It was long presumed that minute colonial organisms known as bryozoans emerged tens of millions of years subsequent to the Cambrian explosion. However, remarkable fossil discoveries unearthed from 520-million-year-old strata in China unequivocally demonstrate the presence of these creatures during that epoch.
A depiction of the Early Cambrian seabed, illustrating colonies of Protomelission gatehousei and Dayingomelission hexaclitia coexisting with archaeocyath reefs in shallow marine environments approximately 520 million years ago. Attribution: Zhifei Zhang.
“Bryozoans, which are diminutive, filter-feeding colonial invertebrates inhabiting the world’s oceans today, presented a perplexing void in the fossil record for many decades regarding their origins,” stated Dr. Timothy Topper, a paleontologist affiliated with Northwest University and the Swedish Museum of Natural History, alongside his research collaborators.
“While nearly every other major animal lineage materialized during the Cambrian explosion, which occurred approximately 530 million years ago, the fossil evidence for bryozoans remained conspicuously absent until the Ordovician period, an interval some 50 million years later.”
Within a novel investigation, the paleontological team meticulously examined exceptionally preserved bryozoan fossils recovered from the Early Cambrian Xiannüdong Formation situated in China.
These fossil specimens encompass two distinct species: the previously identified Protomelission gatehousei and a newly documented taxon, Dayingomelission hexaclitia.
“The absence of bryozoans from the Cambrian paleontological record has been a significant unresolved issue for a considerable duration,” Dr. Topper commented. “All other principal animal phyla possessed representation during the Cambrian, with the sole exception of bryozoans. These newly discovered fossils definitively resolve this longstanding enigma.”
A specimen of Protomelission gatehousei originating from the Xiannüdong Formation, showcasing the preservation of its membranous sacs. Credit: Song et al., doi: 10.1038/s41586-026-10590-9.
Beyond merely rectifying an omission in the paleontological record, these findings carry substantial implications for our understanding of the evolutionary tree of life.
A comprehensive phylogenetic analysis firmly situates both Protomelission gatehousei and Dayingomelission hexaclitia within the crown group Stenolaemata, identified as one of the three predominant classes of extant bryozoans.
Given that these fossils represent an already evolved branch of the bryozoan lineage, their existence suggests that the diversification of this entire group extends even further back in time, potentially to the Ediacaran period, predating the onset of the Cambrian explosion.
Furthermore, the study challenges prior hypotheses that questioned the classification of Protomelission gatehousei as a bryozoan, with some researchers proposing it might alternatively be classified as a green alga or isolated sclerites from an unrelated organism.
The newly acquired soft-tissue data, coupled with meticulous comparisons of colony dimensions, morphology, and internal structure, effectively debunks these alternative interpretations and establishes an irrefutable link to bryozoans.
Specimens of Dayingomelission hexaclitia from the Xiannüdong Formation, revealing the colony structure and cystids. Credit: Song et al., doi: 10.1038/s41586-026-10590-9.
“These findings do not depict rudimentary precursors; rather, they represent sophisticated, modular colonies,” remarked Dr. Baopeng Song, a paleontologist at Northwest University.
“The amalgamation of skeletal architecture and internal anatomy provides definitive validation that these are indeed true bryozoans, and that this phylum was actively undergoing diversification during the Cambrian radiation event.”
“Collectively, the two Chinese taxa alongside previously documented Cambrian material from South Australia indicate that bryozoans were not only more widely distributed in Early Cambrian seas than previously understood but had already achieved a high degree of complexity.”
“The colonial body plan, wherein genetically identical individuals referred to as polypides collaborate within a shared skeletal framework, appears to have emerged not as a recent innovation, but as a fundamental evolutionary development of the Cambrian explosion itself.”
The research paper authored by the team was published today in the esteemed 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
