"Ladatheca" cylindrica (Grabau, 1900) was a eurytopic metazoan with a calcareous operculum and an elongate, gently tapering conch up to 15+ cm long and with an apical angle of approximately 0.75-2.0°. This apparent polychaete was geographically widespread in the sub-trilobitic Lower Cambrian of middle latitude, siliciclastic (middle and upper Placentian Series, Avalon Zone) and low latitude, carbonate platform (Tommotian Stage, Siberia(?) and Kazakhstan; and, probably, the upper Meishucunian Stage, southwestern China) sequences. In situ conchs (vertically to steeply inclined, apex down) of "Ladatheca" cylindrica are common in deep subtidal, siliciclastic mudstones (Chapel Island Formation) and in a peritidal algal mud mound(?) limestone (West Centre Cove Formation) in southeastern Newfoundland. Closely juxtaposed conchs locally form a worm reef at the top of the West Centre Cove Formation. This structure is older than the oldest archaeocyathan build-ups and is the earliest known metazoan-constructed biostrome. "Ladatheca" cylindrica was a dominant element of the sessile benthos prior to its apparent ecological displacement by Coleoloides typicalis Walcott, 1889, in the upper Placentian Series.