UNVEILING DIVERSE BIOMINERALIZATION PATHWAYS IN CRYOGENIAN ARCELLINID PROTISTS

The emergence of biomineralization marks a major evolutionary innovation within Eukarya, having arisen independently across multiple clades. However, the fossil record of early biomineralized structures is sparse and often lacks clear biological affinity prior to the terminal Ediacaran. Arcellinid testate amoebae are widely distributed in Proterozoic rocks, and stand out by the diversity of their vase-shaped tests. Here, we present evidence of at least two different biomineralization processes in Neoproterozoic vase-shaped microfossils (VSMs), based on high-resolution synchrotron imaging techniques, within the context of Arcellinid crown group diversification. Exceptionally well-preserved VSMs were recovered from stromatolitic clasts within a diamictite at the base of the Cryogenian Urucum Formation (Jacadigo Group, western Brazil). Specimens of the Genus Cycliocyrillium are consistently preserved as organic-rich calcareous tests, with minor contributions of Fe, Ni and Zn. In contrast, specimens of Taruma rata display a unique architecture with a rigid external siliceous test enclosing a delicate internal organic wall. The unusual fidelity of preservation is attributed to unusual taphonomic conditions driven by the distinctive ocean chemistry prevailing between the two Cryogenian "snowball Earth" glaciations. Successive early diagenetic cementation within peritidal microbialites, likely related to supersaturated marine waters, encapsulated the trapped and bound tests, protecting them from later degradation. This distinctive preservation mode, resembling a “Matryoshka effect” (after the nested Russian dolls), provides a rare window into early eukaryotic biomineralization. The combined mineralogical and morphological data not only suggest that different Arcellinid genus employed contrasting biomineralization strategies, but also lend support to recent phylogenetic models for this group.