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

¹ Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
² Centre for Ecostratigraphy and Palaeobiology, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
³ Division of Earth Sciences, School of Environmental and Rural Science, University of New England, Armidale NSW 2351, Australia
⁴ Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden

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.

Key Words: Tommotiida • phylogeny • Brachiopoda • Phoronida • Lower Cambrian