Ministromatolite modular framework reef, ~1.5 Ga, North China

A small lensoid mound, with cavernous framework formed by microbial carbonate, occurs in the ~1500-million-year-old (Myr) Wumishan Formation at Yesanpo, northern China. It consists of column and cones composed of innumerable conjoined ministromatolites, each ≤2 cm wide. The irregular, decimetric, framework cavities contain carbonate mud, pisoids, and marine cement. Geochemical signatures, together with traces of organic filaments in the ministromatolites, are consistent with slight oxygenation. The role of ministromatolites in creating this modular framework can broadly be compared with that of polyps in present-day scleractinian coral colonies. Cavernous habitats create extraordinary biodiversity in present-day tropical coral reefs. Yesanpo, currently the oldest known framework reef, predates earliest reported reef-dwelling animals by ~600 Myr. As oxygenation increased in the Late Proterozoic, microbial reef frameworks - similar in structure to Yesanpo - may have been occupied by cryptic biota. These habitats provide prime targets in the continued search for early marine animal life.