Cretaceous biological soil crusts: a glimpse of ancient dryland microbial life and soil development

Biological soil crusts (BSCs) are bio-sedimentary structures that play vital ecological roles in modern extreme environments. As microbial communities thriving in disturbed settings under hydrological stress and erosion, they function as both ecosystem engineers and pioneer colonizers of bare soils, particularly in arid and polar regions. Despite their significance for soil development and ecological succession and their widespread occurrence in contemporary environments, BSCs remain markedly underrepresented in paleopedological records. This study presents a detailed macro and micromorphological analysis of Cretaceous biological soil crusts preserved in paleosols of Marília Formation (Bauru Basin, Brazil), elucidating their biogenic origin and paleoenvironmental significance. The studied crusts exhibit characteristic microbial sedimentary structures, including wrinkle marks, gas domes, fenestral porosity, and desiccation cracks, alongside well-preserved coccoid microbial filaments, extracellular polymeric substances (EPS), and carbonate spherulites. These features closely resemble modern humipedons, particularly crusto-humus systems characterized by organic (CruO) and organo-mineral (CruOA) AC horizons. Such structures reflect enhanced organic matter accumulation and biological activity during soil development, further supporting their interpretation as cyanobacteria-dominated microbial mats (BSCs) that stabilized floodplain. Micromorphological and SEM analyses confirm bioweathering signatures, calcified root cells, and microbial-induced CaCO₃ precipitation, reinforcing their biogenicity. The association of BSCs with rhizoliths and invertebrate trace fossils indicates periods of landscape stability and incipient ecological succession following sedimentary deposition. Paleoenvironmental reconstruction points to seasonally semi-arid conditions punctuated by intermittent flooding events, with BSCs developing during intervals of subaerial exposure. This study underscores the significance of BSCs for interpreting paleosols and sedimentation dynamics, providing new insights into Cretaceous terrestrial ecosystems.