Billion-year-old seaweed fossil may be oldest ancestor of land plants
Two paleontologists at Virginia Tech have found one billion year old fossils of green seaweed in China. Called Proterocladus antiquus, the tiny fossils are the oldest green seaweed found to date – 200 million years older than the previous titleholder – and may be related to the first plants to colonize the land in Earth's distant past, 450 million years ago.
When you read popular science stories about the evolution of life on Earth, it's easy to find big events like the first fish to set foot (or fin) on dry land, the first mammals, or the first humans. However, there are other stories that are equally important, like the emergence of plants moving from the sea to the land.
We tend to take green plants for granted, but without them on the land, the first fish or even the first insect to leave the sea might as well have stayed home because there wouldn't be anything to eat and a lot less oxygen to breathe. Exactly how these plants got on land isn't very well-known, with some believing that they came directly from the sea and others saying that they came indirectly via freshwater rivers and lakes, but what is known is that they made the jump some 450 million years ago.
But where did these green plants come from? According to Professor Shuhai Xiao and post-doctoral researcher, Qing Tang from the Department of Geosciences of the Virginia Tech College of Science, the new micro-fossil found near the city of Dalian in the Liaoning Province of northern China may be the oldest relative of the first land plants that emerged later, over many millions of years.
There are three main types of seaweed – brown (Phaeophyceae), green (Chlorophyta), and red (Rhodophyta) with the red dating back to 1.047 billion years ago. The newly discovered green seaweed fossils, Proterocladus antiquus, may also date back a billion years, but Xiao says that there are modern green seaweed that appear very similar in size and shape, such as a group called siphonocladaleans.
The 2 mm-long fossils were found using an electronic microscope to examine stone sections treated with mineral oil to increase contrast. Xiao and Tang say that the seaweed lived in shallow ocean water and was preserved when it "cooked" under a thick layer of sediment. Later geological activity lifted the seabed, leaving it as part of dry land.
"[Proterocladus antiquus] display multiple branches, upright growths, and specialized cells known as akinetes that are very common in this type of fossil,” says Xiao. "Taken together, these features strongly suggest that the fossil is a green seaweed with complex multicellularity that is circa one 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 one billion years."
The study was published in Nature Ecology & Evolution.
Source: Virginia Tech