500-million-year-old fossils reveal the origins of moss animals
The newly discovered bryozoans were only a few millimetres in size and lived attached to the seabed in shallow tropical seas. The image is a reconstruction of what they may have looked like. Illustration: Zhifei Zhang
New fossils from China show that bryozoans evolved as early as the Cambrian explosion. The finds push their origins back by 20 million years and show that they evolved at the same time as most other animal phyla first appeared. The study has been published in the journal Nature.
Today, bryozoans are among the most common colony-forming animals in the oceans and live in almost all aquatic environments, from lakes and rivers to the sea – one species is even found in the Fyrisån River here in Uppsala. Despite their great ecological importance, their evolutionary origins have long been a mystery. The bryozoan fossil record had previously remained silent until the Ordovician period, around 480 million years ago, making them an exception among today’s animal phyla, which otherwise emerged during what is known as the Cambrian explosion.
New, well-preserved fossils from the early Cambrian period found in China now show that bryozoans actually emerged just over 500 million years ago.
“This solves one of palaeontology’s classic problems. Bryozoans have long been an evolutionary enigma because they appeared to emerge much later than other animal phyla. Our new fossils show that they actually evolved at the same time as the rest of the modern animal phyla,” says Lars Holmer, professor at Uppsala University and co-author of the study.
The study is based on fossils from the Xiannüdong Formation in southern China. In a previous study of these fossils, researchers also investigated another possible bryozoan species, Protomelission gatehousei, whose relationship has recently been the subject of much debate. The new, better-preserved finds clearly demonstrate a relationship with the bryozoans.
Fossil of the early bryozoan Protomelission gatehousei from the Cambrian period, just over 500 million years ago. The fossil preserves both the colony’s structure and details of the soft tissues. The images in the middle shows the openings from which the tentacled individuals of the bryozoans would have emerged. Photo: Baopeng Song
Unique fossils reveal soft tissues
What makes these finds particularly significant is that the fossils preserve not only the skeletons of the microscopic colonies, but also details of their soft tissues. The researchers were able to identify anatomical structures such as muscles, membrane sacs and partitions between individuals within the colony – characteristics that clearly demonstrate that the animals are indeed bryozoans.
“It is extremely rare to find fossils where such delicate structures are still intact. Thanks to the special chemical conditions during fossilisation, we have been able to study the anatomy in detail and definitively confirm the Cambrian origin of bryozoans,” says Lars Holmer.
The researchers also show that bryozoans had already developed advanced colonies with specialised structures during the Cambrian period.
Early colonies on the seabed
The newly discovered bryozoans were only a few millimetres in size and lived attached to the seabed in shallow tropical seas. The colonies consisted of many small individuals that together built leaf-like or honeycomb-like structures.
In addition, the new taxon, Dayingomelission hexaclitia, represents a completely new type of early bryozoan and shows that the phylum already exhibited great variation in colony form and structure at an early stage.
“Perhaps the most surprising thing is just how advanced these colonies already were. It shows that the development of cooperative animal colonies proceeded very rapidly during the Cambrian explosion,” says Lars Holmer.
Sandra Gunnarsson
Facts: Bryozoans
- Bryozoans are small, colony-forming animals that live in the sea and in fresh water.
- Each colony consists of many microscopic individuals that work together.
- Most bryozoans filter plankton from the water using a tentacled structure called a lophophore.
- Fossil bryozoans are very common from the Ordovician period onwards, but there has long been a lack of confirmed fossils from the Cambrian period.
- The new fossils from China are over 500 million years old and show that bryozoans emerged during the Cambrian explosion.
Publication
Song, B., Zhang, Z., Strotz, L.C. et al. High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa. Nature (2026). DOI: 10.1038/s41586-026-10590-9