3D-printed 541-million-year-old fossil algae reveal modern-looking origin of the plant kingdom

Reconstruction of cross-section with Protocodium sinense, showing similarity with modern Codium. Credit Shu Chai

University of Toronto

Paleontologists have identified a new genus and species of algae called Protocodium sinense that predates the origin of modern land plants and animals and provides new insight into the early diversification of the plant kingdom.

Discovered at a site in China, this 541-million-year-old fossil is the first and oldest green alga of this age to be preserved in three dimensions, enabling researchers to investigate its internal structure and identify the new specimen with unprecedented accuracy. The study, published today in the journal BMC Biology, opens a window into a world of evolutionary puzzles that scientists are just beginning to solve.

says co-author Cedric Arya, a postdoctoral fellow in the Department of Ecology and Evolution Biology in the University of Toronto’s College of Arts and Sciences and is based at the Royal Ontario Museum (ROM). “His discovery touches on the origin of the entire plant kingdom and puts a familiar name on the organisms that predated the Cambrian explosion more than half a billion years ago, when the world’s first modern ecosystems appeared.”

The newly discovered Protocodium fossils were found by a team led by Honghua, professor of geology, including Xu Zhai, a postdoctoral researcher, both from Northwest University, Xi’an, China. It is part of the Gujiashan flora, the name given to a large group of exceptionally well-preserved fossils, in the Dinging Formation in southern Shaanxi Province. In the past 20 years, this geological formation has produced important fossil species documenting the end of the Ediacaran period 541 million years ago.

Organisms and their parts that do not originally absorb minerals – unlike shells or bones – require exceptional conditions for their preservation. In this case, the entire fossil and its fine cellular detail were preserved in three dimensions due to the replacement of the original organic matter by phosphate. This mode of preservation allowed researchers to use electron microscopy and X-ray techniques to roughly slice the fossil, revealing its precise internal structure and eventually identifying it as a relative of modern Codium, a type of seaweed.

The fossils of the protocodium are small balls half a millimeter wide, like large grains of pollen, covered with a large number of smaller domes. Thanks to the 3D scan, the researchers determined the domed surface to be part of a complex single cell containing thin filaments called a siphon. This shape is typical of some modern unicellular seaweeds that have many nuclei.

The discovery of Protocodium may require caution when identifying general Ediacaran spherical fossils and may indicate that organisms such as Codium are in fact older and more widespread. The famous Doushantuo fossil embryos, also from China and 3D preserved, have

She was at the center of discussions about the profound origin of certain animal groups. The specific stages of some of these animal-like embryos resemble unicellular Protocodium on the outside, but the 3D slicing reveals how they consist of so many cells. On the other hand, several round, 2D fossils of unconfirmed algae or other affinities are also known from the Ediacaran and earlier periods, but in less detail.

“We know that the seagrass-like fossils are at least a billion years old,” says Chai, the study’s first author. “But so far, the flat, granular two-dimensional preservation has made it difficult to identify more than general morphological structures.”

Green algae are photosynthetic organisms, which means they convert light and carbon dioxide into sugars and oxygen. So potentially important foundations for Earth’s early ecosystems, the study indicates that green algae did indeed originate in the world’s shallow waters as recyclers of carbon dioxide and oxygen producers before the Cambrian explosion.

Aside from its smaller size, Protocodium looks surprisingly identical to modern Codium, a type of green algae found in many seas around the world. Certain types of this seaweed are notorious for being invasive – such as the fragile subspecies Codium tomentosoides, dubbed “dead man’s fingers” because of their appearance, and are common alongside commercially farmed oysters. From an evolutionary perspective, green algae such as ancient Protocodium and land plants share a common ancestor that was thought to be 1 billion to 1.5 billion years old, but is now likely to be older – designating Protocodium very close to a modern group going back in time in the history of the entire plant kingdom .

“It is critical that such an organism has remained practically unchanged for at least 540 million years,” Arya says. “By Ediacaran, evolution has pushed it toward a stable adaptive zone—it has been comfortable there ever since, and more than that, it has been so successful. So much so that Codium nowadays takes advantage of global trade to easily outcompete other algal species.”

Funding support for research and field work came from the National Natural Science Foundation of China and the National Research and Development Programme. The Aria Postdoctoral Fellow is funded by Albert and Barbara Milstein & The Polk Family Foundations (ROM) and an NSERC Discovery Grant awarded to Dr. Jean-Bernard Caron and Richard M. Ivy, Curator of Invertebrate Paleontology at ROM.

A Codium alga stem group from the most recent Ediacaran of southern China provides a taxonomic view of the early diversification of the plant kingdom.BMC . Biology