High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa

Article High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa https://doi.org/10.1038/s41586-026-10590-9 Received: 10 February 2026 Accepted: 23 April 2026 Baopeng Song (宋宝鹏)1, Zhifei Zhang (张志飞)1 ✉, Luke C. Strotz1,2, Timothy P. Topper1,3, Andrej Ernst4, Junye Ma (马俊业)5, Zhiliang Zhang (张志亮)1,5, Mei Luo (罗梅)1, Lars E. Holmer1,6, Yue Liang (梁悦)1, Yazhou Hu (胡亚洲)1, Caibin Zhang (张彩彬)1, Yanlong Chen (陈延龙)1 & Glenn A. Brock1,7 Published online: xx xx xxxx Open access Check for updates The major animal body plans originated during the Cambrian explosion, yet the phylum Bryozoa has remained a conspicuous exception to this pattern1. The initial discovery of Protomelission gatehousei2 provided compelling evidence for a Cambrian origin for the Bryozoa, together with other major metazoan phyla and compatible with independent molecular clock estimates3–7. Nevertheless, the scarcity of definitive soft-tissue anatomy and diagnostic skeletal microstructure has left its phylogenetic affinities ambiguous and debated8,9. Here we report exquisite fossils of P. gatehousei and a new taxon, Dayingomelission hexaclitia gen. et sp. nov., from the early Cambrian Xiannüdong Formation of China. These specimens preserve in situ phosphatized soft tissues in modular skeletons, revealing critical anatomical structures, including styles, annular muscles, membranous sacs and ring septa. This suite of traits provides definitive evidence that these taxa belong to the Bryozoa. Phylogenetic analysis incorporating these new features identifies them as crown group stenolaemates. These results confirm a Cambrian origin for the phylum and reveal an unexpected early disparity in colonial architecture, demonstrating that bryozoan diversification was an integral component of the Cambrian radiation. Moreover, the early appearance of a differentiated stenolaemate crown group indicates a still deeper origin for the bryozoan stem lineage than was first apparent. The origin of the colonial lophotrochozoan phylum Bryozoa has often been considered an evolutionary enigma, because a diverse fossil record comprising six of the eight recognized bryozoan orders appears suddenly in the Early Ordovician Epoch10. The timing of this event is in stark contrast to molecular clock analyses, which have consistently indicated an origin for bryozoans in the early Cambrian period (Terreneuvian Epoch)3–6 and is inconsistent with almost all other animal phyla, which first appeared during the Cambrian evolutionary radiation6,7,11. Several putative Cambrian bryozoans have been previously proposed (such as Pywackia12,13, Archaeotrypa14 and Marcusodictyon15) but have generally been discredited16–20. Resolution of this conundrum seemingly came in the form of the modular, bilaminate colony of P. gatehousei, described from the lower Cambrian of South China and South Australia2 and recognized as the first well-supported candidate for a stem-group bryozoan on the basis of its mosaic of well-defined character traits shared with both gymnolaemates and mineralized stenolaemates. The bryozoan affinities of P. gatehousei have been widely accepted21–26, aligning the origins of the Bryozoa with other skeletonized clades and integrating bryozoans into the broader context of the Cambrian radiation. However, this interpretation has been subsequently challenged by alternative hypotheses that have relied on the absence of definitive bryozoan soft-tissue anatomy and diagnostic skeletal microstructure to suggest that the phylogenetic position of P. gatehousei may lie outside the Bryozoa8,9. Here we describe new specimens from the early Cambrian Xiannüdong Formation (southern Shaanxi, China; Extended Data Fig. 1), representing two bryozoan morphotypes: the modular bilaminate colony of P. gatehousei (Fig. 1) and a unilaminate colony ascribed to a new genus, Dayingomelission hexaclitia gen. et sp. nov. (Fig. 2). Crucially, these specimens preserve not only the skeletal traits previously used to establish the bryozoan affinities of P. gatehousei but also soft-tissue features of exceptional fidelity, including internal moulds of membranous sacs in the zooid chambers (Fig. 3). This combined set of characters unequivocally validates the assignment of these two taxa to the Bryozoa, reaffirming a Cambrian origin for the phylum. The presence of two distinct genera indicates that bryozoans were already diversifying during the Cambrian radiation. A collection of 38 modular fossils (see Methods section ‘Materials and occurrences’ for a full catalogue of specimens) are preserved as millimetric, secondarily phosphatized colonies (Figs. 1a–d, 2a–c,f–g and 3a–d). P. gatehousei has an erect, bilaminate, oligoserial colony form, 1–2 mm in width and up to 3 mm in height, tapering distally (Figs. 1a–d and 3j 1 State Key Laboratory of Continental Evolution and Early Life, Shaanxi Key Laboratory of Early Life and Environments, Department of Geology, Northwest University, Xi’an, China. 2Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands. 3Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden. 4Institut für Geologie, Universität Hamburg, Hamburg, Germany. 5State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China. 6Department of Earth Sciences, Palaeobiology, Uppsala University, Uppsala, Sweden. 7School of Natural Sciences, Macquarie University, Sydney, New South Wales, Australia. ✉e-mail: Nature | www.nature.com | 1 Article a b e c d f j g h i k * l Fig. 1 | Specimen of P. gatehousei from the Xiannüdong Formation in which the membranous sacs are preserved (ELI DYCX 8-001). a, Front side of the colony. The outlined area is magnified in h. b, Back side of the colony. The outlined area is magnified in j. c, Lateral view of the bifoliate colony. d, Oblique lateral view of the bifoliate colony showing the hollow arched mesotheca (arrow). e, Partial enlargement of c showing the staggered budding pattern. f,g, X-ray tomographic microscopy images showing the longitudinal section of the colony and the orifice of autozooids (arrowheads) (f, oblique lateral view; g, lateral view). h, Quincuncial arrangement of sub-hexagonal membranous sacs with elliptical orifice. Note the 10-μm gap present between adjacent membranous sacs, indicating the loss of skeletal walls during the taphonomic 2 | Nature | www.nature.com m processes. The outlined area is the membranous sac magnified in i. i, Enlarged view of a membranous sac showing the orifice (asterisk), circular fibres (arrow) and longitudinal fibres (arrowhead). These features suggest muscle preservation in the membranous sac. j, Enlarged view of a zooid. Note that the aperture is coated with secondary phosphate (the energy-dispersive spectroscopy analysis of this aperture is shown in Extended Data Fig. 5). k, Enlarged view of a zooid. (...truncated)