Supposed animal fossil turns out to be algae

Supposed animal fossil turns out to be algae

Fossils of the putative bryozoans. © Zhang Xiguang

Until now, paleontologists have assumed that the first bryozoans – like all other major groups in the animal kingdom – arose during the rapid speciation during the Cambrian Explosion. The approximately 500 million year old fossils of a bryozoan species seemed to prove this. But now new, better preserved finds of the same species show that this is a mistake. Because the supposed bryozoans turn out to be early green algae on closer analysis. The bryozoans could thus be the only group of animals that did not already develop in the course of the Cambrian explosion.

More than 500 million years ago, at the beginning of the Cambrian Age, biodiversity on earth increased rapidly. In the course of this “Cambrian explosion” the blueprints of almost all animal groups known today developed. These include, for example, the mollusks with relatives such as snails and mussels, but also the group of arthropods, which includes crabs and insects, among others. Until now, paleontologists had also assumed that the first moss animals, also called bryozoans, had developed in the Cambrian. Bryozoans are microscopic animals with tentacles that live in “skyscraper-like” underwater colonies.

New fossils reveal plant traits

The oldest evidence of primeval bryozoans were fossils more than 500 million years old, which were assigned to the bryozoan species Protomelission gateshousei. But new finds from the Hongjingshao Formation near the Chinese city of Kunming now challenge this assumption. Unlike previous Protomelission remains, these newly discovered fossils also have soft tissues preserved. This allowed paleontologists led by Yunnan University’s Jie Yang to examine the fragile, thin-walled tissue more closely using powerful microscopes and fluorescence photography. They also compared the fossil tissue samples to other fossil finds to determine their exact place in the tree of life.

In their analyses, the researchers came across several characteristics that did not match bryozoans and animal organisms in general. “Instead of the tentacles that we would expect in bryozoans, we discovered simple sheet-like flanges,” reports Yang’s colleague Zhang Xiguang. From this and other characteristics, the paleontologists conclude that these must not be animal remains, but plant remains. This means that the oldest evidence of bryozoans in the world is in fact none at all. Instead, Yang’s team places the Protomelission fossils in a group of early green algae called Dasycladales.

Evolutionary laggards

This correction is also important for the chronological classification of bryozoan evolution. Because what is now the oldest fossil of these animals is around 40 million years younger than the incorrectly assigned Protomelission relics. According to paleontologists, this suggests that the bryozoans probably evolved after the Cambrian. This makes them the only group of fossil animals that did not form during the Cambrian explosion. This knowledge has far-reaching consequences for the reconstruction of the history of the earth. Until now it was assumed that the blueprints of all animal life go back to this phase of intensive speciation and were then simply modified again and again. “But if bryozoans did indeed evolve after the Cambrian, it shows that evolution has remained creative even after this critical period of innovation – perhaps half a billion years ago, the path of life wasn’t set in stone,” explains co-author MartinSmith.

Jie Yang (Yunnan University, China) et al., Nature, doi: 10.1038/s41586-023-05775-5

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