US and Chinese palaeontologists have discovered what they believe are the oldest green seaweeds ever found.
The micro-fossils of a form of algae known as Proterocladus antiquus are around a billion years old, they say, and likely related to the ancestor of the earliest land plants and trees.
Barely two millimetres long, they were imprinted in rock taken from an area of dry land that was formerly ocean near the city of Dalian in the Liaoning Province of northern China.
Despite its age, the species shows a range of features consistent with present-day green algae, including multicellularity, differentiated, branched cells and root-like structures.
“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,” says Shuhai Xiao, who led the research with Qing Tang, a colleague at Virginia Tech in the US.
“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 one 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?”
Writing in the journal Nature Ecology and Evolution, the researchers suggest the tiny seaweeds once lived in a shallow ocean, died, 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 scientists from Virginia Tech and China’s Nanjing Institute of Geology.
Xiao says the current hypothesis is that land plants from green seaweeds, which were aquatic plants. Over millions of years, they moved out of the water and became adapted to a new environment where they prospered.
However, he notes that not everyone agrees. “Some scientists think 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.
“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.”