The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny
The first partially articulated scleritome of a tommotiid, Eccentrothecasp., is described from the Lower Cambrian of South Australia.The Eccentrotheca scleritome consists of individual scleritesfused in a spiral arrangement, forming a tapering tube-shapedskeleton with an inclined apical aperture and a circular tosubcircular cross section. Traditionally, tommotiid scleriteshave been assumed to form a dorsal armor of imbricating phosphaticplates in slug-like bilaterians, analogous to the calcareoussclerites of halkieriids. The structure of the Eccentrothecascleritome is here reinterpreted as a tube composed of independent,irregularly shaped sclerites growing by basal-marginal accretionthat were successively fused to form a rigid, protective tubularstructure. The asymmetrical shape and sometimes acute inclinationof the apical aperture suggests that the apical part of thescleritome was cemented to a hard surface via a basal disc,from which it projected vertically. Rather than being a vagrantmember of the benthos, Eccentrotheca most likely representeda sessile, vermiform filter feeder. The new data suggest thatthe affinities of Eccentrotheca, and possibly some other problematictommotiids, lie with the lophophorates (i.e., the phoronidsand brachiopods), a clade that also possesses a phosphatic shellchemistry and a sessile life habit.