Seaweed fossils that are one billion years old are believed to be the oldest green algae discovered.
They could also be related to the ancestor of the earliest land plants and trees that first developed 450 million years ago, researchers say.
The micro-fossil seaweeds are hardly visible to the naked eye, at 2mm in length. The specimens were imprinted in rock taken from an area of dry land near the city of Dalian in the Liaoning Province of northern China, which used to be ocean.
Before the discovery, published in 'Nature Ecology and Evolution', the earliest fossil records of green seaweeds were found in rock dated at about 800 million years old.