While hiking on a hillside in Morocco in 2016, geologist Rowan Martindale noticed something that stopped him in his tracks. It was a slab of sedimentary rock covered in a wrinkled texture reminiscent of elephant skin.

“When I saw the wrinkles, I thought, ‘This can’t be in this rock. What’s going on here?'” said Martindale, an associate professor at the Jackson School of Geosciences at the University of Texas at Austin.

A rock’s texture contains clues about the geological activity that formed it. For Martindale, these wrinkles in time were textbook examples of microbial mat fossils. They captured exudate-rich communities of bacteria that lived during the early Jurassic period, more than 180 million years ago.

Thanks to a colleague in her lab who specializes in early Triassic microbial fossils, while in graduate school she had seen dozens of photographs and samples that had the same texture as the stone tablets.

There was just one problem. The geological environment was all wrong.

The deposits from which the wrinkles were formed were originally in the deep ocean, about 600 feet below the surface. However, the prevailing explanation in the geological community has been that wrinkle structures created by microbes are restricted to shallow water environments that are either stressful or post-extinction events. In both situations, the microorganisms have access to sunlight and there are no marine organisms that can easily prey on them.

For deep-sea environments, the usual explanation for wrinkle-like textures is that they are traces formed by underwater landslides, which pushed sediment into ridges and grooves. But Martindale wasn’t buying it. The wrinkles on the hillside had a microbial appearance.

“Knowing what to look for and having a ‘search image’ of the wrinkle structure in my head was one of the reasons I wanted to stop and dig into this,” she said.

In a recent paper published in geologyshe and her co-authors propose a new explanation of wrinkle structure that connects the worlds of biology and geology. These wrinkles were not formed by physical forces during the submarine landslide. But the landslide carried nutrients to the ocean floor, setting the stage for microbial growth.

In their paper, the researchers propose that the wrinkles were formed not by sunlight but by microbial mats that were sustaining themselves on nutrients swept into the deep ocean by landslides, an energy-producing method called chemosynthesis. Ingestion of chemicals may also have enabled these microbial communities to release toxic sulfur compounds that would have kept marine life at bay.

Microbial communities have now been found living in a similar way in the deep ocean, with microbial mats covering whale carcasses floating on the ocean floor, supporting a seed-rich and ephemeral “whale fall” ecosystem.

Jake Bailey, a professor at the University of Minnesota who studies how microbes shape the Earth’s environment, said the study challenges the assumption that ancient wrinkle structures came from just one type of microbial community.

“Some of the largest microbial ecosystems on Earth are now found in the dark ocean,” said Bailey, who was not involved in the study. “The research here shows that certain ancient sedimentary structures may record the presence of these chemotrophs rather than photosynthetic organisms (organisms that require sunlight to make energy).”

Professor Martindale said the discovery was important because it suggested fossils of chemosynthetic microbial communities may be more widespread in the fossil record than previously thought. Additionally, a bias toward interpreting wrinkles as purely physical structures can lead geoscientists to misclassify fossils as natural geological formations. This bias is further exacerbated by the lack of specific language to describe rock wrinkles.

“The terminology is pretty loose,” Martindale says. “There’s so much meanness about wrinkles that we lack diagnostic language.”

Martindale’s usual research interests include ancient coral reefs and mass extinctions. She never expected to take a detour to study deep-sea microbial mats. But when a thorny question presents itself, sometimes you need to see where the science is leading you.

“I’m really happy that it went in a direction that I didn’t expect at all,” she said. “We didn’t have any hypothesis that we would find these microbial mats here. We just had the right search image in the right place at the right time. And we were stubborn enough to not let it go.”

This research was funded by the National Science Foundation.

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