Cryptogamic ground covers as analogues for early terrestrial biospheres: Initiation and evolution of biologically mediated proto‐soils

Modern cryptogamic ground covers (CGCs), comprising assemblages of bryophytes (hornworts, liverworts, mosses), fungi, bacteria, lichens and algae, are thought to resemble early divergent terrestrial communities. However, limited in situ plant other fossils the rock record, a lack CGC-like soils reported pre-Silurian sedimentological have hindered understanding structure, composition interactions within earliest CGCs. A key question is how organisms drove weathering on primordial surfaces (regolith), leading stages soil development as proto-soils, subsequently contributing large-scale biogeochemical shifts Earth System. Here, we employed novel qualitative, quantitative multi-dimensional imaging approach through X-ray micro-computed tomography, scanning electron, optical microscopy investigate whether different combinations modern CGC from primordial-like settings Iceland develop organism-specific forming features at macro- micro-scales. Additionally, analysed CGCs growing hard rocky substrates initiation processes non-destructively 3D. We show that thalloid (liverworts, hornworts) thin organic layers surface (<1 cm) with subsurface structural development, whereas leafy mosses communities mixed form profiles thicker (up ~ 7 cm), structurally more complex, organic-rich. term these proto-soils. Component analyses tomography data thickness structure proto-soils determined by type colonising organism(s), suggesting evolution complex Palaeozoic may been driven shift body plan flattened appressed upright leafy. Our results provide framework for identifying record new proxy organism–soil ancient biospheres their contribution formation.