Molecular clock studies have suggested that the Phylum Bryozoa, or Moss Animals, first appeared in the Early Cambrian, which is consistent with the appearance of nearly all other Animal phyla at this time. However, for a long time the earliest known fossil Bryozoans came from the Early Ordovician, at which point six of the eight known orders of Bryozoans appear abruptly. Several putative Cambrian Bryozoan fossils, such as Pywackia, Archaeotrypa, and Marcusodictyon, were described, but none of these was universally accepted as a Bryozoan. In 2021 a more plausible Brozoan, Protomelission gatehousei, was described from the Early Cambrian of Australia and South China. In this it was possible to identify several Brozoan traits, including monomorphic zooid capsules, modular construction, organic composition, and a simple linear budding growth geometry, leading to the conclusion that this was probably a stem-group Bryozoan.
However, while Protomelission gatehousei shows enough Bryozoan-like features that most palaeontologists have accepted it to be at least a stem group Bryozoan, the specimens used to describe the species lacked the definitive Bryozoan soft-tissue anatomy and diagnostic skeletal microstructure which would be necessary for complete conformation, leaving the identity of these fossils open to challenge.
In a paper published in the journal Nature on 3 June 2026, Baopeng Song (宋宝鹏) and Zhifei Zhang (张志飞) of the Department of Geology at Northwest University, Luke Strotz, also of the Department of Geology at Northwest University and also of the Department of Earth Sciences at Utrecht University, Timothy Topper, again of the Department of Geology at Northwest University, and of the Department of Palaeobiology at the Swedish Museum of Natural History, Andrej Ernst of the Institut für Geologie at Universität Hamburg, Zhiliang Zhang of the Department of Geology at Northwest University, and the Institut für Geologie at Universität Hamburg, Mei Luo (罗梅), again of the Department of Geology at Northwest University, Lars Holmer, again of the Department of Geology at Northwest University, and of the Department of Earth Sciences at Uppsala University, Yue Liang (梁悦), Yazhou Hu (胡亚洲), Caibin Zhang (张彩彬), and Yanlong Chen (陈延龙), all of the Department of Geology at Northwest University, and Glenn Brock, once again of the Department of Geology at Northwest University, and of the School of Natural Sciences at Macquarie University, describe new specimens of Protomelission gatehousei from the Early Cambrian Xiannüdong Formation of southern Shaanxi Province, China, as well as a second new species of Bryozoan from the same formation.
Notably, these fossils preserve soft-tissue features in exceptional fidelity, including internal moulds of membranous sacs in the zooid chambers, which allow the unequivocal placement of these taxa within the Phylum Bryozoa. The presence of two separate Bryozoan taxa within these Early Cambrian deposits pushes the origin of the group still earlier, confirming that this group appeared during the Cambrian explosion.
These new specimens show Protomelission gatehousei as forming upright colonies with two curved lamellar sheets of zooids back-to-back, with the largest colonies being 1-2 mm in width and about 3 mm high, tapering towards their tip. Each of these lamellae has six-to-eight rows of zooids, with budding originating from a planar mesotheca.
The new species described is named Dayingomelission hexaclitia, where 'Dayingomelission' means 'honeycomb from Daying' and 'hexaclitia' means 'six slopes' in reference to the sloped, hexagonal apertures of its autozooids. Colonies of Dayingomelission hexaclitia form a sheet-like grown covering the substrate. This sheet is interpreted as having spread by linear branching, with a single row of zooids diverging to form two new rows. Each autozooid is hexagonal and box-like, between 200 µm to 400 µm in diameter, and separated from its neighbours by a double-walled structure. All vertical walls show this double-walled structure, while the basal wall is planar, sometimes showing a slight curvature.
Specimens of Dayingomelission hexaclitia from the Xiannüdong Formation showing the colony and cystids. (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). Song et al. (2026).Both species have hexagonal zooids with a box-shaped profile and a non-porous phosphatized or silicified skeleton. These are more-or-less uniform in size, and angled at 30-75° to the median lamina or basal exterior wall. They have preserved phosphatized internal structures interpreted as membranous sacks, the outer end of which comprises an elliptical orifice surrounded by an undulating fold. These are made up of densely packed circular and longitudinal fibres interpreted as annular and longitudinal muscles. Longitudinally aligned cylindrical structures, possibly representing protective shields or a broad operculum are present in some specimens. In others sac is attached to the cystid wall in the inner part of the zooid cell.
Both Protomelission gatehousei and Dayingomelission hexaclitia show most of the key features associated with Palaeozoic Bryozoans, including aspects of their colony morphology, their skeletal architecture, the presence of soft-tissue structures such as membranous sacs, as well as annular and longitudinal musculature. Notably they contain a number of features associated with the Class Stenolaemata, including styles and a free-walled colony organisation, which would make both species crown-group Brozoans. This makes it more likely that they were biomineralized in life, although it is impossible to determine the initial composition of their skeletons. Brozoans are known to have undergone a number of independent biomineralization events, with a molecular clock analysis indicating that the first of these was likely to have happened in the Early Cambrian. These results also imply that the common ancestor of the organicwalled Gymnolaemata and the mineralized Stenolaemata probably originated in the early Cambrian (Terreneuvian) or even perhaps in the Ediacaran Period.
The presence of two species of Bryozoan in the Early Cambrian Xiannüdong Formation of Shaanxi Province, as well as one of these species being present in the lower Wirrealpa Limestone of South Australia makes it likely that Bryazoans had already diversified and become widespread in the Early Cambrian. This lends support to the idea that the tentative mineralised Bryomorphs from the Lower Cambrian of Nevada recently described by Pruss et al. (2022) are also Bryozoans, and that Moss Animals were therefore widespread in shallow Cambrian seas, particularly Archaeocyath reef-associated carbonate platform settings.
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