Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland

The Stoer Group and Diabaig Formation of the Torridonian succession in northwest Scotland are late Mesoproterozoic to early Neoproterozoic (ca. 1200-1000 Ma). Features preserved on the top surfaces of fine- to medium-grained sandstone beds in a number of stratigraphically and geographically separated localities are attributable to microbially induced sedimentary structures; these include wrinkle structures, remnants of apparent microbial crusts, and indications of original cohesiveness and pliancy in sand-sized sediment. The surfaces on which the microbial structures formed were exposed subaerially (abundant, deep desiccation cracks and locally pedogenic structures) in alluvial, interfluve, and lacustrine margin settings, and many of the structures developed in areas well away from the perennially wetted regions adjacent to shorelines and fluvial channels. Thus, these features indicate that Earth's biosphere had adapted to and colonized land surfaces many hundreds of millions of years before the dawn of the Phanerozoic.