GoKawiilThe 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)3,4,5,6 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 discredited16,17,18,19,20.
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 accepted21,22,23,24,25,26, 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.
Fig. 1: Specimen of P. gatehousei from the Xiannüdong Formation in which the membranous sacs are preserved (ELI DYCX 8-001). The alternative text for this image may have been generated using AI. Full size image 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 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. Note that the secondary phosphate coating of the aperture is partially stripped away. l,m, Enlarged view of the membranous sac showing the longitudinal fibres (l, arrowhead) and circular fibres (m, arrow). These features suggest muscle preservation in the membranous sac. Scale bars, 500 μm (a–d), 50 μm (e,i–k), 200 μm (f), 150 μm (g), 100 μm (h) and 30 μm (l,m).
Fig. 2: Specimens of D. hexaclitia gen. et sp. nov. from the Xiannüdong Formation showing the colony and cystids. The alternative text for this image may have been generated using AI. Full size image a,b, ELI ZJBX 10-001 (holotype). a, Oblique view of the front side of a unilaminate colony form clearly showing the regular hexagonal, compactly arranged, honeycomb-shaped cystids. The outlined area is shown in b. b, Hexagonal cystid with vertical wall and ring septa clearly evident (arrow). c–e, ELI ZJBX 10-002. c, Front side of a unilaminate colony form. The bottom outlined area shows the cystids magnified in d; whereas the top outline shows the cystids magnified in e. d, Enlarged view of adjacent cystids. Note the hexagonal vertical wall (arrow) and the basal exterior wall of cystids (arrowheads). e, Row bifurcation showing change in zooid width along rows. f–i, ELI ZJBX 3-001. f, Front side of a unilaminate colony form with styles indenting the zooidal chambers. g, Oblique view showing hexagonal cystids with styles. h, Oblique view of colony surface. Note that the styles arise in the endozone and extend through most of exozone. i, Enlarged view of the vertical wall with planar spherulitic fabric. Scale bars, 500 μm (a,c), 80 μm (b), 100 μm (d), 200 μm (e), 300 μm (f,g), 100 μm (h) and 25 μm (i).
Fig. 3: Membranous sacs preserved in situ in the autozooid cystids of P. gatehousei and D. hexaclitia gen. et sp. nov. and colonial growth reconstruction of P. gatehousei. The alternative text for this image may have been generated using AI. Full size image a,b, P. gatehousei ELI DYCX 8-016. a, Front side of a bifoliate colony showing the eight series of zooids. The outlined area is magnified in b. b, Enlarged view of a zooid. Note that the membranous sac (arrow) is preserved in the cystid (arrowhead). c–g, D. hexaclitia ELI DYCX 8-004. c, Front side of a unilaminate colony, with ten series of zooids, all with membranous sacs and cystids. The outlined area is magnified in g. d, Back side of the colony showing the membranous sacs of the zooids and the gap between the sacs. The outlined area is magnified in e. e, Enlarged view showing capsule-like membranes and gaps. f, X-ray tomographic microscopy image showing the longitudinal section of zooids with membranous sacs and cystids. g, Enlarged view highlighting that the membranous sacs (arrow) are captured in the cystids (arrowhead), and the membranous sacs are in contact with the cystids 20 μm from the apertures (ligamentous attachment, asterisks). h, Three-dimensional reconstruction of a bryozoan zooid with protruding lophophore. i, Longitudinal section of reconstructed bryozoan zooid. Greyish white, cystid; translucent white, membranous sac and tentacles; pink, polypide excluding tentacles. j, Reconstruction of P. gatehousei, front surface view. Illustration in j adapted from ref. 2, under a Creative Commons licence CC BY 4.0. Scale bars, 500 μm (a,c,d), 40 μm (b), 200 μm (e,f) and 100 μm (g).
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 and Extended Data Fig. 2a–c). Each colony consists of two curved sheets of zooids arranged back to back, comprising six to eight rows of autozooids per side, with budding originating from a planar mesotheca (Extended Data Fig. 3a–c) or a flattened axial cylinder (Fig. 1d). D. hexaclitia is characterized by a unilaminate colony (Fig. 2a,c) (see ‘Systematic palaeontology’ section). Its autozooid chambers align within a single plane, growing almost perpendicular to the basal exterior wall (Fig. 2c–e). The colony surface displays a distinct zooidal arrangement, with colony growth proceeding through bifurcation (Fig. 2e). One of the unilaminate specimens (ELI ZJBX 3-001) preserves elongated, elliptical–cylindrical styles that originate in the endozone and extend prominently throughout the exozone (Fig. 2f–h).
All zooids of both taxa are hexagonal in outline and box-shaped in profile, with a phosphatized or silicified skeleton (cystid) present in all colony forms (Fig. 2a–e and Extended Data Fig. 3a–c). On the frontal surface, these autozooids have an average width of 208.4 μm and a length of 121.7 μm (Extended Data Fig. 4 and Supplementary Data 1). The cystids are uniform in size, short and box-shaped, angled at 30–75° to the median lamina (mesotheca) (Extended Data Fig. 3a–c,f) or basal exterior wall (Fig. 2d). The skeleton is composed of non-porous basal and vertical walls. Although the vertical walls of adjacent zooids form a double wall (Fig. 2a–e and Extended Data Fig. 3d–f), the frontal (exterior) skeleton walls are structurally absent. The internal cystid walls of D. hexaclitia have a planar spherulitic fabric (Fig. 2i), whereas its vertical walls preserve ring septa (Fig. 2b).
The zooids of P. gatehousei and D. hexaclitia contain phosphatized structures interpreted as membranous sacs, featuring thin walls and smooth outlines (Figs. 1a,e,h,i and 3b,g). The distal end of each membranous sac consists of a long, elliptical orifice surrounded by an undulating fold (Fig. 1h,i and Extended Data Fig. 5i). A consistent, uniform, 10–20-µm-wide gap separates adjacent sacs (Fig. 1h,i). The densely packed, well-developed circular and longitudinal fibres on the membranous sac surfaces represent annular and longitudinal muscles (Fig. 1i,l,m). The sac membranes are composed of crystalline apatite particles (Extended Data Figs. 6 and 7). In ELI DYCX 8-001, the phosphatized sacs and secondarily filled orifices (Extended Data Fig. 5) are overgrown by a subsequent phosphate coating, forming smooth bulges (Fig. 1b,j,k), indicating a multiphase phosphatization history. Longitudinally aligned cylindrical structures, possibly representing protective shields or a broad operculum, are preserved on both sides of the secondary orifice coatings (Extended Data Fig. 2a–f). The exceptional preservation of specimens ELI DYCX 8-016 and ELI DYCX 8-004 reveals that the membranous sacs are preserved in situ in the autozooid cystids (Fig. 3a–c,g–j). The sac is attached to the cystid wall in the distal apertural area, with a consistent spacing of 10–20 µm (Fig. 3g), matching the thickness interval of adjacent sacs in specimen ELI DYCX 8-001 (Fig. 1h,i). Transmission electron microscopy confirms the absence of a replacement relationship between the phosphatized sac and the silica-replaced cystid wall in ELI DYCX 8-004 (Extended Data Fig. 6), indicating their independent preservation pathways.