Tiny colonial animals called bryozoans were long thought to have appeared tens of millions of years after the Cambrian explosion. Extraordinary fossils found 520-million-year-old rocks in China prove these animals were there all along.
Reconstruction of the Early Cambrian seafloor, depicting colonies of Protomelission gatehousei and Dayingomelission hexaclitia living among archaeocyath reefs in shallow seas approximately 520 million years ago. Image credit: Zhifei Zhang.
“Bryozoans are tiny, filter-feeding colonial invertebrates that thrive in the world’s oceans today, yet for decades their origins presented a puzzling gap in the fossil record,” said Dr. Timothy Topper, a paleontologist at Northwest University and the Swedish Museum of Natural History, and his colleagues.
“While nearly every other major animal group made its first appearance during the Cambrian explosion roughly 530 million years ago, the bryozoan fossil record remained stubbornly silent until the Ordovician period, some 50 million years later.”
In a new study, the paleontologists examined exquisite bryozoan fossils from the Early Cambrian Xiannüdong Formation of China.
The specimens represent two species: the previously known Protomelission gatehousei and an entirely new taxon, Dayingomelission hexaclitia.
“Bryozoa has been the elephant in the room of Cambrian paleontology for a long time,” Dr. Topper said.
“Every other major animal phylum had a Cambrian representative, except bryozoans. These fossils, finally close that chapter for good.”
Specimen of Protomelission gatehousei from the Xiannüdong Formation in which the membranous sacs are preserved. Image credit: Song et al., doi: 10.1038/s41586-026-10590-9.
Beyond simply filling a gap in the fossil record, the findings have profound implications for the tree of life.
A phylogenetic analysis places both Protomelission gatehousei and Dayingomelission hexaclitia firmly within the crown group Stenolaemata, one of the three main classes of living bryozoans.
Because these fossils represent an already-advanced branch of the bryozoan family tree, their existence pushes the origin of the entire group even deeper, perhaps as far back as the Ediacaran period, before the Cambrian explosion even began.
The study also refutes previous theories that had questioned whether Protomelission gatehouse is a bryozoan at all, with some researchers suggesting it might instead be a green alga or isolated sclerites from an unrelated organism.
The new soft-tissue data, combined with detailed comparisons of colony size, shape, and internal structure, refute these alternative interpretations providing an unequivocal link to bryozoans.
Specimens of Dayingomelission hexaclitia from the Xiannüdong Formation showing the colony and cystids. Image credit: Song et al., doi: 10.1038/s41586-026-10590-9.
“These aren’t just simple precursors; they are complex, modular colonies,” said Dr. Baopeng Song, a paleontologist with Northwest University.
“The combination of skeletal architecture and internal anatomy provides definitive evidence that these are true bryozoans, and that the phylum was already diversifying during the Cambrian radiation.”
“Together, the two Chinese taxa and previously reported Cambrian material from South Australia suggest that bryozoans were not only more widespread in Early Cambrian seas than previously recognized, but were already highly sophisticated.”
“The colonial body plan, in which genetically identical individuals called polypides cooperate within a shared skeleton, appears to have arisen not as a late-arriving novelty, but as a core innovation of the Cambrian explosion itself.”
The team’s paper was published today in the journal Nature.
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B. Song et al. High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa. Nature, published online June 3, 2026; doi: 10.1038/s41586-026-10590-9
