Diverse vase-shaped microfossils within a Cryogenian glacial setting in the Urucum Formation (Brazil)

Abstract

Vase-shaped microfossils (VSMs) attributed to testate amoebae occur globally in diverse assemblages in Tonian rocks. These microfossils have thus been considered a potential biostratigraphic tool, especially for the interval between 789 and 729 Ma. Here we report a diverse and well-preserved in situ VSM assemblage, including several taxa previously considered as Tonian, within glacially influenced deposits for which sedimentological data support a Cryogenian age. However, the more robust recent multi-proxy correlation proposed by Freitas et al. (2021) indicates a Marinoan age for the studied succession. Detrital zircon data provide a maximum depositional weighted mean age of 749 ± 3 Ma for the VSM-bearing, organic-rich, fine-grained deposits within the Marinoan sequence in the Urucum Formation. Nine taxa are described from the fine-grained deposits in the upper Urucum Formation, Jacadigo Group, Brazil: Cycliocyrillium simplex, Bonniea dacruchares, Bonniea pytinaia, Bombycion micron, Limeta lageniformis, Palaeoarcella athanata, Trigonocyrillium horodyskii, Pakupaku kabin and cf. Taruma rata. The discovery of well-preserved in situ VSMs attributable to specific Tonian taxa within a Cryogenian succession challenges previous thinking that these organisms disappeared from marine ecosystems at the end of the Tonian.