Ancient China green seaweed micro-fossils

Paleontologists have made a remarkable discovery in China -one billion-year-old micro-fossils of green seaweeds that could be related to the ancestor of the earliest land plants and trees that first developed 450 million years ago.

A team of scientists from the Nanjing Institute of Geology and Palaeontology in China and the Department of Geosciences and Global Change, Virginia Tech College of Science in the USA carried out the research, which has been published in the journal Nature Ecology & Evolution.

Professor Shuhai Xiao said that 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.

The micro-fossil seaweeds — a form of algae known as Proterocladus antiquus — are barely visible to the naked eye at 2 millimeters in length, or roughly the size of a typical flea. Xiao said the fossils are the oldest green seaweeds ever found. They were imprinted in rock taken from an area of dry land — formerly ocean — near the city of Dalian in the Liaoning Province of northern China. Previously, the earliest convincing fossil record of green seaweeds was found in rock dated at roughly 800 million years old.

Current scientific thinking is that land plants — the trees, grasses, food crops, and bushes — evolved from green seaweeds, which were aquatic plants. Through millions of years of geological time they moved out of the water and became adapted to and prospered on dry land, their new natural environment. The scientists said that these fossils are related to the ancestors of all the modern land plants we see today. However, the origins of green plants remains at topic of scientific debate, with some scientists arguing that green plants started in rivers and lakes, and then conquered the ocean and land later.

There are three main types of seaweed: brown (Phaeophyceae), green (Chlorophyta), and red (Rhodophyta), and thousands of species of each kind. Fossils of red seaweed, which are now common on ocean floors, have been dated as far back as 1.047 billion years old.

The researchers said there are some modern green seaweeds that look very similar to the fossils that they found found in northern China. A group of modern green seaweeds, known as siphonocladaleans, are particularly similar in shape and size to the fossils they found.

Photosynthetic plants are, of course, vital to the ecological balance of the planet because they produce organic carbon and oxygen through photosynthesis, and they provide food and the basis of shelter for untold numbers of mammals, fish, and more. Yet, going back 2 billion years, Earth had no green plants at all in oceans, Xiao said.

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. Xiao said that the 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, and raises the question of what kind of seaweeds supplied food to the ancient marine ecosystem?

“There are some modern green seaweeds that look very similar to the fossils that we found,” Xiao said. “A group of modern green seaweeds, known as siphonocladaleans, are particularly similar in shape and size to the fossils we found.”

Green seaweed fossil dating back 1 billion years. The image was captured using a microscope as the fossil itself is 2 millimeters long.

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

According to the researchers, the tiny seaweeds once lived in a shallow ocean, died, and then became “cooked” beneath a thick pile of sediment, preserving the organic shapes of the seaweeds as fossils. Many millions of years later, the sediment was then lifted up out of the ocean and became the dry land where the fossils were retrieved by the paleontologists from Nanjing Institute of Geology and Paleontology.

Source: SciTechDaily, 4 July 2023.

Scientific Reference: “A one-billion-year-old multicellular chlorophyte” by Qing Tang, Ke Pang, Xunlai Yuan and Shuhai Xiao, 24 February 2020, Nature Ecology & Evolution.
DOI: 10.1038/s41559-020-1122-9