; Pickering et al, 2011b). In contrast to all other hominin deposits in the CoH, the deposits that host H. naledi in Rising Star Cave are composed of largely unconsolidated, mud-clast breccia in a mud matrix with no evidence of coarse clastic sediment being carried in by water flow.…”

age of fossil StW573 (‘Little Foot’): Reply to comments by Stratford et al. (2017)

“…On that occasion the flowstones around the fossil (F2, F3 and F4) were examined and J.K. pointed out that these flowstones would most likely turn out to be fracture fillings once the fossil was excavated and the outcrop clear. This was subsequently confirmed by Bruxelles et al 11 (therefore the statement that we 'admit' the intrusive character of these flowstones is strange and inconsistent with the actual facts). However, despite having both visited the locality several times, we have seen no evidence to convince us that the lowest flowstone, F1, is not a stratigraphic one.…”

“…We did not, and do not, explicitly posit that this upper cave was separate from the one that contained Member 4 (now in the open excavation), but from outcrop observations this cannot be excluded either. Point 6: Whilst Bruxelles et al 11 do not interpret the sediments surrounding StW573 as being debris flows, we point out that their descriptions of the sediments are entirely consistent with debris flows. The fact that the body was mummified does not exclude this interpretation.…”

“…However, despite having both visited the locality several times, we have seen no evidence to convince us that the lowest flowstone, F1, is not a stratigraphic one. Also, Bruxelles et al 11 hardly describe this flowstone and present no such evidence. In our discussion of the palaeomagnetic results of Herries and Shaw 10 we clearly mention both possibilities for F1 and discuss the consequences.…”