Wrinkle structures — tiny ridges and pits pressed into sediment by microbial mats — are a familiar signature of ancient life in shallow, sunlit waters. Photosynthetic microbes colonized tidal flats, bound the sediment with their sticky biofilms, and left textured surfaces preserved in rock. The association is strong: wrinkle structures mean shallow water, and shallow water means light.
Martindale and colleagues found wrinkle structures in turbidite deposits from Morocco's Dadès Valley in the Central High Atlas. The sediments are one hundred and eighty million years old, from the Jurassic. And they were deposited at depths of at least one hundred and eighty meters — far below the reach of sunlight.
The microbes that made these structures were not photosynthetic. They were chemosynthetic, deriving energy from chemical reactions in the sediment rather than from light. The wrinkle morphology is indistinguishable from the shallow-water version, but the metabolism driving it is completely different.
There's a second surprise: at one hundred and eighty million years ago, animal bioturbation should have prevented microbial mats from forming on the deep seafloor. Burrowing organisms churn sediment, disrupting the surface layer and destroying the mats. The Jurassic deep ocean had active bioturbators. The mats shouldn't have survived.
They did. Either the chemosynthetic productivity was intense enough to outpace bioturbation, or the specific chemical environment — perhaps toxic to burrowers — created a refuge where mats could establish. The wrinkle structures record a microbial foothold in hostile territory, preserved against both the absence of light and the presence of predators.
The diagnostic was wrong. Wrinkle structures don't mean shallow water. They mean microbial mats. The mats can run on chemistry instead of light, and when they do, they can form anywhere.