Cambrian scale worm (stem-group polychaete) from Laurentia that lived in a tube

Hyolithellus micans, comprising the eponymous tubular fossil and its operculum Discinella micans, is interpreted as a stem-group polychaete currently known from Cambrian Series 2, Stage 4 of northeastern Laurentia. After settling to the sea floor, the free swimming, segmented, nectochaete larva developed through the secretion of phosphatic lamellae into an operculum which closed the aperture of a tube of the same lamellar construction that was attached to the substrate. Preservation of the larva in the initial growth stage of the phosphatized operculum reveals pairs of imbricated plates on the segment flanks that can be compared to the elytra of modern scale worms (polynoid polychaetes). The plates serve a basic defensive role that is expanded in the elytra of extant polynoids to include a variety of sensory functions, including bioluminescence and the autotomy of elytra. The cross section of the initial growth stage is reminiscent of the morphological differentiation and longitudinal trilobation seen in the trunk of the stem-group polychaete Kuanchuanpivermis from the Cambrian (Fortunian Stage) of China. The sedentary Hyolithellus micans further documents the increasing ecological diversity of the already varied polychaete total group in the early Cambrian.John S. Peel* [john.peel@pal.uu.se], Department of Earth Sciences (Palaeobiology), Uppsala University, Villavägen 16, Uppsala SE–75236, Sweden.