Exceptionally well-preserved specimens of the reticulosan sponge Cyathophycus loydelli from the Sandbian (Late Ordovician) Llanfawr Mudstones Formation of Llandrindod, Waes, UK, have been examined using scanning electron microscopy (SEM). The specimens include exquisitely detailed pyritized spicules, and granular pyritization of surrounding soft tissues. Spicules frequently show axial canals of diameter similar to those of modern siliceous sponges, with hexagonal symmetry typical of modern demosponges rather than hexactinellids. In one case, the axial filament is also preserved. The largest spicules (ray diameter >20 μm) show a complex structure, with a laminar external region similar to that of the extant hexactinellid Monorhaphis. Some spicules preserve sub-micron detail of the spicule surface, resembling the reticulate collagenous sheath of Monorhaphis. The hexagonal symmetry of the canal confirms that at least some Reticulosa are not crown-group hexactinellids, but stem-group Hexactinellida or Demospongea, or stem-group Silicea. This suggests that a square canal is a sufficient diagnostic feature of total-group Hexactinellida, but that hexagonal canals were more widely distributed among Early Silicea and were probably not restricted to demosponges. Alternatively, comparison with the structure of modern verongiid fibres suggests that these may be homologous with the outer layers of Cyathophycus spicules, and Cyathophycus may instead be a stem-group demosponge. The preserved detail of the surface layer shows that pyritization can preserve certain material with extraordinarily fine resolution.