Small colonial animals known as bryozoans, once believed to have evolved millions of years after the Cambrian explosion, have now been proven to have existed much earlier. Stunning fossils discovered in 520 million-year-old rocks in China reveal that these fascinating creatures have been part of our planet’s history all along.
Reconstruction of the early Cambrian ocean floor featuring Protomerision Gatehouse and Daingomellission Hexacritia approximately 520 million years ago in the shallow waters of Archaeossias Reef. Image credit: Zhifei Zhang.
“Bryozoans are small, filter-feeding colonial invertebrates flourishing in the world’s oceans today. However, their origins have puzzled scientists for decades,” said paleontologist Dr. Timothy Topper from Northwest University and the Swedish Museum of Natural History.
“While almost all other major animal groups first appeared during the Cambrian explosion about 530 million years ago, the fossil record for bryozoans remained elusive until the Ordovician period, roughly 50 million years later.”
A recent study examined an intricate bryozoan fossil from the early Cambrian Sennudo Formation in China.
This study reveals two species: Protomerision Gatehouse and a newly identified taxon, Daingomellission Hexacritia.
“For a long time, bryozoans were the missing link in Cambrian paleontology,” Dr. Topper stated.
“Aside from bryozoans, every major animal phylum had representatives during the Cambrian. These fossils bring clarity to this long-standing gap.”
Specimen of Protomerision Gatehouse excavated from the sacrolactoid layer where a membranous sac is preserved. Image credit: Song et al., doi: 10.1038/s41586-026-10590-9.
This discovery is not just a minor addition to the fossil record; it significantly impacts the understanding of the evolutionary tree of life.
Phylogenetic analysis places both Protomerision Gatehouse and Daingomellission Hexacritia firmly within the Crown Group Stenolaemata, encompassing one of the three primary classes of living bryozoans.
These fossils indicate an already advanced branch of the Bryonidae family tree, pushing the origin of the entire group back potentially to Ediacaran times, before the Cambrian explosion.
Furthermore, while some researchers have questioned whether Protomerision Gatehouse is indeed a bryozoan, suggesting it could be a sclerotid isolated from green algae or an unrelated organism, new soft tissue data and detailed comparisons of colony dimensions, shape, and internal structure support its classification as a true bryozoan.
Specimen of Daingomellission Hexacritia, showing colonies and cystids from the Xiannüdong Formation. Image credit: Song et al., doi: 10.1038/s41586-026-10590-9.
“These fossils represent not just precursors, but complex, modular colonies,” explains paleontologist Baopeng Song of Northwest University.
“The combination of skeletal structures and internal anatomy provides conclusive evidence that these are genuine bryozoans, indicating that this phylum was already diversifying during the Cambrian radiation.”
“The findings suggest that bryozoans were more widespread and sophisticated than previously acknowledged in early Cambrian oceans,” he added. “Colonial body plans, consisting of genetically identical polypids cooperating within a shared skeleton, appear to have originated as a fundamental innovation of the Cambrian explosion itself.”
The team’s paper has been published in the latest edition of Nature.
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B. Song et al., The high-fidelity modular skeleton confirms the Cambrian origin of bryozoans. Nature published online on June 3, 2026. doi: 10.1038/s41586-026-10590-9
Source: www.sci.news