Who hasn’t walked along a previously flooded area and seen flakes of dried mud cakes magically curled up in geometrical shapes? At times, the curls are so pronounced that they make complete scrolls. The area looks like a field of shards. No big deal, just dried mud you might say (as you reach down, tempted to pick up some pieces to play with). Au contraire, such curlicue structures, known as roll-ups, are of considerable interest to geologists. Roll-ups are sedimentary structures capped with a surface layer of clay and organic material that form when the surface dries out. Once formed, they are quite resistant to wetting and weathering, the open curls much less so.
To explain why we're playing with mud cakes on this blog, it turns out that the roll-ups are largely biogenic. Their surface includes distinct populations of cyanobacteria, which are much less common in the adjacent, non-rolled soil. The cyanobacteria are largely filamentous and of the non-heterocyst forming variety. How do they contribute to the curling? According to H. Beraldi-Campesi and F. Garcia-Pichel of Arizona State, the authors of a recent study, they provide the organic material in the form of bacterial extracellular polysaccharide (EPS). Enough there and it alone can be the reason for the curvature. With less biomass present (<1% of the total), the microbial filaments in combination with their EPS and some fine sediment suffice to do the curling. The authors could reproduce credible roll-up formations by using a clay that swells on watering (smectite) or agar (we hope they enjoyed doing it).
Fossil roll-ups similar to modern ones are found in Precambrian rocks where they serve as interesting biosignatures for ancient life. Their very presence suggests that microbes existed at that time and place. But can we be sure that the ancient roll-ups are biogenic in origin? One argument is that because microbial roll-ups are strong enough to withstand wetting and transport, they had a better chance of being preserved. The oldest fossil roll-up dates from 1.8 billion years. They were thought to be of biogenic origin based on the cohesive strength inferred for the muddy layers that form them. The work reported on modern roll-ups supports this interpretation more strongly.
Fossil evidence from a couple of billion years ago is not exactly abundant. Therefore, anything that can add to that record concerning early life is welcome, even if it be rolled-up cakes of mud.
Beraldi-Campesi H, & Garcia-Pichel F (2010). The biogenicity of modern terrestrial roll-up structures and its significance for ancient life on land. Geobiology PMID: 21040397