Billion-Year-Old Seaweed Fossils May Be Ancestors of Land Plants

(CN) – Scientists announced Monday the discovery of 1-billion-year-old microfossils of green seaweeds that could be the precursor to the earliest land plants.

The discovery, published in the journal Nature Ecology & Evolution, details a form of algae called Proterocladus antiquus. The microfossil seaweeds are only 2 millimeters in length, or about the size of a flea.

A photo of a green seaweed fossil dating back 1 billion years. The image was captured using a microscope as the fossil itself is 2 millimeters long, roughly the size of a flea. The dark color of this fossil was created by adding a drop of mineral oil to the rock in which it’s embedded, to create contrast. (Photo courtesy of Virginia Tech)

Shuhai Xio, a postdoctoral researcher at Virginia Tech, said the fossils represent the oldest seaweeds ever found. They were found imprinted in rock in northern China near the city of Dalian. The previous earliest fossils of green seaweed dated back to 800 million years.

“These new fossils suggest that green seaweeds were important players in the ocean long before their land-plant descendants moved and took control of dry land,” professor Xiao said.

He added: “The entire biosphere is largely dependent on plants and algae for food and oxygen, yet land plants did not evolve until about 450 million years ago. Our study shows that green seaweeds evolved no later than 1 billion years ago, pushing back the record of green seaweeds by about 200 million years. What kind of seaweeds supplied food to the marine ecosystem?”

Scientists hypothesize that land plants evolved from green seaweeds. Over the course of millions of years, the seaweeds moved to dry land and adapted to the new environment. Xiao, however, said not all scientists believe modern land plants evolved from ocean plants.

“Not everyone agrees with us; some scientists think that green plants started in rivers and lakes, and then conquered the ocean and land later,” Xiao said.

Co-author Qing Tang, a postdoctoral researcher at Virginia Tech, said he discovered the microfossils using an electronic microscope and mineral oil, which helped identify them.

“These seaweeds display multiple branches, upright growths, and specialized cells known as akinetes that are very common in this type of fossil,” Tang said. “Taken together, these features strongly suggest that the fossil is a green seaweed with complex multicellularity that is circa 1 billion years old. In short, our study tells us that the ubiquitous green plants we see today can be traced back to at least 1 billion years.”

The researchers say the seaweeds lived in a shallow ocean, died and then became “cooked” underneath a pile of sediment that preserved their organic shapes. Millions of years later, the sediment became the dry land where the researchers found the fossils.

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