In 2016, geologist Rowan Martindale was hiking across a hillside in Morocco when something unusual caught her eye. A slab of sedimentary rock was covered in a wrinkled texture that looked strikingly like elephant skin. The pattern was so unexpected that it immediately stood out.
"I looked at the wrinkles and I was like, 'These aren't supposed to be in rocks like this. What the heck is going on?'" said Martindale, an associate professor at The University of Texas at Austin's Jackson School of Geosciences.
Rock textures can reveal the processes that shaped them over millions of years. To Martindale, the wrinkled surface looked exactly like fossilized microbial mats. These structures form when communities of microbes grow across sediment, leaving behind distinctive patterns. In this case, the textures appeared to preserve a dense layer of microbial life that existed more than 180 million years ago during the Early Jurassic.
Martindale recognized the pattern right away. During graduate school, she had studied similar textures through photos and samples shared by a lab mate who focused on microbial fossils from the Early Triassic.
A Puzzle in the Wrong Place
There was one major issue. The setting did not match what scientists expected.
The rock layer where the wrinkles formed originated in deep ocean water, nearly 600 feet below the surface. However, scientists have long believed that these kinds of microbial wrinkle structures only formed in shallow environments. In those settings, microbes could rely on sunlight for energy and avoid animals that would otherwise consume them, especially during stressful periods or after mass extinctions.
In deeper water, similar patterns are usually explained in a different way. Geologists often attribute them to underwater landslides that push sediment into ridges and grooves. But Martindale was not convinced. The patterns she saw had the clear signature of microbial activity.
"It was one of those things, knowing what to look for and having that 'search image' of wrinkle structures in my head, that made me want to stop and dig into this," she said.
A New Explanation for Deep-Sea Wrinkle Structures
In a recent study published in Geology, Martindale and her colleagues propose a new interpretation that connects geological processes with biological activity. They suggest that while an underwater landslide did occur, it did not directly create the wrinkles. Instead, it delivered nutrients to the seafloor, allowing microbes to grow and form the structures.
According to the team, these microbes did not depend on sunlight. Instead, they likely relied on chemicals for energy, a process known as chemosynthetic. The influx of nutrients from the landslide may have supported these communities, while the release of toxic sulfur compounds could have discouraged other marine life from disturbing them.
Modern Clues From Deep Ocean Ecosystems
Similar ecosystems exist in today's oceans. Some microbial mats thrive in deep, dark environments by feeding on chemical energy rather than sunlight. One example can be found on whale carcasses that sink to the seafloor. These "whale fall" sites create temporary but rich ecosystems where microbes quickly colonize and flourish.
Jake Bailey, a professor at the University of Minnesota who studies how microbes influence Earth's systems, said the findings challenge long-standing assumptions about these rock structures.
"In the present, some of the largest microbial ecosystems on our planet are found in the dark ocean," said Bailey, who was not involved with the research. "The research here shows that certain ancient sedimentary structures may record the presence of these chemolithotrophs rather than phototrophs (organisms that need sunlight to make energy). "
Rethinking Fossils in the Rock Record
Martindale explained that this discovery could have broad implications. If chemosynthetic microbial communities were more widespread than previously believed, their fossils may be more common as well. However, scientists may have overlooked them by interpreting wrinkled rock textures as purely physical formations.
Part of the challenge lies in the language used to describe these features. Without clear terminology, it can be difficult to distinguish between structures formed by physical forces and those created by living organisms.
"The terminology is pretty lax," Martindale said. "Wrinkly can be mean lots of things, so there's a lack of diagnostic language."
Following an Unexpected Scientific Path
Martindale typically studies ancient coral reefs and mass extinctions. She did not expect this observation to lead her into the study of deep-sea microbial mats. But the mystery proved too compelling to ignore.
"It's really cool to have gone in this direction that I totally wasn't expecting," she said. "There was no hypothesis that I would find these microbial mats here. It was just being in the right place at the right time, with the right search image. And then being so stubborn as to not let go of it."
The research was funded by the National Science Foundation.
