A new study shows how wrinkle structures can form on a bed of sand when waves and microorganisms are present. Wrinkle structures on sandy bed surfaces are rare on Earth today, but were more common in ancient sedimentary environments. These ancient sediments often have trace fossils and imprints of early animals, and appear in the geological record after some of the largest mass extinctions on Earth.

Some scientists have theorized that wrinkle structures are the remnants of dense colonies of microbes known as microbial mats, but the new study proposes a different origin.

In their experiment, researchers placed microbial aggregates on bare sand in a wave tank. Microbial mats are dense colonies of microorganisms that are often many layers thick and attached to a surface. Microbial aggregates are basically broken-up pieces of microbial mats, which are produced when the mats are damaged in events like storms or strong currents. Animals that feed on microbial mats also help break them apart, making them more susceptible to storms and helping to produce free-floating microbial aggregates.

The team showed that the wrinkle structures are formed in the interaction between microbial aggregates and sandy sediments when waves are present. The waves do not move sand grains directly. Instead, they act on the microbial aggregates to produce these features. Aggregates of microorganisms (about a millimeter in size) are pushed around by waves, which results in the formation of features like ridges and pits.

The team concluded that wrinkle structures are indeed biosignatures (signs of life’s presence), but that they form when microorganisms are present at the interface between sediment and waves, and not beneath microbial mats.

The study was supported in part by the NASA Astrobiology Institute and published in the journal Nature Geosciences.

For a short Q&A with the researchers, read more here.