The operculum and mode of life of Hyolites

A reexamination of Hyolithes carinatus Matthew from the Middle Cambrian Burgess shale [of British Columbia] has shown that the features considered by C.D. Walcott to be supports for the fins of pteropods actually appear to be outgrowths of the operculum. If the dorsal point of the operculum was hinged at the dorsal point of the aperture a push of the body outward would have swung it upward. The supports may then have served to prop up the operculum and keep it open. Because of the supports and other features of shell morphology, Hyolithes is interpreted as a virtually sessile, benthonic organism, the longest side of the shell being ventral.