Protocodium sinense is a new species of algae that was discovered by paleontologists and provides new insight into the early evolution of plants.
This 541 million-year-old fossil is the first and oldest green alga from this era to be preserved in three dimensions, allowing researchers to investigate its internal structure and identify the new specimen. Today's publication of the study opens a window into a world of evolutionary puzzles that are just beginning to be untangled.
The co-author of the study says that Protocodium was already well diversified before the end of the Ediacaran period. The discovery touches the origin of the entire plant kingdom and puts a familiar name on the organisms that preceded the Cambrian explosion.
The Protocodium fossils were found by a team led by Hong Hua, professor of geology, and including Shu Chai, a graduate student. The Dengying Formation in the southern Shaanxi Province is home to a group of extremely well-preserved fossils. This geological formation has yielded important fossil species in the past two decades.
Organisms that don't originally absorb minerals need special conditions to be preserved. The fossils were preserved in three dimensions because of the replacement of the original organic material byphosphate. This mode of preservation allowed the researchers to use various electron and X-ray microscopy techniques to slice the fossil, reveal its internal structure and identify it as a close relative of the modern Codium alga.
Protocodium fossils are half a millimeter wide and covered by many smaller domes. The domed surface was found to be part of a single cell with thin strands called siphons. Modern single-celled seaweeds have a lot of nuclei.
Protocodium's discovery could mean that organisms like Codium are much older and widespread. The famous fossil embryo from China, preserved in 3D, has been at the center of debates about the deep origins of certain animal groups. Some of the animal-like embryo stages look similar to the Protocodium on the outside, but 3D slicing shows how they are made up of many cells. There are many 2D round fossils of uncertain algal or other affinity that are known from the Ediacaran and older periods.
Chai says that seaweed-like fossils are at least one billion years old. It was difficult to recognize more than general structures until now.
Light and carbon dioxide are converted into sugars and oxygen by greenalgae. Green algae were already established in the world's shallow waters as carbon dioxide recyclers and oxygen producers before the Cambrian explosion.
Protocodium is remarkably similar to the modern Codium, a type of green algae found in many seas around the world. The Codium fragile subspecies tomentosoides, dubbed "dead man's fingers" for its appearance, is one of the seaweeds that are notoriously Invade. The ancient Protocodium and land plants share a common ancestor that was thought to be one billion to one billion and a half years old, but now likely older.
Over the course of at least 540 million years, the organisms has not changed. Since the Ediacaran, evolution has driven it toward a stable adaptive zone. Nowadays Codium takes advantage of global trade to easily out compete other algal species.
More information: A stem-group Codium alga from the latest Ediacaran of South China provides taxonomic insight into the early diversification of the plant kingdom, BMC Biology (2022). DOI: 10.1186/s12915-022-01394-0 Journal information: BMC Biology
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