EVIDENCE FOR A POST-STURTIAN EUKARYOTIC BIOSPHERE FROM CAP CARBONATES OF NORTHERN NAMIBIA AND ZAMBIA

The Cryogenian Period, an interval of climatic and geochemical extremes, remains one of the most important but poorly understood intervals in the history of life. Recently, microbially laminated carbonates of the Rasthof Formation, a ~716 Ma Sturtian cap carbonate exposed in northern Namibia, have produced abundant and diverse agglutinated testate and tubular microfossils consistent with lobose testate amoebae and monothalamous foraminifera, respectively. Here, we document the occurrence of testate microfossils at new localities of the Rasthof Formation and in coeval microbial laminites of the Kakontwe Formation, Zambia. The dominant forms at the new Rasthof localities (Okakuyu, Okatjovandu, and Otjomatemba) are spherical and similar to previously documented tests from the South Ombepera locality. The new localities also preserve less abundant oval, dorso-ventrally compressed and heart-shaped spheroidal forms, similar to those from other Rasthof localities. These fossils, typically found in thinly laminated microbial laminites, were present in all but 6 out of 17 residues with 5 residues preserving numerous tests (> 50). The microbial facies of the Kakontwe Formation of Zambia also contain structures that resemble agglutinated tests of the Rasthof Formation. Interestingly, elemental compositions of minerals on the surfaces of Zambian structures are consistent with biotite and titanium-rich minerals, suggesting that different minerals were available and incorporated during biological agglutination or preservation in the Kakontwe and Rasthof formations. Thin sections and residues from Kakontwe samples also contain some silicified tests, although silicification is not a common mode of preservation in the Rasthof. The discovery of silicified tests in cap carbonates of Zambia establishes yet another preservational pathway for these structures. The new reports of fossiliferous Rasthof localities and tests in Zambian carbonates support the notion that a widespread and prevalent eukaryotic biosphere thrived in the aftermath of Sturtian glaciation.