Fossil evidence from approximately 520-million-year-old rocks in China finally places bryozoans where they belong, at the heart of the Cambrian explosion.
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. 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. Now, an international research team has uncovered extraordinary fossils that bridge this evolutionary gap, confirming that bryozoans were active participants in the Cambrian explosion after all.
The Evolutionary Enigma Solved
Reporting in Nature, scientists from China, Sweden, Australia and Germany describe exquisitely preserved fossils from the Xiannüdong Formation of southern Shaanxi Province, China. The fossils include new specimens of the previously known Protomelission gatehousei and an entirely new taxon, Dayingomelission hexaclitia gen. et sp. nov. Both lived during the early Cambrian, approximately 520 million years ago.
"Bryozoa has been the elephant in the room of Cambrian palaeontology for a long time." said co-author Dr Timothy Topper of Northwest University and the Swedish Museum of Natural History. "Every other major animal phylum had a Cambrian representative, except bryozoans. These fossils, finally close that chapter for good."
What makes these fossils exceptional is not just their age, but their preservation. The tiny colonies, no bigger than a few millimetres, are preserved in three dimensions with their internal soft tissues intact, mineralised by phosphate. Using advanced imaging techniques, the researchers identified delicate soft-tissue anatomy, including membranous sacs, diagnostic structural spines called styles, and even individual muscle fibres. Equally striking is the diagnostic hexagonal, modular arrangement of the zooid skeletons, a hallmark of bryozoan colonies. This combination of skeletal architecture and soft-tissue anatomy is consistent with a bryozoan affinity, and together the evidence leaves little room for doubt.
"These specimens are remarkable, to have soft tissues mineralised inside their original skeletal housing, half a billion years later, is nothing short of extraordinary," said Professor Zhifei Zhang of Northwest University, the study's corresponding author. "These bryozoans lived in shallow, clear-water reef environments, which may explain why they have eluded discovery for so long; the Cambrian fossil sites best known for soft-tissue preservation invariably represent deeper-water settings."
Rewriting the timeline
Beyond simply filling a gap in the fossil record, the findings have profound implications for the tree of life. A phylogenetic analysis places both Cambrian taxa 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 P. gatehousei 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.
"These aren't just simple precursors; they are complex, modular colonies," explains Baopeng Song, the study's lead author. "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.
