The Scleritome of 'Eccentrotheca' from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny

Abstract

The first partially articulated scleritome of a tommotiid, 'Eccentrotheca' sp., is described from the Lower Cambrian of South Australia. The 'Eccentrotheca' scleritome consists of individual sclerites fused in a spiral arrangement, forming a tapering tube-shaped skeleton with an inclined apical aperture and a circular to subcircular cross section. Traditionally, tommotiid sclerites have been assumed to form a dorsal armor of imbricating phosphatic plates in slug-like bilaterians, analogous to the calcareous sclerites of halkieriids. The structure of the 'Eccentrotheca' scleritome is here reinterpreted as a tube composed of independent, irregularly shaped sclerites growing by basal-marginal accretion that were successively fused to form a rigid, protective tubular structure. The asymmetrical shape and sometimes acute inclination of the apical aperture suggests that the apical part of the scleritome was cemented to a hard surface via a basal disc, from which it projected vertically. Rather than being a vagrant member of the benthos, 'Eccentrotheca' most likely represented a sessile, vermiform filter feeder. The new data suggest that the affinities of 'Eccentrotheca', and possibly some other problematic tommotiids, lie with the lophophorates (i.e., the phoronids and brachiopods), a clade that also possesses a phosphatic shell chemistry and a sessile life habit.