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The operculum of the problematic tubular fossil Cupitheca holocyclata Bengtson in Bengtson et al., (1990) is described for the first time based on collections from South Australia and North China. The phosphatized sub-circular operculum exhibits well defined cardinal processes and a narrow cardinal shield unequivocally demonstrating that Cupitheca is a hyolith, probably an orthothecid. C . holocyclata has an almost global distribution in Cambrian Stages 3–4. The apical structure of the operculum is an elevated, disc-shaped platform with a concave base and a marginal rim that could represent the scar of a specialized larval attachment structure, perhaps anchoring the larval hyolith to a sediment grain, algae or other benthic substrate. Cupitheca probably had a pelagic larval stage and settled on the seafloor by attachment of the apical disc to suitable substrates before developing a free-living benthic adult lifestyle. This contrasting mode of life compared to other hyolith genera suggests that the group had already evolved a range of distinct lifestyles in the Cambrian, providing significant clues into their ecology and distribution. [ABSTRACT FROM AUTHOR]

Helens, the curved lateral spines inserted between the conch and operculum of some hyoliths, are a unique morphological adaptation characterizing the order Hyolithida. These structures are paired, movable and had a mechanical function, probably related to orienting the hyolith conch and lifting its aperture above the sea floor. We show that helens are intimately associated with the hyolith opercula and are structurally comparable to the rod like units that constitute the clavicles, internal wall-like structures of the hyolithid operculum that probably evolved to secure the operculum from lateral displacement in the conch aperture. In some early Cambrian hyolith taxa that lack helens, such as Paramicrocornus , new clavicle rods are added in the gap separating the clavicles from the cardinal processes, the same position where helens are inserted in later hyolithids. We also show that the size of incipient helens at the earliest ontogenetic stage matches the size of the clavicles in associated opercula. We propose that helens are modified clavicle rods that were detached from the operculum and developed into lateral spines through allometric growth during early ontogeny. Further, we suggest a four-step model for the evolution of hyolithid hyoliths from orthothecid ancestors: 1, Externally fitting operculum; 2, Stabilizing, radially arranged structures on the inside of the operculum; 3, Ligula and folded operculum; 4, Detachment of clavicle rods and origin of helens. • Hyolithids are extinct benthic invertebrates with a conch and an external operculum. • Helens, lateral processes of Hyolithids, helped stabilize, re-orient and lift the conch. • Helens share a fibrous ultrastructure with the clavicles of hyolith opercula. • Helens evolved from detached clavicle rods. • Through allometric growth, clavicle rods developed into functional clavicles. [ABSTRACT FROM AUTHOR]