A recent study suggests that a species previously believed to be the oldest known bryozoan, which lived around 520 million years ago, is actually a type of colony-forming algae. Bryozoans are animals that live in colonies on seafloors or lake bottoms and are characterized by their filter-feeding and tentacle-bearing abilities. However, other organisms, including algae, can also inhabit similar modular constructions. Although the species Protomelission gatehouse was initially classified as a bryozoan in 2021, new analyses of even better-preserved fossils suggest otherwise. Martin Smith, a paleobiologist at Durham University in England, led this research.
According to a recent report published in Nature, new fossils found in southern China have revealed soft parts of the organism that were not present in previously discovered fossils. While past fossils had only preserved the skeletal framework of colonies, these new fossils contain additional features. Surprisingly, the fossils exhibit simple leaflike flanges rather than the expected tentacles of a well-preserved bryozoan. The research was conducted by Martin Smith and his colleagues, and the report was published on March 8th.
The recent discovery, if confirmed, would indicate that the oldest bryozoan fossils, without any doubt, are only around 480 million years old. Martin Smith argues that this finding positions bryozoans as the only significant animal group that did not first emerge during the Cambrian Period. This period is widely considered a significant era of biological diversification, often referred to as "life's Big Bang," and it ended approximately 488 million years ago
The researchers have concluded that the Cambrian Period is not the exclusive time of evolutionary innovation when all animal life blueprints were laid out, as previously believed. Martin Smith states that the question is whether evolution lost the ability to create new body plans, and the new findings suggest that it did not. However, not everyone is in agreement that the new fossils are not bryozoans. According to Paul Taylor, an invertebrate paleontologist at London's Natural History Museum, who was not involved in the study, the leaflike flanges described by Smith and his colleagues could just as easily be interpreted as body parts of individual animals within the bryozoan colony.
Taylor points out that the absence of tentacles in the new fossils is not unexpected, as these soft-tissue structures tend not to fossilize well. Therefore, he believes that the new findings are not conclusive enough to dismiss P. gatehousei as a Cambrian bryozoan. However, they do highlight the difficulties involved in identifying fossils with simple body plans. To resolve the question, Taylor suggests that further fossils that preserve additional features, including those of the organism's early growth stages, are necessary.
J. Yang et al. Protomelission is an early dasyclad alga and not a Cambrian bryozoan. Nature. Published online March 8, 2023. doi: 10.1038/s41586-023-05775-5.